US20060116588A1 - Method and apparatus for calculating blood pressure of an artery - Google Patents

Abstract

The present invention is a method for calculating blood pressure of an artery having a pulse. The method includes applying a varying pressure to the artery. Pressure waveforms are sensed to produce pressure waveform data. Waveform parameters are derived from the sensed pressure waveform data. Blood pressure is then determined using the derived parameters.

Description

CROSS-REFERENCE TO RELATED APPLICATIONS
• This patent is a continuation of and claims the benefit of prior U.S. patent application Ser. No. 10/439,445, filed May 16, 2003, entitled “METHOD AND APPARATUS FOR CALCULATING BLOOD PRESSURE OF ANY ARTERY,” which is a continuation of application Ser. No. 09/594,051, filed Jun. 14 2000, entitled “METHOD AND APPARATUS FOR CALCULTING BLOOD PRESSURE OF AN ARTERY,” which is a continuation of application Ser. No. 09/070,311, filed Apr. 30, 1998, issued as U.S. Pat. No. 6,099,477 and entitled “METHOD AND APPARATUS FOR CALCULATING BLOOD PRESSURE OF AN ARTERY,” which is a continuation of application Ser. No. 08/388,751, filed Feb. 16, 1995, issues as U.S. Pat. No. 5,797,850 and entitled “METHOD AND APPARATUS FOR CALCULATING BLOOD PRESSURE OF AN ARTERY,” which is a continuation-in-part of application Ser. No. 08/227,506, filed Apr. 14, 1994, issued as U.S. Pat. No. 5,450,852 and entitled “CONTINUOUS NON-INVASIVE BLOOD MONITORING SYSTEM,” which is a continuation-in-part of application Ser. No. 08/150,382, filed Nov. 9, 1993 entitled “CONTINUOUIS NON-INVASIVE BLOOD PRESSURE MONITORING SYSTEM⇄, abandoned all of which are hereby incorporated herein in their entirety by reference thereto.
BACKGROUND OF THE INVENTION
• The present invention relates to systems for measuring arterial blood pressure. In particular, the invention relates to a method and apparatus for measuring arterial blood pressure in relatively continuous and non-invasive manner.
• Blood pressure has been typically measured by one of four basic methods: invasive, oscillometric, auscultatory and tonometric. The invasive method, otherwise known as an arterial line (A-Line), involves insertion of a needle into the artery. A transducer connected by a fluid column is used to determine exact arterial pressure. With proper instrumentation, systolic, mean and diastolic pressure may be determined. This method is difficult to set up, is expensive and involves medical risks. Set up of the invasive or A-line method poses problems. Resonance often occurs and causes significant errors. Also, if a blood clot forms on the end of the catheter, or the end of the catheter is located against the arterial wall, a large error may result. To eliminate or reduce these errors, the set up must be adjusted frequently. A skilled medical practitioner is required to insert the needle into the artery. This contributes to the expense of this method. Medical complications are also possible, such as infection or nerve damage.
• The other methods of measuring blood pressure are non-invasive. The oscillometric method measures the amplitude of pressure oscillations in an inflated cuff. The cuff is placed against a cooperating artery of the patient and thereafter pressurized or inflated to a predetermined amount. The cuff is then deflated slowly and the pressure within the cuff is continually monitored. As the cuff is deflated, the pressure within the cuff exhibits a pressure versus time waveform. The waveform can be separated into two components, a decaying component and an oscillating component. The decaying component represents the mean of the cuff pressure while the oscillating component represents the cardiac cycle. The oscillating component is in the form of an envelope starting at zero when the cuff is inflated to a level beyond the patient's systolic blood pressure and then increasing to a peak value where the mean pressure of the cuff is equal to the patient's mean blood pressure. Once the envelope increases to a peak value, the envelope then decays as the cuff pressure continues to decrease.
• Systolic blood pressure, mean blood pressure and diastolic blood pressure values can be obtained from the data obtained by monitoring the pressure within the cuff while the cuff is slowly deflated. The mean blood pressure value is the pressure on the decaying mean of the cuff pressure that corresponds in time to the peak of the envelope. Systolic blood pressure is generally estimated as the pressure on the decaying mean of the cuff prior to the peak of the envelope that corresponds in time to where the amplitude of the envelope is equal to a ratio of the peak amplitude. Generally, systolic blood pressure is the pressure on the decaying mean of the cuff prior to the peak of the envelope where the amplitude of the envelope is 0.57 to 0.45 of the peak amplitude. Similarly, diastolic blood pressure is the pressure on the decaying mean of the cuff after the peak of the envelope that corresponds in time to where the amplitude of the envelope is equal to a ratio of the peak amplitude. Generally, diastolic blood pressure is conventionally estimated as the pressure on the decaying mean of the cuff after the peak where the amplitude of the envelope is equal to 0.82 to 0.74 of the peak amplitude.
• The auscultatory method also involves inflation of a cuff placed around a cooperating artery of the patient. Upon inflation of the cuff, the cuff is permitted to deflate. Systolic pressure is indicated when Korotkoff sounds begin to occur as the cuff is deflated. Diastolic pressure is indicated when the Korotkoff sounds become muffled or disappear. The auscultatory method can only be used to determine systolic and diastolic pressures.
• Because both the oscillometric and the auscultatory methods require inflation of a cuff, performing frequent measurements is difficult. The frequency of measurement is limited by the time required to comfortably inflate the cuff and the time required to deflate the cuff as measurements are made. Because the cuff is inflated around a relatively large area surrounding the artery, inflation and deflation of the cuff is uncomfortable to the patient. As a result, the oscillometric and the auscultatory methods are not suitable for long periods of repetitive use.
• Both the oscillometric and auscultatory methods lack accuracy and consistency for determining systolic and diastolic pressure values. The oscillometric method applies an arbitrary ratio to determine systolic and diastolic pressure values. As a result, the oscillometric method does not produce blood pressure values that agree with the more direct and generally more accurate blood pressure values obtained from the A-line method. Furthermore, because the signal from the cuff is very low compared to the mean pressure of the cuff, a small amount of noise can cause a large change in results and result in inaccurate measured blood pressure values. Similarly, the auscultatory method requires a judgment to be made as to when the Korotkoff sounds start and when they stop. This detection is made when the Korotkoff sound is at its very lowest. As a result, the auscultatory method is subject to inaccuracies due to low signal-to-noise ratio.
• The fourth method used to determine arterial blood pressure has been tonometry. The tonometric method typically involves a transducer including an array of pressure sensitive elements positioned over a superficial artery. Hold down forces are applied to the transducer so as to flatten the wall of the underlying artery without occluding the artery. The pressure sensitive elements in the array typically have at least one dimension smaller than the lumen of the underlying artery in which blood pressure is measured. The transducer is positioned such that at least one of the individual pressure sensitive elements is over at least a portion of the underlying artery. The output from one of the pressure sensitive elements is selected for monitoring blood pressure. The pressure measured by the selected pressure sensitive element is dependent upon the hold down pressure used to press the transducer against the skin of the patient. These tonometric systems measure a reference pressure directly from the wrist and correlate this with arterial pressure. However, because the ratio of pressure outside the artery to the pressure inside the artery, known as gain, must be known and constant, tonometric systems are not reliable. Furthermore, if a patient moves, recalibration of the tonometric system is required because the system may experience a change in gains. Because the accuracy of these tonometric systems depends upon the accurate positioning of the individual pressure sensitive element over the underlying artery, placement of the transducer is critical. Consequently, placement of the transducer with these tonometric systems is time-consuming and prone to error.
• The oscillometric, auscultatory and tonometric methods measure and detect blood pressure by sensing force or displacement caused by blood pressure pulses as the underlying artery is compressed or flattened. The blood pressure is sensed by measuring forces exerted by blood pressure pulses in a direction perpendicular to the underlying artery. However, with these methods, the blood pressure pulse also exerts forces parallel to the underlying artery as the blood pressure pulses cross the edges of the sensor which is pressed against the skin overlying the underlying artery of the patient. In particular, with the oscillometric and the auscultatory methods, parallel forces are exerted on the edges or sides of the cuff. With the tonometric method, parallel forces are exerted on the edges of the transducer. These parallel forces exerted upon the sensor by the blood pressure pulses create a pressure gradient across the pressure sensitive elements. This uneven pressure gradient creates at least two different pressures, one pressure at the edge of the pressure sensitive element and a second pressure directly beneath the pressure sensitive element. As a result, the oscillometric, auscultatory and tonometric methods produce inaccurate and inconsistent blood pressure measurements.
SUMMARY OF THE INVENTION
• The present invention is an improved method for determining blood pressure of an artery having a pulse. As a varying pressure is applied to the artery, pressure waveforms are sensed to produce sensed pressure waveform data. The sensed pressure waveform data are then analyzed to derive waveform parameters. One or more blood pressure values are derived based upon the waveform parameters.
BRIEF DESCRIPTION OF THE DRAWINGS
• FIG. 1 is a perspective view of a blood pressure monitoring system having a sensor assembly mounted upon the wrist of a patient.
• FIG. 2 is a side view of the wrist assembly of the blood pressure monitoring system ofFIG. 1.
• FIG. 3 is an end view of the wrist assembly.
• FIG. 4 is a cross-sectional view of the wrist assembly.
• FIG. 4A is an expanded cross-sectional view of the sensor interface alongsection4A-4A ofFIG. 4.
• FIG. 5 is a top view of the wrist assembly and cylinder of the system ofFIG. 1.
• FIG. 6 is a bottom view of the wrist assembly and cylinder with a portion removed.
• FIG. 7 is an electrical block diagram of the blood pressure monitoring system ofFIG. 1.
• FIG. 8 is a front elevational view of a monitor of the blood pressure monitoring system ofFIG. 1.
• FIG. 9 is a graph illustrating blood pressure waveforms.
• FIG. 10 is a graph illustrating a curve fit from points taken from the waveforms ofFIG. 9.
• FIG. 11 is a graph illustrating a corrected and scaled waveform taken from the waveforms ofFIG. 9.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTSI. Overview
• FIG. 1 illustrates bloodpressure monitoring system20 for measuring and displaying blood pressure within an underlying artery within wrist22 of a patient.Monitoring system20 includeswrist assembly24, monitor26,cylinder28,cable30 andcable32.
• Wrist assembly24 is mounted on wrist22 for applying a varying hold down pressure to an artery within wrist, and for sensing blood pressure waveforms produced in the artery.Wrist assembly24 includesswivel mount34, hold downassembly36,sensor interface assembly38,waveform pressure transducer40, hold downpressure transducer42,connection tube44,wrist mount46 andwrist pad48.
• Cylinder28, under the control ofmonitor26, supplies fluid pressure throughcable32 towrist assembly24 to produce the varying hold down pressure.Cylinder28 includes a movable piston which is driven by stopper motor or linear actuator.
• Electrical energization towrist assembly24 and pressure waveform sensor signals to monitor26 are supplied over electrical conductors extending betweenmonitor26 and wrist assembly throughcable30,cylinder28 andcable32. Drive signals tocylinder28 are supplied frommonitor26 through electrical conductors withincable30.
• Monitor26 receives the pressure waveform sensor signals fromwrist assembly24, digitizes the signals to produce pressure waveform data for a plurality of beats, and performs waveform analysis on the data. The waveform analysis extracts a plurality of waveform parameters, which preferably include waveform shape, relative amplitude and gain parameters. From the waveform parameters, monitor26 calculates or otherwise derives blood pressure values, such as mean blood pressure, diastolic blood pressure and systolic blood pressure.Monitor26 then displays the derived blood pressure values.
• As shown inFIG. 1, monitor26 includes control switches orinput keys50a-50g, digital displays52a-52canddisplay screen54.Input keys50a-50ccomprise hard keys for controllingmonitor32.Input keys50d-50gconsist of software programmable keys which are adaptable for various functions. Digital displays52a-52ccontinually display systolic, diastolic and mean blood pressure, respectively.Display screen54 displays the blood pressure pulse waveforms and prompts to guide the operator.
• In operation,sensor interface assembly38 is positioned over the radial artery.Wrist mount46 maintains the position ofwrist assembly24 includingsensor interface assembly38 on wrist22. In response to fluid pressure supplied fromcylinder28 throughcable32, hold downassembly36 applies force and movessensor interface assembly38 to vary the pressure applied to wrist22 above the radial artery.
• As this pressure is varied, distinct arterial pressure waveforms are exhibited by the blood pressure pulse within the underlying artery. Each waveform corresponds to a cardiac cycle. Each arterial pressure waveform or shape is obtained by sensing and measuring pressures exhibited by the pulse of the underlying artery versus time during an individual cardiac cycle. Arterial pressure applied tosensor interface assembly38 and is transferred as a fluid pressure frominterface assembly38 towaveform pressure transducer40 throughtube44. The electrical sensor signals fromtransducer40 are supplied to monitor26 for digitization and analysis.
• The amplitude of each sensed waveform is a function of the applied pressure applied to the artery bysensor interface assembly38 and the amplitude of the arterial pulse. The shape characteristics of at least one waveform and other parameters derived from the sensed waveforms are used by digital signal processing circuitry ofmonitor26 to determine systolic, mean and diastolic pressure. The calculated pressures are displayed by displays52a-52canddisplay screen54.
II.Wrist Assembly24
• Wrist assembly24 is shown in further detail inFIGS. 2-6.Swivel mount34 and hold downassembly36 are side-by-side, and are pivotally connected by swivel joint60. Swivel mount34 carriestransducers40 and42 andwrist pad48.Sensor interface assembly38 is pivotally connected to and is positioned below hold downassembly36.Wrist mount46, which includesflexible wrist band62 andwire loops64 and66, is connected between an outer end ofswivel mount34 and teetermount68 at an opposite outer end of hold downassembly36.
• FIG. 2 is a side elevational view illustratingwrist assembly24 in greater detail.Swivel mount34 is a U-shaped body. Swivel joint60 is formed by asocket70 ofswivel mount34 andswivel ball72 of hold downassembly36.Socket70 extends into a channel within the U-shaped configuration ofswivel mount34 and is sized for receivingswivel ball72 which projects from an inner end wall of hold downassembly36. The ball socket swivel joint provided byball72 andsocket70 permit swivel mount34 and hold downassembly36 to rotate and pivot in virtually any direction so as to better conform to wrist22. To aid in pivotingswivel mount34 with respect to hold downassembly36, swivel mount34 includes an arcuate or beveledlower edge74 along its inner end. Bevelededge74 permits hold downassembly36 to pivot downward so as to wrap around wrist22 (or alternate anatomy) of a patient.
• Swivel mount34 further includes a tighteningscrew76 which extends across swivel mount34adjacent socket70 andball72. Tighteningscrew76permits socket70 of swivel mount34 to be tightened aboutball72 so as to increase friction betweensocket70 andball72 to adjust the level of force necessary to readjust the positioning ofswivel mount34 and hold downassembly36.Untightening screw76permits ball72 to be released fromsocket70 such that hold downassembly36 andsensor interface assembly38 may be disassembled fromswivel mount34.
• FIG. 3 is a end elevational view of bloodpressure monitoring system20 ofFIG. 1, illustrating teeter mount68 in greater detail. As shown byFIG. 3, teetermount68 includesfulcrum80 and tighteningscrew82.Fulcrum80 is generally a triangular shaped member having two opposing slanted top surfaces.Fulcrum80 is coupled toloop66 and thereby towrist band62.Fulcrum80 teeters about hold downassembly36 andpermits loop66 andwrist band62 to be adjustably positioned so as to better conform to wrist22. Tighteningscrew82 extends throughfulcrum80 and threadably engages hold downassembly36. Tighteningscrew82 tightensfulcrum80 against hold downassembly36 so that the position offulcrum80 may be frictionally set. InFIG. 3,fulcrum80 is shown in a middle position, and can be rotated either a clockwise or counterclockwise direction as needed.
• Wrist assembly24 stably and securely positionssensor interface assembly38 over the underlying artery of the patient.Swivel mount34 may be rotated and pivoted in practically all directions aboutsocket70 andball72. Furthermore, teeter mount68permits wrist band62 to be teetered or adjusted so as to better conform with wrist22 of the patient.Wrist band62 wraps around wrist22 to securesensor interface assembly38 andwrist pad48 adjacent wrist22 of the patient. Becausesensor interface assembly38 is more securely and stably positioned above the underlying artery of wrist22, patient movement is less likely to repositionsensor interface assembly38. As a result,sensor interface assembly38 can be reliably located over the underlying artery so that more accurate and consistent blood pressure measurements may be taken.
• As shown inFIG. 4, swivel mount34 carrieswaveform pressure transducer40, hold downpressure transducer42, andwrist pad48.Waveform pressure transducer40 senses blood pressure waveforms from the artery which is transmitted to transducer40 fromsensor interface assembly38 through fluid tube44 (FIG. 1). Hold downpressure transducer42 senses fluid pressure supplied bycylinder28 to hold downassembly36, and is used as a safety feature to detect an excess hold down pressure condition.Wrist pad48 is preferably adhesively secured to plate90 at a bottom surface ofswivel mount34.Pad48 is preferably made of a soft flexible and compressible material so that swivel mount34 better conforms to the wrist of a patient.Plate90 is preferably made of a metal such as brass and is screwed to swivelmount34 byscrew92. Conductive plate94 is secured withinswivel mount34 and is spaced fromplate90 so thattransducer40 is positioned betweenplates90 and94.Transducer40 preferably has a metallic conductive surface such as brass which contacts conductive plate94, which is electrically grounded. As a result, brass plate94electrically grounds transducer40 so as to drain static charge fromtransducer40.
• As shown byFIG. 4, hold downpressure assembly36 includesswivel ball72,housing100,diaphragm102,ring104,piston106,piston rod108,pin110 andpin mount112.Diaphragm102 comprises a generally circular sheet of flexible material such as reinforced rubber.Diaphragm102 is spaced from and cooperates withinterior cavity114 formed withinhousing100 to definepressure chamber116.Pressure chamber116 extends generally above and partially aroundpiston106.Pressure chamber116 receives pressurized fluid fromcylinder28 throughfluid passage118 such thatdiaphragm102 expands and contracts to drivepiston106 andpiston rod108 up and down. As a result, a selected pressure may be applied topiston106 andpiston rod108 so as to selectively apply a pressure tosensor interface assembly38, which is pivotally mounted to the lower end ofpiston rod108. By varying the volume of fluid withinpressure chamber116, bloodpressure monitoring system20 applies a varying hold down pressure tosensor interface assembly38 and the underlying artery.
• Diaphragm102 is supported in place byring104.Ring104 encircles the outer perimeter ofdiaphragm102 and captures an outer perimeter or edge portion ofdiaphragm102 betweenring104 andhousing100 so as to sealdiaphragm102 againsthousing100.Ring104 is preferably adhesively secured tohousing100 anddiaphragm102.
• Piston106 is preferably a disk or cylinder shaped member which has its top surface preferably fixedly coupled (such as by an adhesive) todiaphragm102. Consequently, as fluid is supplied tochamber116, the volume ofchamber116 expands by movingpiston106 downward.Bore120 extends from top to bottom ofpiston106 and is sized for receiving a portion ofpiston rod108.Piston106 mates withpiston rod108 and exerts pressure uponpiston rod108 andsensor interface assembly38.
• Piston rod108 is coupled topiston106 andsensor interface assembly38.Piston rod108 includesplug122,flange124,stem126,ball128 andpin hole130.Plug122 is cylindrically shaped and is press fit withinbore120 to securepiston rod108 topiston106.Flange124 projects outwardly fromplug122 and fits within a depression formed in the bottom surface ofpiston106. As a result,piston106 presses againstflange124 ofpiston rod108 to drivepiston rod108. Alternatively, becausepiston rod108 is secured topiston106 byplug122,piston106lifts piston rod108 as pressure is decreased withinpressure chamber116.Stem126 integrally extends downward fromflange124 and has a length extending intointerface assembly38.Ball128 is integrally formed at the lower end ofstem126 and is received withinsocket132 ofsensor interface assembly38. As a result,sensor interface assembly38 pivots aboutball128 ofpiston rod108.
• Pin hole130 axially extends throughpiston rod108 and is sized for receivingpin110.Pin110 is fixedly secured tohousing100 bypin mount112 and extends throughhousing100 intopin hole130.Pin110 has a diameter smaller than the diameter ofpin hole130 and extends intostem126.Pin110 guides the up and down movement ofpiston106 andpiston rod108 as pressure withinpressure chamber116 is varied.Pin110 prevents lateral movement ofpiston106 andpiston rod108 so thatpiston106 andpiston rod108 apply only a perpendicular force tosensor interface assembly38. As a result, pin110permits piston106 andpiston rod108 to move up and down whilepin110 remains fixedly supported bypin mount cap112 tohousing100.Pin110 is preferably made from a hard rigid material such as stainless steel.
• As shown byFIG. 4, hold downpressure assembly28 further includespressure supply passage118, which extends frompressure chamber116 throughswivel ball72 where it connects withflexible tubes140 and142 (shown inFIGS. 5 and 6).Flexible tube140 extends throughcable32 fromcylinder28 topassage118 inswivel ball72.Flexible tube142 connectspassage118 totransducer42 inswivel mount34. This allows transducer to monitor the fluid pressure inchamber116.Fluid supply tube140 applies pressurized fluid fromcylinder28 intopressure chamber116 to vary the pressure withinchamber116 so as to drivepiston106 andpiston rod108.
• FIGS. 4 and 4A illustratesensor interface assembly38 in detail.FIG. 4 is a cross-sectional view ofwrist assembly24.FIG. 4A is an enlarged cross-sectional view ofsensor interface assembly38, taken alongsection4A-4A ofFIG. 4.Sensor interface assembly38 includestop plate150,upper V mount152, lower V mount154,diaphragm lock156,inner mounting ring158, outer mountingring160,side wall diaphragm162, dampingring164,inner diaphragm166 andouter diaphragm168.
• Top plate150 is a generally flat annular platform having acentral bore200,shoulder202,shoulder204, and side bore206. Central bore200 receives and holds lower V mount154.Upper V mount152 engagesshoulder202 and extends downward intobore200 and into lower V mount154.Rings158 and160 and the upper outer end ofside wall diaphragm162 are mounted inshoulder204.
• Side bore206 is defined withintop150 and extends through top150 so as to be in communication withfluid passage208 defined between upper and lower V mounts152 and154 and betweenupper V mount152 anddiaphragm lock156. Side bore206 receives an end oftube44 so thattube44 is in fluid communication withfluid passage208 and sensor interface chamber210 (which is defined bydiaphragms166 and168).Fluid passage208 andtube44 provide fluid communication betweensensor interface chamber210 andtransducer40 eccentric tosocket132. As a result,piston rod108 may be pivotally connected tosensor interface assembly38 at a lower pivot point.
• Upper V mount152 is a funnel-shaped socket which is sized for receiving the lower or distal end ofpiston rod108. Preferably,upper V mount152 extends throughcentral bore200 oftop plate150 to a location nearsensor interface chamber210.Upper V mount152 is fixedly secured to an upper portion of top plate atshoulder202.Upper V mount152 is supported bytop plate150 such thatupper V mount152 is spaced from lower V mount154 to defineannular fluid passage208.Fluid passage208 is in fluid communication with ansensor interface chamber210. A fluid coupling medium fillschamber210,passage208 andtube44 all the way totransducer40.Upper V mount152, which is made from a material such as nylon and forms detent220 andsocket132 for pivotally receiving aball member128 ofpiston rod108. As a result,sensor interface assembly38 may be pivoted aboutsocket132 so as to better conform to the anatomy of the patient. Furthermore, becausesocket132 is adjacent tosensor interface chamber210,sensor interface assembly38 is pivotally coupled topiston rod108 about a low pivot point. This permitssensor interface assembly38 to be stably positioned above the underlying artery. In addition, the low pivot point enables hold downassembly36 to apply a more direct, uniform force ondiaphragm168. Thus, the hold down pressure applied by hold downpressure assembly36 is more uniformly applied to the anatomy of the patient above the underlying artery.
• Lower V mount154 is a generally cylindrical shaped member including step or spar230 and bore232. An outer surface or perimeter of lower V mount154 projects outwardly to form spar230. Spar230 engages the lower surface oftop plate150 to partially supportside wall diaphragm162 which is partially captured betweentop plate150 and spar230. In the preferred embodiment, adhesive is used between the lower surface oftop plate150 and spar230 to fixedly secure the portion ofside wall diaphragm162 trapped therebetween. Alternatively, spar230 may be press fit against the lower surface oftop plate150 to secure and supportside wall diaphragm162. Spar230 further divides the outer perimeter of lower V mount154 into two portions, anupper portion234 and alower portion236.Upper portion234 fits withinbore200 oftop plate150.Upper portion234 is preferably adhesively secured totop plate150 withinbore200.Lower portion236 extends below spar230.Lower portion236, spar230 andside wall diaphragm162 defineexpansion cavity240.Expansion cavity240 enablesupper diaphragm166 to initially change shape while only experiencing a small change in volume.
• Diaphragm lock156 is a thin, elongated, annularring including bore250 andlower lip252.Bore250 extends throughdiaphragm lock156 and withupper V mount152, defines a portion offluid passage208.Lip252 projects outwardly from a lower end ofdiaphragm lock156.Diaphragm lock156 fits within bore232 of lower V mount154 until an inner edge ofdiaphragm lock156 is captured between inserts,lip252 and the lower end of lower V mount154.Diaphragm lock156 is preferably adhesively affixed to lower V mount154. Alternatively,diaphragm lock156 may be press fit within lower V mount154.
• Side wall diaphragm162, rings158 and160 andtop plate150 define an annulardeformable chamber260 coupled betweentop plate150 andring164.Side wall diaphragm162 is preferably formed from a generally circular sheet of flexible material, such as vinyl, and is partially filled with fluid.Diaphragm162 has a hole sized to fit aroundupper portion234 of lower V mount154.Diaphragm162 includesouter edge portion162aand inner edge portion162b.Outer edge portion162ais trapped and held betweenouter ring160 andtop plate150. Inner edge portion162bis trapped and supported betweentop plate150 and spar230 of lower V mount154.Diaphragm162 is made from a flexible material and is bulged outward whenchamber260 is partially filled with fluid.Chamber260 is compressible and expandable in the vertical direction so as to be able to conform to the anatomy of the patient surrounding the underlying artery. As a result, the distance betweentop plate150 and the patient's anatomy can vary around the periphery ofside wall diaphragm162 according to the contour of the patient's anatomy. Furthermore, because fluid is permitted to flow through and aroundchamber260, pressure is equalized around the patient's anatomy.
• Dampingring164 generally consists of an annular compressible ring and is preferably formed from a foam rubber or other pulse dampening material such as open celled foam or closed cell foam.Ring164 is centered about and positioned betweenside wall diaphragm162 anddiaphragms166 and168. Dampingring164 is isolated from the fluid coupling medium withinchamber210. Becausering164 is formed from a compressible material,ring164 absorbs and dampens forces in a direction parallel to the underlying artery which are exerted by the blood pressure pulses onsensor interface assembly38 as the blood pressure pulse crossessensor interface assembly38. Becausebottom ring164 is isolated from the fluid coupling medium, the forces absorbed or received byring164 cannot be transmitted to the fluid coupling medium. Instead, these forces are transmitted acrossring164 andside wall diaphragm162 totop plate150. Because this path is distinct and separate from the fluid coupling medium,chamber210 and the fluid coupling medium are isolated from these forces. In addition,ring164 also presses tissue surrounding the artery to neutralize or offset forces exerted by the tissue.
• Upper diaphragm166 is an annular sheet of flexible material having an inner portion166a, anintermediate portion166b, anouter portion166cand an inner diameter sized to fit arounddiaphragm lock156. Inner portion166ais trapped or captured betweenlip252 ofdiaphragm lock156 and the bottom rim of lower V mount154. Inner portion166A is preferably adhesively affixed betweenlip252 and lower V mount154.
• Intermediate portion166blies between inner portion166aandouter portion166c.Intermediate portion166bis adjacent toexpansion cavity240 and is isolated fromring164 andchamber260. Becauseintermediate portion166bis positioned adjacent toexpansion cavity240,intermediate portion166bis permitted to initially move upward intoexpansion cavity240 aschamber260,ring164 andouter diaphragm168 conform to the anatomy of the patient surrounding the underlying artery while the experiences only a small change in volume. Asring164 is pressed against the anatomy of the patient surrounding the artery to neutralize or offset forces exerted by the tissue,diaphragm168 is also compressed. However, becauseintermediate portion166bis permitted to roll intoexpansion cavity240,chamber210 does not experience a large volume decrease and a large corresponding pressure increase. Thus,sensor interface assembly38 permits greater force to be applied to the anatomy of the patient throughring164 to neutralize tissue surrounding the artery without causing a corresponding large change in pressure withinchamber210 as the height of the side wall changes. As a result,sensor interface assembly38 achieves more consistent and accurate blood pressure measurements.
• Outer diaphragm168 is a generally circular sheet of flexible material capable of transmitting forces from an outer surface to fluid withinchamber210.Outer diaphragm168 is coupled toinner diaphragm166 and is configured for being positioned over the anatomy of the patient above the underlying artery.Outer diaphragm sheet168 includes non-active portion or skirt168aandactive portion168b. Skirt168aconstitutes the area ofdiaphragm168 whereinner diaphragm166, namelyouter portion166c, is bonded toouter diaphragm168. Skirt168aandouter portion166care generally two bonded sheets of flexible material, forces parallel to the underlying artery are transmitted acrossskirt168aandouter portion166cand are dampened by the compressible material ofring164.
• Active portion168bis constituted by the portion ofouter diaphragm sheet168 which is not bonded toinner diaphragm166.Active portion168bis positioned below and within the inner diameter ofring164.Active portion168bis the active area ofsensor interface assembly38 which receives and transmits pulse pressure totransducer40.Active portion168bofdiaphragm168,intermediate portion166bofdiaphragm166 anddiaphragm lock156 definesensor interface chamber210.
• The coupling medium withinchamber210 may consist of any fluid (gas or liquid) capable of transmitting pressure fromdiaphragm168 totransducer40. The fluid coupling medium interfaces betweenactive portion168bofdiaphragm168 andtransducer40 to transmit blood pressure pulses totransducer40. Because the fluid coupling medium is contained withinsensor interface chamber210, which is isolated from the side wall ofsensor interface assembly38, the fluid coupling medium does not transmit blood pressure pulses parallel to the underlying artery, forces from the tissue surrounding the underlying artery and other forces absorbed by the side wall totransducer40. As a result,sensor interface assembly38 more accurately measures and detects arterial blood pressure.
• Sensor interface assembly38 provides continuous external measurements of blood pressure in an underlying artery. Becausesensor interface assembly38 senses blood pressure non-invasively, blood pressure is measured at a lower cost and without medical risks. Becausesensor interface assembly38 is relatively small compared to the larger cuffs used with oscillometric and auscultatory methods,sensor interface assembly38 applies a hold down pressure to only a relatively small area above the underlying artery of the patient. Consequently, blood pressure measurements may be taken with less discomfort to the patient. Becausesensor interface assembly38 does not require inflation or deflation, continuous, more frequent measurements may be taken.
• Furthermore,sensor interface assembly38 better conforms to the anatomy of the patient so as to be more comfortable to the patient and so as to achieve more consistent and accurate blood pressure measurements. Becausechamber260 is deformable and partially filled with fluid,chamber260 better conforms to the anatomy of the patient and equalizes pressure applied to the patient's anatomy. Becausering164 is compressible and becausediaphragm168 is flexible and is permitted to bow or deform inwardly,ring164 anddiaphragm168 also better conform to the anatomy of the patient. At the same time, however,sensor interface assembly38 does not experience a large sudden increase in pressure insensor interface chamber210 asring164 anddiaphragm168 are pressed against the anatomy of the patient.Chamber260 andring164 apply force to the anatomy of the patient to neutralize the forces exerted by tissue surrounding the underlying artery. Becausechamber260 andring164 are both compressible, the height of the side wall decreases as side wall is pressed against the patient.Diaphragms166 and168 are also conformable. However, becauseintermediate portion166bofinner diaphragm166 is permitted to move upward intoexpansion cavity240,sensor interface chamber210 does not experience a large volume decrease and a corresponding large pressure increase. Thus, the side wall is able to apply a greater force to the anatomy of the patient without causing a corresponding large, error producing increase in pressure withinsensor interface chamber210 due to the change in height of the side wall and the change in shape ofouter diaphragm168.
• At the same time,sensor interface assembly38 permits accurate and consistent calculation of blood pressure. Because of the large sensing area through which blood pressure pulses may be transmitted totransducer40,sensor interface assembly38 is not as dependent upon accurate positioning ofactive portion168bover the underlying artery. Thus,sensor interface assembly38 is more tolerant to patient movement as measurements are being taken.
• Moreover,sensor interface assembly38 achieves a zero pressure gradient across the active face orportion168bof the sensor, achieves a zero pressure gradient between the transducer and the underlying artery, attenuates or dampens pressure pulses that are parallel to the sensing surface of the sensor, and neutralizes forces of the tissue surrounding the underlying artery.Sensor interface assembly38 contacts and applies force to the anatomy of the patient acrossskirt168aandactive portion168b. However, the pressure withininterface chamber210 is substantially equal to the pressure applied acrossactive portion168b. The remaining force applied bysensor interface assembly38 acrossskirt168awhich neutralizes or offsets forces exerted by the tissue surrounding the underlying artery is transferred through the side wall (ring164 and chamber260) totop plate150. As a result, the geometry and construction ofsensor interface assembly38 provides the proper ratio of pressures betweenskirt168aandactive portion168bto neutralize tissue surrounding the underlying artery and to accurately measure the blood pressure of the artery. In addition, because the fluid coupling medium withinsensor interface chamber210 is isolated from the side wall, pressure pulses parallel to the underlying artery, forces from tissue surrounding the underlying artery and other forces absorbed by the side wall are not transmitted through the fluid coupling medium totransducer40. Consequently,sensor interface assembly38 also achieves a zero pressure gradient betweentransducer40 and the underlying artery.
• FIG. 5 is a top view ofwrist assembly24.FIG. 5 further illustrates portions ofswivel mount34 andcable30 in greater detail.Fluid tube140 has one end connected topassage118 inswivel ball72 and its other end connected tocylinder28.
• Fluid tube142 extends betweentransducer42 andpassage118 inball72.Fluid tube142 fluidly connectspressure chamber116 andtransducer42. As a result,transducer42 senses the pressure withinpressure chamber116.Transducer42 produces electrical signals representing the sensed hold down pressure withinpressure chamber116. These electrical signals are transmitted byelectrical wires280 which extend withincables30 and32 to monitor26 (shown inFIG. 1). As a result, monitor26 may continuously verify that the actual pressure withinpressure chamber116 is within a safe range.
• As further shown byFIG. 5,cable32 additionally encloseselectrical wires290 from transducer40 (shown inFIG. 4).Electrical wires290 transmit electrical signals representing blood pressure amplitudes sensed bytransducer40.Cable32 also encloses anelectrical grounding wire300 which is electrically connected through resistor302 (FIG. 6) to brass plate94 (shown inFIG. 4) and which electricallygrounds transducers40 and42.
• FIG. 6 is a bottom view ofwrist assembly24.FIG. 6 illustratesswivel mount34 withpad48 and plate90 (FIG. 4) removed.FIG. 6 illustrates the electrical connection betweentransducers40 and42 andelectrical wires280 and290, respectively. As shown byFIG. 6, swivel mount34 contains electrical connector304. Electrical connector304 receives leads306 oftransducer40. Leads306 transmit the electrical signals produced bytransducer40 representing the pressures and transmits the electrical signals toelectrical wires290. Electrical connector304 further includes anelectrical resistor302 electrically coupled to brass plate94.Resistor302 is further electrically coupled to groundedelectrical wire300. As a result, static charge is drained throughresistor302 through electrical connector304 and through groundedwire300. Electrical connector304permits transducer40 to be removed and separated fromswivel mount34.
• Similarly,transducer42 includes fourelectrical leads310 which are electrically connected toelectrical wires280. In contrast totransducer40, however,transducer42 is generally fixed and mounted withinswivel mount34. As shown byFIG. 6, swivel mount34 electrically connectstransducers40 and42 to monitor26 byelectrical wires280 and290 carried withincables30 and32.
III.Monitor26
• FIG. 7 shows a block diagram of bloodpressure monitoring system20. As best shown byFIG. 7, monitor26 further includesinput signal processor350, analog-to-digital converter352, microprocessor (and associated memory)354,inputs50a-50g,cylinder drive356, displays52a-52cand54, andpower supply358. In operation,microprocessor354 receives inputted signals frominputs50a-50g.Inputs50a-50gmay also consist of a keyboard or other input mechanisms.Inputs50a-50gpermit microprocessor354 to perform a calibration.
• Microprocessor354controls cylinder drive356 to vary hold down pressure applied by hold downpressure assembly36 ofwrist assembly24. Hold down pressure is applied to the anatomy of the patient directly above the artery. The hold down pressure applied by hold downpressure assembly36 onsensor interface assembly38 is increased over time. As the force or hold down pressure applied bysensor interface assembly38 increases, the amplitude or relative pressure of the blood pressure pulse also increases until a maximum amplitude results. Once the maximum amplitude or maximum energy transfer results, the amplitude of the blood pressure pulse begins to decrease as the artery begins to flatten out beyond the point of maximum energy transfer.
• Transducer40 ofwrist assembly24 senses the amplitude and shape of the blood pressure pulses within the underlying artery.Transducer40 creates electric sensor signals representing the pressures exerted by the sensed blood pressure pulses. The sensor signals are transmitted to inputsignal processor350 ofmonitor26.Input signal processor350 processes the sensor signals and filters any unwanted or undesirable noise and other effects. The sensor signals are then transmitted frominput signal processor350 to analog-to-digital convertor352. Analog-to-digital convertor352 converts the sensor signal into digital form. A digital signal representing the pressures of the sensed blood pressure pulses is sent tomicroprocessor354.
• Based upon the digital sensor signals representing the sensed pressures and shape of the blood pressure pulses,microprocessor354 determines wave shape information by measuring amplitude and shape versus time of individual cardiac cycles. The arterial wave shape information is determined by sampling the arterial waves at a rate significantly above heart rate so that a good definition of the arterial pressure wave is measured. From wave shape information and other parameters derived therefrom,microprocessor354 calculates systolic, diastolic and mean blood pressures.
IV. Method for Locating Sensor Interface Assembly Over Artery
• FIG. 8 illustrates digital displays52a-52canddisplay screen54 ofmonitor26 in greater detail. As shown byFIG. 8,display screen54 further includespressure scale400,horizontal guidelines410 and digital readout430.Monitor26 also is used to display blood pressure pulse waveforms so as to guide the operator in positioning and locatingsensor interface assembly38 directly over the underlying artery having a blood pressure pulse so that more accurate blood pressure values may be determined.
• To placesensor interface assembly38 over an underlying artery,sensor interface assembly38 is located or positioned above a known approximate location of the underlying artery. Assensor interface assembly38 is positioned over the underlying artery, a constant hold down pressure is applied tosensor interface assembly38 and to the underlying artery. Preferably, the pressure applied tosensor interface assembly38 should be as high as possible without thediastolic portion440 ofblood pressure waveforms450 distorting.
• In response to the applied pressure, the underlying artery exhibits a blood pressure pulse waveform for each cardiac cycle.Sensor interface assembly38 senses or receives the force exerted by the blood pressure pulse as the pulse travels beneath the sensing surface and transmits the pressures through the fluid coupling medium totransducer40.Transducer40 in turn senses the changes in pressure and converts the pressures into electrical signals which represent the arterial pressure waveforms. The signals are then transmitted throughcables30 and32 to monitor36.Monitor36 samples the signals preferably at a rate of128 samples per second.Monitor36 then visually displays the sampled signals received fromtransducer40 and displays the signals representing arterial pressure waveforms ondisplay screen54.Display screen54 is preferably indexed so as to provide avertical scale400 withhorizontal guidelines410 for displaying pressure.Guidelines410 permit the maximum pressure amplitude of blood pressure pulse waveforms at the particular location and at a constant hold down pressure to be determined. A representative series of bloodpressure pulse waveforms450 is illustrated onscreen54 inFIG. 8.
• To further aid the operator in determining the maximum amplitude of blood pressure pulse waveforms,display screen54 further includes a digital readout430 which digitally displays the maximum pressure amplitude exerted by the pulse in response to the hold down pressure applied to the artery. As shown inFIG. 8, the artery exhibits pressures which are in the form of bloodpressure pulse waveforms450 when a constant hold down pressure of 80 mmHg is applied to the underlying artery. Bloodpressure pulse waveforms450 exhibit a maximum amplitude of approximately 18 mmHg.
• Once the maximum pressure amplitude exerted by the pulse at a particular hold down pressure at the particular location is determined and noted,sensor interface assembly38 is repositioned at a second location above the known approximate location of the artery. The same constant hold down pressure is applied tosensor interface assembly38 and to the underlying artery of wrist22. The constant hold down pressure applied to the underlying artery is preferably as close as possible to the constant hold down pressure applied at the first location as indicated bydisplay screen54. This can be done by applying a hold down pressure tosensor interface assembly38 at a constant force equal to one ofguidelines410.
• The maximum pressure amplitude exerted by the pulse in response to the hold down pressure applied to the artery at the second location can be determined from the analog display of theblood pressure waveforms450 ondisplay screen54 or the digital readout430 ondisplay screen54. The maximum pressure amplitude at the second location is then noted or recorded for comparison with maximum pressure amplitudes at other locations. Typically,sensor interface assembly38 will be repositioned at a plurality of locations above a known approximate location of the artery while applying a constant hold down pressure to the artery. At each location, the maximum pressure amplitude exerted by the pulse in response to the constant hold down pressure will be displayed ondisplay screen54 and noted. At each location, the maximum pressure amplitude indicated bydisplay screen54 is compared with maximum pressure amplitudes exerted by the pulse in response to the constant hold down pressure applied to the artery and indicated bydisplay screen54 at the plurality of other locations. After the maximum pressure amplitude corresponding to each of the plurality of locations are compared,sensor interface assembly38 and its sensing surface are positioned at the particular location which corresponds to the location at which the largest of the maximum pressure amplitudes is exerted by the pulse in response to the constant hold down pressure applied to the artery.
V. Method for Determining Blood Pressure Values
• Once the sensor is properly positioned over the underlying artery, bloodpressure monitoring system20 determines blood pressure values from the sensed waveform pressure amplitudes sensed bysensor interface assembly38 and from other parameters derived from the pressure amplitudes using a stored set of coefficients. A pressure amplitude is determined at each sample point.
• Bloodpressure monitoring system20 calculates a systolic blood pressure valve (S), a mean blood pressure (M) and a diastolic blood pressure (D) based upon the following formulas:
  M=F_m(P₁^m, . . . , P_n^m, . . . , C_n^m
  S=F_s(P₁^s, . . . , C₁^s, . . . , C_n^s)
  D=F_d(P₁^d, . . . , P_n^d, C₁^d, . . . , C_n^d)
  wherein F_m, F_s, F_dare linear or non-linear functions, P₁^m, P₁^s, P₁^d, . . . , P_n^m, P_n^s, P_n^dare parameters derived from waveform pressure amplitudes and C₁^m, C₁^s, C₁^d, . . . , C_n^m, C_n^s, C_n^dare coefficients obtained during training processes based upon clinical data.
• In particular, bloodpressure monitoring system20 calculates a systolic blood pressure value (S), a mean blood pressure value (M), a diastolic blood pressure value (D) based upon the following formulas:
  M=C₁^mP₁^m+C₂^mC₂^m+ . . . +C_n^mP_n^m
  S=C₁^sP₁^s+C₂^sP₂^s+ . . . +C_n^sP_n^s
  D=C₁^dP₁^d+C₂^dP₂^d+ . . . +C_n^dP_n^d
  wherein P₁^m, P₁^s, P₁^d. . . P^nm, P_n^s, P_n^dare parameters derived from waveform pressure amplitudes. Such parameters may be calculated from shape characteristics of the waveform or parameters calculated from functions such as curves based upon relationships between particular points of several waveforms. The parameters may be further based upon hold down pressure values and time periods between particular points on the waveforms. The value C₁^m, C₁^s, C₁^d. . . C_n^m, C_n^s, C_n^dare coefficients obtained during training processes based upon clinical data.
• In addition, the pulse rate (PR) may also be determined using the formula:$\mathrm{PR}=\frac{{\mathrm{<figure-callout\; label="PR"\; filenames="US20060116588A1-20060601-D00008.png"\; state="\{\{state\}\}">PR</figure-callout>}}_1+{\mathrm{<figure-callout\; label="PR"\; filenames="US20060116588A1-20060601-D00012.png"\; state="\{\{state\}\}">PR</figure-callout>}}_2+{\mathrm{PR}}_3+{\mathrm{<figure-callout\; label="PR"\; filenames="US20060116588A1-20060601-D00012.png"\; state="\{\{state\}\}">PR</figure-callout>}}_4}{4}$
• To determine pulse rate, four individual waveforms or beats are sensed and are time averaged to determine pulse rate. Preferably, the waveforms used to determine pulse rates include the waveform having largest maximum pressure amplitude, the two waveforms prior to the waveform having the largest maximum pressure and the waveform succeeding the waveform having the largest maximum pressure. Once the four waveforms are identified, the pulse rate of each waveform is determined. The sum of the pulse rate of the four waveforms is then divided by four to calculate pulse rate PR. The pulse rate (PR) for each waveform is based upon the following formula:${\mathrm{PR}}_N\mathrm{beats}\mathrm{per}\mathrm{minute}\left(N=1,2,3,4\right)=\frac{128\mathrm{samples}/\sec }{\mathrm{No}.\mathrm{samples}/{\mathrm{beat}}_N}.$
• FIGS. 9, 10 and11 illustrate representative parameters which may be used to calculate blood pressure values.FIG. 9 illustrates a sample series of waveforms exhibited by the underlying artery as a varying pressure is applied over time. The vertical scale indicates pressure in mmHg while the horizontal scale indicates individual sample points at which the blood pressure values exerted by the pulse are measured over time. In the preferred embodiment,transducer40 produces continuous electrical signals representing waveform pressures which are sampled 128 times per second.
• In the preferred embodiment, the hold down pressure applied by hold downpressure assembly36 to sensor interface assembly38 (shown inFIG. 1) is swept over a preselected range of increasing hold down pressures. Preferably, the sweep range of hold down pressures is begun at approximately 20 mmHg. The hold down pressure applied by hold downpressure assembly36 is then steadily increased until two individual waveforms are sensed following the sensed waveform having the largest pressure amplitude. Alternatively, once the waveform having the largest maximum pressure is sensed and identified, successive sweeps may alternatively have a varying hold down pressure applied until a preselected multiple of the mean hold down pressure of the waveform having the largest maximum pressure amplitude is reached. Preferably, each sweep range extends between the initial hold down pressure of 20 mmHg and a final hold down pressure of approximately 150% of the mean hold down pressure of the waveform having the largest maximum pressure amplitude during the previous sweep. In addition, the sweep range may alternatively have an initial hold down pressure of approximately 20 mmHg to a final hold down pressure having a preselected absolute value. Alternatively, the sweep could start at a high pressure and sweep low. As a safety measure, the pressure within pressure chamber (sensed by transducer42) and interface chamber210 (sensed by transducer40) are continually monitored bymonitor26. If the ratio of the pressures withinpressure chamber116 andchamber210 fall outside of a defined range of limits, an alarm is signaled.
• After each hold down pressure sweep, bloodpressure monitoring system20 begins a successive new sweep to calculate new, successive blood pressure values. As a result, bloodpressure monitoring system20 continually measures blood pressure within the underlying artery without causing discomfort to the patient. As can be appreciated, the sweep range of hold down pressure applied by hold downpressure assembly36 may have various initial and final points. Furthermore, the hold down pressure applied by hold downpressure assembly36 may alternatively be intermittently varied. For example, the hold down pressure may be increased or decreased in a step-wise fashion.
• Based upon sensed and sampled pressure waveform signals or data produced bytransducer40 and sent to monitor26 during each sweep of hold down pressures, monitor26 derives preselected parameters for calculating blood pressure values from the derived parameters and a stored set of coefficients. As indicated inFIG. 9, parameters maybe derived directly from the absolute waveform pressures which vary as hold down pressure is varied over time. Such parameters may be derived from the shape of the waveforms including a particular waveform's slope, absolute pressure at a selected sample point, a rise time to a selected sample point on a waveform and the hold down pressures corresponding to a particular sample point on a waveform. As can be appreciated, any of a variety of parameters may be derived from the absolute waveform pressures shown inFIG. 9. Parameters may further be based upon particular points or functions of the sample points.
• FIG. 10 illustrates an example of how values or parameters ofmultiple waveforms500 shown inFIG. 9 may be used to derive additional parameters.FIG. 10 shows several data points510. Eachdata point510 represents a selected waveform taken from the sweep shown inFIG. 9.Curve520 is derived byfitting points510 to a preselected function or relationship. Parameters such as thepeak530 are then derived fromcurve520. As can be appreciated, various other parameters such as slope may also be derived fromcurve520. Parameters derived fromcurve520 are ultimately based uponpressure waveforms500 shown inFIG. 9 which are produced from sensed pressure waveform data or signals fromtransducer40. However, becausecurve520 is derived using a plurality ofwaveforms500, parameters derived fromcurve520 represent the overall relationship between the plurality ofwaveforms500. In other words, parameters derived fromcurve520 represent the way in which the plurality of waveforms500 (shown inFIG. 9) are related to one another. Data points510 represent corrected, relative waveform pressures. As can be appreciated, functions such as curves may also be derived using absolute waveform pressure values which are shown inFIG. 9.
• A waveform is “corrected” by subtracting the hold down pressure from the absolute pressure of the waveform to produce relative waveform pressures (otherwise known as amplitudes). Correcting a waveform eliminates characteristics of the waveform which result from a continuously increasing hold down pressure being applied to the artery during each waveform or cardiac cycle.
• FIG. 11 further illustrates other parameters which may be derived from waveform pressure values as shown inFIG. 9.FIG. 11 illustrateswaveform600 selected fromwaveforms500.Waveform600 is preferably the waveform having the largest peak or maximum pressure amplitude. Alternatively,waveform600 may be any of the waveforms500 (shown inFIG. 9) such as waveforms immediately preceding or succeeding the waveform having the largest maximum pressure. As shown inFIG. 11,waveform600 is corrected such that thebeginning point602 and anending point604 have the same absolute waveform pressure value. As further shown byFIG. 11,waveform600 is horizontally and vertically scaled to eliminate gain from parameters derived fromwaveform600. Preferably,waveform600 is scaled from zero to twenty-one beginning atbeginning point602 and ending at endingpoint604 ofwaveform600 on the horizontal b axis. Preferably,waveform600 is vertically scaled from zero to one beginning at its base and ending at its peak. Becausewaveform600 is horizontally and vertically scaled, parameters may be derived fromwaveform600 for calculating blood pressure values without the gain of the particular patient affecting the calculated blood pressure value. Gains are caused by the differences between the actual pressure exerted within the artery and the pressures sensed at the surface of the wrist or anatomy which is caused by varying characteristics of the intermediate tissue.Scaling waveform600 eliminates any gains exhibited by individual patients. By using scaled values to locate corresponding points or waveform pressure amplitudes onwaveform600, points onwaveform600 uniformly correspond to the same points on waveforms exhibited by other patients.
• As shown byFIG. 11, various parameters may be derived from scaled, correctedwaveform600. As shown byFIG. 11, such parameters include widths ofwaveform600 at selected points along the vertical y axis, ratios of individual waveform pressure amplitudes at selected points along the horizontal b axis and the amplitude of the waveform, the rise time or time elapsed from the start ofwaveform600 atpoint602 to a selected point along the vertical y axis. In addition, several other parameters may also be derived fromwaveform600, such as slope and other shape characteristics.
• Once the parameters to be used in calculating blood pressure values are selected, coefficients corresponding to each parameter must be determined. Coefficients represent the relationship between a particular parameter set and the resulting blood pressure value to be determined from a particular parameter set. Coefficients are initially ascertained from clinical tests upon patients having known blood pressure values. Typically, the known blood pressure value is determined using the A-line method which is generally accurate, although difficult to set up, expensive and medically risky. As the blood pressure is determined using the A-line or other methods,sensor interface assembly38 is positioned over the underlying artery of the patient. Hold downpressure assembly36 applies a varying pressure to the artery of the patient having the known blood pressure value. As discussed above,transducer40 produces sensed pressure waveform signals or data representing arterial pressure waveforms.Monitor26 receives the produced sensed pressure waveform data and derives preselected parameters from the sensed pressure waveform data. Coefficients are then determined using the derived values of the selected parameters and the known blood pressure value. Each coefficient corresponding to each selected parameter is a function of the known blood pressure values and the derived parameters. Preferably, several patients are clinically tested to ascertain the coefficients. Once obtained, the coefficients are stored for use in non-invasively calculating blood pressure values of other patients without the necessity of using the more time consuming, expensive and risky A-line method and without using the generally more inaccurate conventional blood pressure measuring methods. Each particular coefficient is preferably ascertained so as to be applicable for calculating blood pressure values from the derived waveform parameters of all patients. Alternatively, individualized coefficients may be used to calculate blood pressure values from derived waveform parameters of particular patients falling within a particular age group or other specialized groups. The coefficients are preferably determined for use with the same blood pressure monitoring system as will be used to determine the particular blood pressure value of patients having unknown blood pressure values. However, as can be appreciated, the method of the present invention for ascertaining coefficients as well as the method of the present invention for determining blood pressure values may be used in conjunction with any one of a variety of blood pressure monitoring systems including different sensor assemblies and hold down pressure assemblies.
• In addition to illustrating various methods by which parameters may be derived from waveform pressure data,FIGS. 9, 10 and11 illustrate particular parameters for use in calculating a systolic, a mean and a diastolic blood pressure value of a particular patient during an individual hold down pressure sweep. According to the preferred method of the present invention, hold downpressure assembly36 applies a sweeping, continuously varying hold down pressure to the underlying artery. Preferably, the hold down pressure applied by hold downpressure assembly36 during each sweep begins at 20 mmHg and ramps upward over time until at least two waveforms are detected bytransducer40 after the waveform having the largest maximum pressure is identified. Based upon the produced sensed pressure waveform data representing the waveforms as representatively shown byFIG. 9, bloodpressure monitoring system20 calculates systolic, mean and diastolic blood pressure using a stored set of coefficients. Systolic blood pressure (S) is calculated using the formula:
  S=C₁^sP₁^s+C₂^sP₂^s+C₃^sP₃^s+C₄^sP₄^s+C₅^sP₅^s+C₆^sP₆^s+C₇^sP₇^s+C₈^sP_s+C₉^s
• Coefficients C₁^s−C₁^sare stored coefficients ascertained according to the earlier described method of the present invention. C₉^sis an offset value. Parameters P₁^sand P₂^sare derived from relative waveform pressure amplitudes corresponding to scaled values taken from a scaled and corrected beat as represented bywaveform600 inFIG. 11. Preferably, parameter P₁^sis the ratio defined by the waveform pressure amplitude onwaveform600 which corresponds to scale value b₁along the horizontal axis divided by the maximum waveform pressure amplitude or peak (point606) ofwaveform600. Parameter P₂^spreferably is the ratio defined by the waveform pressure amplitude ofpoint608 onwaveform600 that corresponds to scale value b₃along the horizontal b axis divided by the maximum waveform pressure amplitude or peak (point606) ofwaveform600.
• Parameter P₃^sis the rise time or the time elapsed from the start of the waveform to a particular point alongwaveform600 corresponding to a particular vertical scale value. Preferably, parameter P₃^sis the elapsed time from the start ofwaveform600 to apoint610 onwaveform600 which has a vertical height of approximately 0.18 that of a maximum pressure amplitude or peak (point606) ofwaveform600. This rise time or elapsed time is represented as612 inFIG. 11.
• Parameter P₄^sis the mean pressure of theuncorrected waveform500a(shown inFIG. 9) having the highest peak or maximum pressure. Parameter P₄^sis indicated onFIG. 9 bypoint700. Parameter P₅^sis the systolic point of the uncorrected pressure waveform immediately following the uncorrected pressure waveform having the largest maximum pressure. Parameter P₅^sis represented bypoint710 onFIG. 9.
• Parameter P₆^sis a parameter taken from a function such as a curve derived from values of a plurality of waveforms500 (shown inFIG. 9). Preferably, parameter P₆^sis the peak pressure ofcurve520 shown inFIG. 10. The peak is represented bypoint530.Curve520 is preferably generated by fitting the relative waveform pressure amplitude of waveforms500 (shown inFIG. 9) to the function or mathematical expression of:
  AMPLITUDE=exp (ax²+bx+c),
  wherein x=the mean pressure amplitude of each pressure waveform.
• Parameter P₇^sis a time value representing a width of waveform600 (represented bysegment614 betweenpoints616 and618) which corresponds to a selected percentage of the maximum pressure amplitude or peak (point606) ofwaveform600. The time elapsed betweenpoints616 and618 is determined by counting the number of samples taken bymonitor26 which lie abovepoints616 and618 onwaveform600. Preferably, parameter P₇^sis the width ofwaveform600 at a height of about 0.9 A, where A is the maximum waveform pressure amplitude of waveform600 (point606).
• Parameter P₈^sis the maximum slope of the uncorrected waveform500cimmediately following thewaveform500ahaving the largest maximum pressure or peak.
• The mean blood pressure value (M) is calculated using the formula:
  M=C₁^mP₁^m+C₂^mP₂^m+C₃^mP₃^m+C₄^mP₄^m+C₅^m
• Coefficients C₁^m−C₅^mare stored coefficients ascertained according to the earlier described method of the present invention. Coefficient C₅^mis an offset. Parameters P₁^mand P₂^mare derived from relative waveform pressure amplitudes corresponding to scaled values taken from the scaled and corrected beat as represented bywaveform600 inFIG. 11. Preferably, parameter P₁^mis the ratio defined by the waveform pressure (point620) onwavefornm600 which corresponds to the scale value b₉along the horizontal axis divided by the maximum waveform pressure amplitude or peak (point606) ofwaveform600. Similarly, parameter P₂^mis the ratio defined by the waveform pressure onwaveform600 which corresponds to scale value b₁₃along the horizontal axis (point622) divided by the maximum waveform pressure amplitude or peak (point606) ofwaveform600.
• Parameter P₃^mis identical to parameter P₄^sused to calculate systolic blood pressure. Parameter P₄^mis identical to parameter P₆^sused to calculate systolic blood pressure.
• Diastolic blood pressure values (D) are calculated using the formula:
  D=C₁^dP₁^d+C₂^dP₂^d+C₃^dP₃^d+C₄^dP₄^d+C₅^dP₅^d+C₆^dP₆^d+C₇^dP₇^d+C₈^d
• Coefficients C₁^d−C₈^dare stored coefficients ascertained according to the earlier described method of the present invention. Coefficient C₈^dis an offset value. Parameter P₁^dis derived from relative waveform pressure corresponding to scaled values taken from a scaled and corrected beat as represented bywaveform600 inFIG. 11. Preferably, parameter P₁^dis a ratio defined by the waveform pressure amplitude onwaveform600 which corresponds to scale value b₁₂along the horizontal axis (point624) divided by the maximum waveform pressure amplitude or peak (point606) ofwaveform600.
• Parameter p₂^dis identical to parameter P₃^dused to calculate the systolic blood pressure. Preferably, parameter P₃^dis the width ofsegment626 betweenpoints628 and630. Preferably points626 and628 are points alongwaveform600 that are located at a height of 0.875 A, where A is the maximum pressure amplitude (point606) ofwaveform600. The width or time of parameter P₃^dis determined by counting the number of individual waveform pressure amplitude signals or samples generated bytransducer40 and transmitted to monitor26 which lie abovepoints626 and628 onwaveform600. Ifpoints626 and628 fall between individual waveform pressure amplitude signals or samples, interpolation is used to determine the time width of parameter P₃^d.
• Parameter P₄^dis identical to parameter P₄^sused to calculate systolic blood pressure. Parameters P₅^dand P₆^dare calculated from absolute waveform pressures as illustrated inFIG. 9. Preferably, parameter P₅^dis the diastolic pressure value of the uncorrected waveform having the largest maximum pressure value. This diastolic value is represented bypoint720. Parameter P₆^dis the diastolic pressure value of the uncorrected waveform (waveform500c) immediately following the waveform (waveform500a) having the largest maximum pressure amplitude or peak. Parameter P₆^dis represented bypoint730 onFIG. 9.
• Parameter P₇^dis derived from absolute waveform pressures illustrated inFIG. 9. To derive parameter P₇^d, the slopes along the portions of eachindividual waveform500 are determined. Parameter P₇^dis the hold down pressure applied to the underlying artery that corresponds to the point on the particular waveform having the maximum slope corrected amplitude. The slope corrected amplitude of a waveform is obtained by multiplying its amplitude with the maximum slope over allwaveforms500 and dividing the result with the slope corresponding to the individual waveform. As can be appreciated, various alternative parameters may also be used to calculate blood pressure values under the method of the present invention.
VI. Conclusion
• The present invention enables blood pressures of patients to be continuously and non-invasively determined without the complexity, cost, risks, and inaccuracies associated with the prior methods and apparatuses for determining blood pressure.Wrist assembly24 securely mountssensor interface assembly38 upon wrist22 of the patient so that patient movement does not alter the optimal location ofsensor interface assembly38 found. The lower pivot point ofsensor interface assembly38 causes pressure applied by the sidewall ofassembly38 to the tissue above the underlying artery to be uniform around the perimeter of the sidewall. As a result, bloodpressure monitoring system20 samples more accurate signals representing blood pressure pulse waveforms. By deriving parameters from the waveform data and using stored coefficients, blood pressure monitoring system consistently and accurately determines blood pressure values.
• Although the present invention has been described with reference to preferred embodiments, workers skilled in the art will recognize that changes may be made in form and detail without departing from the spirit and scope of the invention. For example, although the determination of pressure values based upon waveform parameters has been described using linear equations and stored coefficients, other methods using non-linear equations, look-up tables, fuzzy logic and neural networks also can be used in accordance with the present invention.

Claims (20)

1. An apparatus for non-invasively sensing a pressure pulse waveform in an artery of a patient underlying body tissue, comprising:

a pressure transmission medium comprising an active surface for placement upon the body tissue with a first dimension generally parallel to the artery;

a force application body comprising a rigid section and a damping section, the rigid section and the damping section having portions coupled to one another for transferring force from the rigid section to the body tissue to offset forces perpendicular to the artery, and the damping section being disposed adjacent to the pressure transmission medium at least along the first dimension for attenuating forces parallel to the artery, the pressure transmission medium being disposed within the force application body for being urged against the body tissue; and

a pressure transducer having a pressure-sensitive port isolated from the force application body and coupled to the pressure transmission medium.

2. The apparatus ofclaim 1 wherein the active surface has a second dimension generally perpendicular to the artery that is greater than a diameter of the artery.

3. The apparatus ofclaim 1 wherein the damping section comprises a compressible foam material.

4. The apparatus ofclaim 1 wherein:

the force application body comprises an expansion cavity; and

the pressure transmission medium comprises a deformable portion for operationally deforming into the expansion cavity upon an operational application of force to the tissue by the force application body.

5. The apparatus ofclaim 1 wherein the force application body comprises a deformable section disposed between the rigid section and the damping section, for conforming to an anatomy of the patient.

6. The apparatus ofclaim 1 wherein for conforming to an anatomy of the patient:

the force application body comprises a deformable section disposed between the rigid section and the damping section for operationally deforming upon an operational application of force to the tissue by the force application body; and

the pressure transmission medium comprises a deformable portion for operationally deforming upon an operational application of force to the tissue by the force application body.

7. The apparatus ofclaim 6 wherein:

the deformable section of the force application body comprises a chamber having flexible solid walls and containing a fluid; and

the pressure transmission medium comprises a chamber having flexible force-transmitting solid walls and containing a fluid.

8. The apparatus ofclaim 1 wherein:

the damping section comprises a compressible foam body; and

the pressure transmission medium is encircled by the foam body.

9. The apparatus ofclaim 8 wherein:

the force application body further comprises a chamber partially filled with a fluid and mounted to the rigid section;

the foam body being mounted to the chamber; and

the pressure transmission medium being encircled by the foam body and by the chamber.

10. The apparatus ofclaim 1 wherein:

the force application body further comprises an annular chamber partially filled with a fluid and mounted to the rigid section;

the damping section comprises an annular compressible foam body mounted to the chamber; and

the pressure transmission medium comprises a fluid-filled chamber encircled by the annular foam body and by the annular chamber.

11. The apparatus ofclaim 10 wherein:

the fluid-filled chamber is operationally deformable; and

the force application body includes an expansion cavity for operationally receiving a portion of the fluid-filled chamber upon deformation thereof.

12. The apparatus ofclaim 11 wherein the fluid-filled chamber comprises:

a generally disk-like body comprising a first diaphragm and a second diaphragm bonded along peripheral portions thereof to form the chamber; the first

diaphragm being operationally deformable into the expansion cavity, and the active surface being on the second diaphragm;

the first diaphragm being bonded to the annular foam body.

13. The apparatus ofclaim 12 wherein:

the annular foam body has a generally constant width; and

the active surface of the second diaphragm has a diameter that is greater than the width of the annular foam body.

14. A sensor for sensing blood pressure pulses within an underlying artery surrounded by tissue as the underlying artery is compressed, the sensor comprising:

a pressure transducer;

an active area having a dimension greater than a diameter of the artery for compressively contacting tissue over the artery;

a pressure transmission path for transmitting pressure operationally present upon the active area to the pressure transducer; and

a compressible member for compressing tissue over the artery, the compressible member being distinct from the pressure transmission path and the pressure transducer, and at least partially surrounding and in proximity to the pressure transmission path for attenuating forces parallel to the artery.

15. The sensor ofclaim 14 wherein:

the compressible member is annular; and

the active area is circular and encircled by the annular compressible member.

16. The sensor ofclaim 15 wherein the annular compressible member comprises a foam material.

17. The sensor ofclaim 14 further comprising:

an annular deformable member;

the compressible member being annular and bonded to the annular deformable member; and

the active area being circular and encircled by the annular compressible member and the annular deformable member.

18. The sensor ofclaim 14 wherein:

the compressible member comprises a foam material; and

the active area is circular and encircled by the compressible member.

19. A method for non-invasively sensing a pressure pulse waveform in an artery of a patient at a measurement site through overlying body tissue, comprising:

applying pressure to the overlying body tissue at least on both sides of the measurement site along the artery with a compressible material, to neutralize forces exerted by the overlying body tissue and to attenuate forces parallel to the artery;

urging a pressure transmission medium upon the overlying body tissue at the measurement site, the pressure transmission medium being distinct from the compressible material; and

detecting a pressure pulse waveform through the pressure transmission medium.

20. The method ofclaim 19 wherein the pressure applying step further comprises applying pressure to the compressible material through a deformable material.

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