VENTILATION SYSTEMS AND METHODS FOR OPTIMIZING LUNG COMPLIANCE
RELATED APPLICATIONS
[0001] This application claims priority from U.S. Provisional Application Serial No.
63/611,575, filed on December 18, 2023, the entire disclosure of which is incorporated herein by this reference.
TECHNICAL FIELD
[0002] The presently-disclosed subject matter relates to ventilation systems and methods for optimizing lung mechanics. In particular, in the systems and methods of the present invention, the pressure provided into the patient’s airway during the exhalation phase is configured to adjust the lung’s functional residual capacity and thereby maximize lung compliance.
BACKGROUND
[0003] Non-invasive ventilation (NIV) involves the delivery of air (often with higher levels of oxygen) via a face mask, eliminating the need of an endotracheal tube. NIV works by creating a positive airway pressure which causes air to be forced into the lungs and achieves comparable benefits to conventional mechanical ventilation by improving gas exchange.
[0004] NIV is an effective treatment for respiratory failure in chronic obstructive pulmonary disease, cardiogenic pulmonary oedema and other respiratory conditions while reducing the likelihood of complications such as respiratory muscle weakness, upper airway trauma, ventilator-associated pneumonia, and sinusitis. In certain applications, NIV is also used as a treatment for sleep apnea, typically referred to in those instances as Constant Positive Airway Pressure (CPAP) or Bi-level Positive Airway Pressure (BPAP). [0005] Although NIV generally, and CPAP and BPAP in particular, are well established treatment methods, there are many well-known issues with efficacy, comfort, and patient adherence. Improved means of ventilating a patient would be both highly desirable and beneficial.
SUMMARY OF THE INVENTION
[0006] The presently-disclosed subject matter relates to ventilation systems and methods for optimizing lung mechanics. In particular, in the systems and methods of the present invention, the pressure provided into the patient’s airway during the exhalation phase is configured to adjust the lung’s functional residual capacity and thereby maximize lung compliance.
[0007] According to the present invention, in some exemplary implementations of the present invention a method for ventilating a patient includes providing pressure into a patient’s airway over a period of time that includes a plurality of inhalation periods and a plurality of exhalation periods. The pressure is at an inspiratory positive airway pressure during the inhalation periods, and the pressure is at an expiratory positive airway pressure during the exhalation periods, the expiratory positive airway pressure less than the inspiratory positive airway pressure. The expiratory positive airway pressure is configured to maximize lung compliance of the patient.
[0008] According to some exemplary implementations, the expiratory positive airway pressure adjusts a functional residual capacity of the patient’s respiratory system to a targeted range such that the lung compliance of the patient is maximized. [0009] According to some exemplary implementations, the expiratory positive airway pressure is configured to minimize the patient’s work of breathing for a given tidal volume and/or minute volume.
[0010] According to some exemplary implementations, the expiratory positive airway pressure is configured to minimize airflow resistance within the patient’s airway during the subsequent inhalation period.
[0011] According to some exemplary implementations, the expiratory positive airway pressure is configured to open the patient’s airway during the subsequent inhalation period.
[0012] According to some exemplary implementations, the expiratory positive airway pressure is configured to minimize the inspiratory positive airway pressure during the subsequent inhalation period while maintaining a given tidal volume and/or minute volume.
[0013] According to some exemplary implementations, the expiratory positive airway pressure is configured to minimize mean airway pressure while maintaining a given tidal volume and/or minute volume.
[0014] According to some exemplary implementations, the compliance of the patient’s lungs is monitored during the period of time pressure is provided into the patient’s airway.
[0015] According to some exemplary implementations, the compliance is measured continuously throughout the period of time pressure is provided into the patient’s airway.
[0016] According to some exemplary implementations, the method for ventilating a patient further includes determining a mean tidal volume of the patient and subsequently monitoring a current tidal volume of the patient. The expiratory positive airway pressure is adjusted, and, if the current tidal volume increases above the mean tidal volume when the expiratory positive airway pressure is increased, the inspiratory positive airway pressure is decreased until the current tidal volume is equal to the mean tidal volume. According to some particular implementations, the expiratory positive airway pressure is increased incrementally until the current tidal volume does not change.
BRIEF DESCRIPTION OF THE DRAWINGS
[0017] FIG. 1 A is a schematic representation of an exemplary positive airway pressure system of the present invention during the inhalation phase of the patient’s respiratory cycle;
[0018] FIG. IB is a schematic representation of the exemplary positive airway pressure system of FIG. IB during a portion of the expiration phase of the patient’s respiratory cycle;
[0019] FIG. 2 is a graph of a pressure profile for a continuous positive airway pressure system (CPAP) in relation to a graph of a patient’s air flow;
[0020] FIG. 3 is a graph of a pressure profile for a bi-level positive airway pressure system (BPAP) in relation to a graph of a patient’s air flow;
[0021] FIG. 4 is an exemplary lung volume diagram illustrating a variety of different volumes related to a patient’s lungs;
[0022] FIG. 5 is an exemplary static lung compliance curve for a patient not receiving ventilation in accordance with the present invention;
[0023] FIG. 6 is an exemplary static lung compliance curve for a patient receiving ventilation in accordance with the present invention;
[0024] FIG. 7A is a flow diagram illustrating one exemplary implementation of the present invention; and
[0025] FIG. 7B is a flow diagram illustrating another exemplary implementation of the present invention. DESCRIPTION OF EXEMPLARY EMBODIMENTS
[0026] The presently-disclosed subject matter relates to ventilation systems and methods for optimizing lung mechanics. In particular, in the systems and methods of the present invention, the pressure provided into the patient’s airway during the exhalation phase is configured to adjust the lung’s functional residual capacity and thereby maximize lung compliance.
[0027] Patients on ventilation require pressure support as they cannot provide adequate levels of minute volume under their own effort. Traditional methods for ventilation consider maintaining a specific minute volume or tidal volume, but fail to consider the effect of work of breathing.
[0028] According to some exemplary systems and methods of the present invention, compliance is monitored during ventilation and the measured compliance is used as feedback to adjust the pressure applied during exhalation periods to affect the patient’s functional residual capacity. By increasing or decreasing the patient’s functional residual capacity such that the patient’s tidal volume occurs where lung compliance is optimized, the required pressure support needed to maintain a targeted tidal volume and/or minute volume is reduced. Providing ventilation to a patient with a maximally compliant lung allows the inspiratory pressure to be minimized since the higher lung compliance permits the same volume delivery at lower pressure support levels (i.e., lower inspiratory pressures). This advantageously reduces work of breathing for the portion of ventilation shouldered by the patient’s efforts. As the pressure needed during the inhalation period is reduced, there is also a reduced mean airway pressure throughout the patient’s breath cycle. [0029] One area in which optimized lung compliance would be beneficial is for patients undergoing ventilation treatment. One general category of patients who require ventilation treatment are home or sub-acute ventilation patients. This category includes, for example, those with chronic obstructive pulmonary disease (COPD), obstructive sleep apnea (OSA), or sleep disordered breathing (SDB), but also any patient who regularly needs some pressure support but who is not in a hospital or other long-term care facility. However, adherence is known to be an issue with this category of ventilation patients. By increasing compliance and therefore reducing work of breathing (WOB) while maintaining a given tidal volume, it is believe that this category of patients will experience more comfort from the ventilation treatment and therefore are more likely to continue with the treatment as intended. For sleep, because the patient typically has normal lung mechanics, a more compliant lung requires less driving force to move the same amount of air into the lungs during inspiration, thus reducing the likelihood of arousal.
[0030] Another general category of patients who require ventilation treatment are acute care patients, who are in the hospital but only require ventilation short-term, such as from a head trauma. The goal for acute care ventilation is to provide adequate ventilation with the lowest pressures possible. Pressure support (PS), i.e., the difference between IPAP and EPAP, is related to tidal volume (Vt) and compliance (C) according to the equation (1):
Vt = PS x C (Equation 1 )
[0031] By optimizing lung compliance (i.e., increasing C), the same tidal volume is achieved with less pressure support. This results in reduced mean airway pressure, which is preferably for patients using mechanical ventilation. To this end, according to some exemplary embodiments, the EPAP is specifically configured to minimize mean airway pressure. [0032] Advantageously, whatever contribution the patient can make toward ventilation, or the level of IPAP needed to maintain appropriate ventilation, is minimized because compliance is optimal.
[0033] According to some exemplary embodiments, and referring to FIGS. 1A and IB, a positive airway pressure system 200 of the present invention includes a flow generator 300 to provide a desired air pressure to a patient 600. As such, the flow generator 300 can also be referred to as a pressure generator. Specifically, the exemplary flow generator 300 includes a fan 302 which draws air in from the environment and through a flow meter 304. The air is then directed through a humidifier 306 before passing from the flow generator 300 into a conduit 400 which has an inlet 402 operably connected to the flow generator 300 and an outlet 404 operably connected to the patient’s respiratory system as well as an exhaust 406 pathway along the conduit 400 but preferably near the outlet 404. The outlet 404 can be in any suitable form for operably connecting air provided by the flow generator 300 to the patient, such as, but not limited to, a full face mask, a partial face mask, a nasal pillow, or any other suitable outlet. By adjusting the speed at which the fan 302 operates, the flow generator 300 is able to affect the pressure applied by the system 200 to the patient’s respiratory system. Other means of adjusting pressure applied to the patient are possible without departing from the spirit and scope of the present invention. For example, another known means of adjusting pressure is an adjustable valve provided on or near the flow generator which bleeds off air flow produced by the fan.
[0034] Referring still to FIG. 1A, during an inhalation period of the patient’s breathing, the diaphragm 608 drops causing the lungs 606 to expand, drawing air through the nasal passage
602 and pharynx 604. By comparison, and referring to FIG. IB, during an exhalation period of the patient’s breathing, the diaphragm 608 and lungs 606 relax pushing air out through the pharynx 604 and nasal passage 602. Throughout both phases of the patient’s respiratory cycle, the flow generator 300 provides pressure through the conduit 400 and the outlet 404 during the inhalation period of the patient’s respiratory cycle as well as during the exhalation period of the patient’s respiratory cycle. As used herein, the nasal passage 602 is inclusive of the mouth. Also, as used herein, the “airway” of the patient 600 is inclusive of the nasal passage 602, pharynx 604, and lungs 606.
[0035] The exemplary airway pressure system 200 further includes a controller 500 for controlling pressure applied to the patient’s airway. The controller 500 includes a computer with a processor for executing instructions stored in a memory component to modulate the flow generator 300 and pressure applied to the patient’s airway. According to some exemplary embodiments, such active control is obtained via an algorithm that controls the device, for example by adjusting the speed of the fan 302, in accordance with the implementations discussed below. The controller 500 is operably connected to a plurality of sensors 502 that measure different aspects of the system 200, the patient 600, and the ambient environment. The sensors 502 can include, for example, a pressure sensor that monitors the pressure of the air in either the flow generator (e.g., the previously mentioned flow meter 304), the conduit 400, the outlet 404, or the exhaust 406. The sensors 502 can also include a temperature sensor, an ambient pressure sensor, a gauge pressure sensor, a patient flow sensor, ambient humidity sensor, microphone, and accelerometer.
[0036] Referring now to FIGS. 2 and 3, throughout a patient’s breath cycle, use of the positive airway pressure system 200 of the present invention results in a pressure profde that includes a pressure applied during a plurality of the patient’s inhalation periods 110 including an end of the patient’s inhalation period 112 (i.e., a beginning of the next exhalation period 120) as well as a plurality of exhalation periods 120 including an end of the exhalation period 122 (i.e., a beginning of the next inhalation period 110).
[0037] In the most basic applications of positive airway pressure therapy, as shown in FIG. 2, a continuous pressure is applied during both the inhalation period and the exhalation period, which is commonly referred to as continuous positive airway pressure (CPAP). In other systems, as shown in FIG. 3, there are two different pressures provided (BPAP), an inspiratory positive airway pressure (IPAP) is provided during the inhalation periods 110 and an expiratory positive airway pressure (EPAP) is provided during the exhalation periods 120. As shown in FIG. 3, typically for ventilation applications, the IPAP is greater than the EPAP.
[0038] Referring now to FIG. 4, a patient’s lungs can be characterized as having a variety of different volumes. The total lung capacity is the volume in the lungs at maximal inflation, but the total lung capacity can be broken down into multiple volumes related to the patient’s breathing as explained below. Although certain volumes in ml/kg are shown in FIG. 4, these are merely exemplary of a typical patient and should not be considered limiting to the present invention.
[0039] Tidal volume (TV or Vt) is the volume of air moved into or out of the lungs during “regular” or “normal” breathing, that is to say, breathing which occurs during a resting state and when there isn’t some abnormal event or episode occurring. The inspiratory reserve volume (IRV) is the maximal volume that can be inhaled above the end-inspiratory level after a normal tidal volume inhalation (i.e., the end of the patient’s inhalation period 112 shown in FIGS. 2 and 3). By comparison, the expiratory reserve volume (ERV) is the maximal volume of air that can be exhaled from the end-expiratory position after a normal tidal volume exhalation (i.e., the end of the patient’s exhalation period 122 shown in FIGS. 2 and 3). The residual volume (RV) is the volume of air remaining in the lungs after such a maximal exhalation, e.g., after a patient pushes as much air out of the their lungs as possible.
[0040] The inspiratory capacity (IC) is the sum of the tidal volume and the inspiratory reserve volume and represents the maximum volume of air a patient can bring into their lungs from the end of a tidal exhale. Likewise, the functional residual capacity (FRC) is the sum of the expiratory reserve volume and the residual volume and represents the volume in the lungs at the end-expiratory position. The vital capacity (VC) is the sum of the inspiratory capacity and the expiratory reserve volume and represents the maximum volume of air moved between a maximum inhale and a maximum exhale. Finally, total lung capacity (TLC) is equal to the sum of the vital capacity and the residual volume, which is the total lung volume at the end of a maximum inhale.
[0041] Referring now to FIGS. 5 and 6, a static lung compliance curve depicts the relationship between static pressure and volume in the patient’s lungs. The curve shown in FIGS. 5 and 6 is merely illustrative and each patient may have their own unique static lung compliance curve. Compliance of the patient’s lungs is defined as the slope of this curve. As shown in FIGS. 5 and 6, compliance is not linear but varies depending on the pressure and volume. Typically, compliance is low (i.e., poor) at lower and higher volumes. Ideally, the patient’s breathing is such that tidal volume is in the middle portion of the curve, where compliance is higher. Although certain values (e g., percent of vital capacity and airway pressures) are shown in FIGS. 5 and 6, these are merely exemplary of a typical patient and should not be considered limiting to the present invention. FIGS. 5 and 6 also provide certain indications of where lung volumes referenced in FIG. 4 typically occur, but once again, these are merely exemplary of a typical patient and should not be considered limiting to the present invention.
[0042] The present invention is designed to optimize compliance which then minimizes the work of breathing for the patient for a given tidal volume. In particular, in accordance with some exemplary embodiments, ventilation is provided through BPAP shown in FIG. 3 and the EPAP is modified to adjust the patient’s expiratory reserve volume such that the patient’s tidal volume occurs where the patient’s lungs are most compliant, i.e., along the region of the static lung compliance curve with the greatest slope. In some embodiments, the EPAP is further configured to minimize the patient’s lung resistance, ensuring the airways are optimally patent to minimize airflow resistance.
[0043] With respect to maximizing compliance, and especially in the cases where patients have reduced expiratory reserve volume such as due to obesity, the EPAP is configured to increase the expiratory reserve volume (e.g., by “re-inflating” that reduced volume) and bring the lung mechanics back into a more compliant region. As the residual volume of a patient is essentially a factor of the patient’s anatomy and physiology and not adjustable through ventilation, increasing the expiratory reserve volume can also be considered the same as increasing the functional residual capacity of the patient’s respiratory system. Accordingly, if FIG. 5 is exemplary of a patient’s unventilated static lung compliance curve, by increasing the expiratory reserve volume (i.e., the functional residual capacity) as shown in FIG. 6, the patient’s tidal volume will occur further along the static lung compliance curve where the slope is greater (i.e., where compliance is greater).
[0044] With respect to minimizing lung resistance, and especially in the case of sleep disordered breathing (SDB), the pressure provided at the end of the expiratory phase is critical to maintaining airway patency. Therefore the EPAP is further configured to maintain the airway during early inspiration until lung volume increases, at which point tracheal traction works to maintain patency of the airway. According to some specific embodiments, rather than maintaining the EPAP at a specific level during the whole of the expiratory phase in the manner shown in FIG. 3, the pressure varies throughout the expiratory phase, but the pressure provided at the end of the expiration periods in particular is configured to maintain the airway during early inspiration until lung volume increases, at which point tracheal traction works to maintain patency of the airway. Further information regarding non-uniform pressures provided across the inhalation periods and exhalation periods in which the pressure at the end of the expiration periods is still effective in treatment are described in International Patent Application No. PCT/US23/83572, filed on December 12, 2023, which is incorporated herein by reference.
[0045] According to some exemplary embodiments of the present invention, the compliance of the patient’s lungs is monitored during the period of time pressure is provided into the patient’s airway. For example, and referring once again to FIG. 3, compliance is monitored during each inhalation period 110 when the IPAP is provided as well as during each exhalation period 120 when the EPAP is provided. In some embodiments, compliance is monitored continuously, or substantially continuously. In other embodiments, compliance is monitored periodically. The particular frequency of measurement required for a given intended use can readily be determined by one skilled in the art. Furthermore, it is contemplated that compliance can be monitored during only the inhalation periods, or during only the exhalation periods without departing from the spirit and scope of the present invention. [0046] Compliance of the patient’s lungs can be measured in a variety of different means. As a non-limiting example, in some exemplary embodiments a “forced oscillation technique” is used in which the flow generator 300 creates an oscillating flow of air at one or more frequencies while the pressure and/or patient flow is measured. A single oscillation frequency is continuously measured to monitor compliance as the imaginary part of the solution phasor. Normally it is possible to determine compliance from a single frequency because the phase variation between pressure and flow results in a complex impedance for which the real part is the physical resistance and the imaginary part is the compliance/elastance. However, this assumes that inertance is negligible, and depending on patient type, inertance may not be ignored. In such instances, two frequencies may be used to first obtain an inertance measurement, and once inertance is determined, it is possible to then determine compliance. Inertance may be monitored occasionally to account for changes which can occur throughout the night, such as a change in body position. Regardless, the frequency signals may be processed by methods well known in the art, including, but not limited to discrete Fourier Transforms (DFT) and/or fast Fourier Transforms (FFT).
[0047] Non-oscillatory methods are also contemplated, including the use of expiratory holds or using a wider band frequency content of the normal breathing pattern. In some embodiments, the use of high |dP/dt| regions of I/E and E/I transitions of PAP can be used to suitably estimate the compliance. In still other embodiments, impulse response and impulse oscillometry can be used to monitor compliance.
[0048] In accordance with some other embodiments, the tidal volume is monitored instead of, or in addition to, monitoring compliance. Once again, when lung compliance is greater, the tidal volume for a given pressure support increases. As such, monitoring tidal volume to determine a maximum volume for a given pressure can bypass the need to determine compliance directly.
[0049] Referring now to FIG. 7A, in one exemplary implementation, the positive airway pressure system 200 is set, for example by a physician, to begin with an initial IPAP and EPAP, and a mean tidal volume of the patient is first determined in a step S102 via methods well known in the art. The tidal volume is then monitored as the EPAP and IPAP are adjusted. Specifically, a first tidal volume (i.e., the current tidal volume) is measured as the patient undergoes one or more breathing cycles in a step SI 04. The EPAP is then adjusted in a step S106. A second tidal volume (i.e., the current tidal volume) is measured in a step S108 as the patient undergoes one or more breathing cycles under the adjusted EPAP, and the second tidal volume is compared against the first tidal volume in a step SI 10. If the tidal volume increases, steps SI 04, SI 06, and SI 08 are then repeated until in step SI 10, the tidal volume stays the same. In other words, the EPAP is increased incrementally until the current tidal volume does not change. The IPAP is then decreased until the current tidal volume is equal to the mean tidal volume. That is to say, in a step SI 12, the IPAP is decreased, and then a third tidal volume (i.e., the current tidal volume) is measured as the patient undergoes one or more breathing cycles in a step SI 14. In a step SI 16, the third tidal volume is compared to the mean tidal volume, and steps SI 12, SI 14, and SI 16 are repeated until the third tidal volume is the same as the mean tidal volume. In this way, a targeted tidal volume is maintained while minimizing pressure support without directly monitoring the compliance of the patient’s lungs.
[0050] In another embodiment, and referring now to FIG. 7B, a mean tidal volume of the patient is first determined in a step S202, and the tidal volume is then monitored as the EPAP and IPAP are adjusted. Specifically, a first tidal volume (i.e., the current tidal volume) is measured as the patient undergoes one or more breathing cycles in a step S204. The EPAP is then adjusted in a step S206. A second tidal volume (i.e., the current tidal volume) is measured in a step S208 as the patient undergoes one or more breathing cycles under the adjusted EPAP, and the second tidal volume is compared against the first tidal volume in a step S210. If the tidal volume increases, in a step S212, the IPAP/PS is then decreased and then a third tidal volume (i.e., the current tidal volume) is measured as the patient undergoes one or more breathing cycles in a step S214. Steps S204-S214 are then repeated until in steps S210, the tidal volume stays the same. In this way, in accordance with the exemplary implementation shown in FIG. 7B, if the tidal volume increases above the previously established mean tidal volume when EPAP is increased, the IPAP is decreased until the current tidal volume is equal to the mean tidal volume. Advantageously, the tidal volume does not excessively increase as the adjustment to EPAP and IPAP are made.
[0051] Although the above explanation focuses on the tidal volume of the patient, the systems and methods of the present invention can operate in substantially the same manner with respect to minute volume. That is to say, rather than looking to maintain a targeted tidal volume, the systems and methods of the present invention can instead operate to maintain a targeted minute volume.
[0052] One of ordinary skill in the art will recognize that additional embodiments and implementations are also possible without departing from the teachings of the present invention or the scope of the claims which follow. This detailed description, and particularly the specific details of the exemplary embodiments disclosed herein, is given primarily for clarity of understanding, and no unnecessary limitations are to be understood therefrom, for modifications will become apparent to those skilled in the art upon reading this disclosure and may be made without departing from the spirit or scope of the claimed invention.