WO1992017150A1 - A combined naso-gastric feeding tube and electrode - Google Patents

Abstract

A combined naso-gastric feeding tube and electrode is described comprising a tube (1) in the region of one end is a luer (2) of conventional construction with a closing cap (3) joined to the luer by a strap (4). The electrode is an electroconductive wire (5) secured to the luer (2) by a plastics sleeve (6) shrunk onto the tube (1), the wire (5) passing down the inside of the tube (1) at one end to emerge through the wall of the tube adjacent its other end. A method of manufacturing the combined feeding tube and electrode is also described.

Description

A COMBINED NASO-GASTRIC FEEDING TUBE AND ELECTRODE

This invention relates to a combined naso-gastric feeding tube and electrode.

Sick new born babies (neonates) or premature babies, e.g. babies sometimes born as early as twenty four weeks gestation, are given intensive care requiring constant monitoring of their electrocardiograph (ECG) and respiration. This is traditionally carried out using three skin electrodes placed on the baby's chest. These electrodes detect the electric potential generated by the heart (the ECG). It is usual to detect the ECG by measuring the voltage difference between two electrodes referred to a third reference electrode (3 lead configuration). However it is also possible to detect the ECG using one electrode referred to a reference electrode (2 lead configuration). The ECG electrodes can also be used to detect respiration by applying a high frequency voltage to two of the electrodes and detecting the impedance changes of the chest during the respiratory cycle. The use of adhesive electrodes on the baby's chest is not only uncomfortable and upsetting to parents but can seriously damage the extremely delicate skin of the preterm baby and also obscures the lung fields at chest x-ray.

All babies in intensive care and most in special care have a naso-gastric feeding tube which is normally passed through the nose until one end of the tube lies in the stomach while the other end has a standard luer fitting. The baby is fed by pushing a syringe containing milk into this luer fitting and holding the syringe above the baby so that gravity forces the milk down the nasogastric tube. There are one or more holes in the side of the tube, allowing the milk to pass directly into the stomach.

Measurements have hitherto been made from the oesophagus by using an electrode dedicated to the measurement. For example, an oesophageal ECG electrode is available which is pushed through the nose and located in the oesophagus at a suitable point.

The heart can also be stimulated, for example, by atrial pacing. These electrodes have no lumen available for feeding purposes, so a separate feeding tube would be required for feeding the babies, however, this would not be ethically acceptable as it would mean two tubes passing through the nose or mouth of the baby. These known electrodes are complex and therefore relatively expensive to manufacture.

An aim of the present invention is to provide a combined naso-gastric feeding tube and electrode to monitor the ECG and respiration while retaining the naso¬ gastric tube for feeding purposes. Only one skin electrode will then be required which can be sited on the buttock or lower back of the baby being monitored.

According to one aspect of the present invention there is provided a combined naso-gastric feeding tube and electrode comprising a tut3 with a coupling at one end and an electroconductive material extending from the region of the coupling to adjacent the other end of the tube. In a preferred construction an electrical connector is removably connected to the coupling to make electric contact with the electroconductive material.

Preferably the electrical connector is a tubular plug of titanium having two electric leads of the same material electrically connected thereto, the other end of each lead having a pin connector to connect the electrode to a monitor.

In a preferred construction the coupling is a luer. Conveniently, the luer is made of plastics material with the electroconductive material embedded in the wall thereof forming an electrical connection for the electrode.

According to a second aspect of the present invention there is provided a method of manufacturing a combined naso-gastric feeding tube and electrode as set forth above, comprising the steps of cleaning a length of stainless steel wire to remove any residue, oiling the wire and passing it through the central lumen of a tube of plastics material having a coupling at one end, making a lower incision and an upper incision from the other end of the tube respectively, threading the wire through the tube wall at the upper incision and threading it back through the lower incision until it re-enters the tube, fixing the end of the wire where it exits from the luer end of the tube with a heat-shrunk plastics sleeve.

According to a third aspect of the present invention there is provided a medico-surgical tube including an electrically-conductive wire extending along its length, wherein the wire is wrapped around the outside of the tube in a region close to the patient end of the tube to form an integral electrode.

According to a fourth aspect of the present invention there is provided a method of feeding and monitoring an electro-cardiogram (ECG) with a medico- surgical tube as set forth in the preceding paragraph.

Embodiments of the present invention will now be described by way of example only with reference to the accompanying drawing illustrating an elevation of a combined naso-gastric feeding tube and electrode according to the invention.

The electrode comprises a standard naso-gastric feeding tube 1. There is available a wide range of sizes of nasograstric feeding tubes but in practice on neonatal units only two sizes are usually used: 4 French Gauge having an external diameter of approximately 1.3 mm is normally used to feed babies under approximately 1500 grams, or a 6 French Gauge tube having an external diameter of approximately 2.0 mm for feeding babies over approximately 1500 grams. Both feeding tubes are 75 cm long while the wall thickness is approximately 0.035 mm for the 4FG and 0.4 mm for the 6FG. Henceforth we shall refer to either of these sizes, but it will be realised that the invention is not so limited.

At one end of the feeding tube is a luer 2 of conventional construction with a closing cap 3 joined to the luer by a strap 4. An electroconductive wire 5 is secured to the luer by a plastics sleeve 6 shrunk onto the feeding tube 1, the wire 5 passing down the inside of the feeding tube at one end to emerge through the wall of the tube adjacent its other end.

The electrode is made of 0.1 mm surgical grade 316 soft stainless steel wire. To assemble the electrode a 1 metre length of this wire 5 is thoroughly cleaned to remove any residue and is then oiled with pure sunflower oil and is passed through the central lumen of the tube 1 from the luer end. Two small incisions 7 and 8 are made through the wall of the tube at 12.5 cm and 13.5 cm from the tube end 9 respectively. The end of the stainless steel wire is pulled through the wall at the incision 8 and is wound round the outside of the tube to form a spiral 10 of two complete turns. The end of the wire is then trimmed and pushed back through the incision 7 such that approximately 1 cm of wire re-enters the tube 1.

Any slack in the spiral 10 is taken up by pulling the free end of the wire protruding from the luer fitting. It should be noted that the incisions are made at

12.5 cm and 13.5 cm in the 6FG naso-gastric tube, and at

9.5 cm and 10.5 cm in the 4FG naso-gastric tube. This produces electrodes centred at 13 cm and 10 cm respectively.

The portion of the tube 1 containing the electrode is then dip-coated with PVC. The coating solution is made from medical grade PVC granules dissolved in tetrahydrofuran. The electrode is dipped several times to ensure that both incisions are fully repaired and to cover the steel wire electrode. The electrode wire is then exposed by scraping the PVC away from the wire using a scalpel. The reason for covering the electrode with

PVC is to ensure that the final electrode does not stand too proud of the tube 1 which may traumatise or damage the baby's oesophagus. The steel wire is brought over the top of the luer 2 and is coated with PVC where it passes down the outside of the luer 2 to provide electrical insulation and added safety, and is held in place by a heat-shrunk plastic sleeve 6.

The combined naso-gastric tube and electrode are connected to a monitoring device which may be of the type 7883 Series Neonatal monitor made by Hewlett Packard Limited, by a male-to-female luer tubular connector 11 made of surgical grade 316 stainless steel. Two electrical leads 12 and 13 are soldered to the outside of the connector 11 at 14 using lead free solder. Each lead has a standard 2 mm straight pin connector 15 and 16, respectively, at its other end to facilitate connection to the monitor.

The outside of steel luer connector 11 is insulated using a PVC heat-shrunk tubing. This sleeve goes over the straight sided part of the luer, the tapered end being inserted into the luer fitting on the tube 1. To feed the baby fitted with the naso-gastric tube electrode, a syringe containing milk can be inserted in the female luer at the top of the steel luer fitting 11, thus the baby may be fed without disconnecting the electrode from the monitoring device.

The reason for attaching two electric leads to the connector is that conventional techniques use three skin electrodes and the naso-gastric electrode replaces two of these. As the monitoring device, i.e. a Hewlett Packard Monitor, is designed to receive three connection leads the combined electrode is compatible with this.

When using a 4FG naso-gastric tube the open end may be slightly narrowed in the manufacturing process of fitting the luer connector, so that the steel electrode wire may substantially occlude this opening which leads to difficulties in getting the feed to drain through the feed tube 1. To overcome this the wire is pulled through the side of the naso-gastric tube just below the luer fitting. The incision in the tube is sealed with PVC, the stainless steel wire being soldered onto two cables and the solder joint and any bare stainless steel wire being electrically insulated. Each cable is provided with a 2 mm connector at its free end so that electrical connection can be made to the monitor without the need of a steel luer connector.

The electrode according to the invention, is used as a unipolar ECG electrode with a reference electrode attached to the baby's buttock or lower back. The buttock or lower back is the best site for the reference electrode because the signals obtained are good, the baby's chest is kept clear of monitoring devices and the movement artefact is minimised.

The tests so far conducted, using the naso-gastric electrode indicate that 10 cm and 13 cm from the stomach end of the feed tube are the best positions for the steel electrode mounted on the feed tube. The best signals are obtained with the end of the electrode, where it protrudes from the stomach end of the tube sited between vertebrae C3 and T3 of the bab" being monitored. It was also noted that good signals are obtained throughout the oesophagus, with the electrode at this level the signals were found to be of good amplitude and there was a minimum of movement artefact. From our tests it wa calculated that electrodes placed at 10 cm on the 4F tube and at 13 cm on the 6FG tube will be suitabl positioned in the oesophagus when in situ. Various modifications may be made to the naso gastric electrode of the present invention, for example, the electrical conductor can be located within the lume as described with reference to the drawing, in which case the conductor must be of a size which does not interfere with the normal use of the feed tube. It has been found unnecessary to insulate the conductor where it is within the lumen of the naso-gastric tube for ECG/respiration use as the walls of the plastics tube provide sufficient electrical insulation. In a modified construction to that described and illustrated, the electrical conductor may be located within the wall of the feed tube while maintaining electrical insulation where required.

The electrical conductor may be a non-toxic cable embedded within the wall of the feed tube or may be made from a conductive polymer such as PVC with the addition of graphite before the material is cured. The conductive material may be manufactured to run through the radio- opaque stripe on the feed tube.

A medico-surgical tube having an electrically conductive wire extending along its length may have the wire wrapped around the outside of the tube in a region close to the patient end of the tube to form an integral electrode. The wire may be exposed on its surface at the electrode or it could be covered by a conductive polymer.

In another construction the conductor or multiple conductors within the naso-gastric tube or the wall of the naso-gastric tube may be brought outside the naso- gastric tube at the luer end as conductive strips embedded into the outside of the tapered part of the female luer 6. A separate ring-shaped connector with corresponding conductive strips on the inside would be passed over the naso-gastric tube and fit over the tapered part of the female luer 6. Cables embedded into the ring-shaped connector would make electrical contact with the conductive strips and thus with the conductor(s) in the naso-gastric tube. If multiple electrodes are used a 'key' would be provided to prevent incorrect connection.

The electrode conductor could alternatively be made of titanium which would be better suited as it is more inert. Noble metals such as platinum would also be suitable. Any metallic part of the male to female luer which is in contact with the conductor in the naso¬ gastric tube should be of the same metal to avoid any electrochemical reaction. In another arrangement two electrodes are mounted on the feed tube close together e.g. 5 cm apart to allow the ECG to be monitored in the 3-lead configuration with the advantage that the effects of interference, e.g. common mode signals, are reduced compared with the 2-lead configuration. In another arrangement two electrodes are mounted on the feed tube such that one electrode is in the upper or middle oesophagus as described above and the other is in the lower half of the oesophagus or stomach. With this arrangement a 2-lead ECG can be obtained using the lower electrode as the reference electrode, thus dispensing with skin electrodes for ECG measurement altogether. In another arrangement three electrodes are mounted on the feed tube such that two are close together in the upper or middle oesophagus and the third electrode is in the lower half of the oesophagus or stomach. With this arrangement a 3-lead ECG recording can be obtained using the lower electrode as a reference electrode, thus dispensing with skin electrodes for ECG measurement altogether and also giving the advantages of a 3-lead configuration. Respiration can be obtained using the lower electrode and one of the upper electrodes.

If only an ECG reading is required then the following arrangement can be used:

1. A cotton wick is run down through the feed tube and would still function even if it was not located outside the tube, in which case the cotton wick is run down to the stomach end of the tube where the side holes are located. The cotton wick functions as an ionic pathway, and must therefore be wet. Connections is made to the cotton wick at the luer end of the feed tube e.g. with a silver luer connector of the same design as the steel one described above. The purpose of the cotton wick is to ensure that an ionic pathway exists throughout the tube because, without it, an air bubble could break the continuity. If the wick does not emerge from the side wall of the feed tube into direct contact with the oesophageal mucosa, there is a risk of a bubble of air forming between the wick and the mucosa disrupting the conductive pathway, thus the electrode is more reliable if it is brought to the outside of the tube. The current flow is due to ionic diffusion through the fluid in the tube or through the fluid in the cotton wick.

The electrode impedance can be defined as the impedance between the monitoring equipment and the measurement site of interest. The impedance of such an electrode is generally several orders of magnitude higher than that of a metal wire. An ECG can be obtained with an electrode impedance of e.g. 100 Kohms. While respiration can be monitored with the above, the impedance technique requires an electrode with a much lower impedance of e.g. several hundred ohms. The impedance of this kind of electrode is too high for respiration, but is suitable for taking readings of ECG. 2. An ECG would also be obtainable with a conductor of e.g. stainless steel wire running down to the stomach end of the feed tube to where the side holes are. The conductor in this case would not need to be brought outside of the feed tube. The electrode would be more reliable if brought outside the tube but the possibility of intermittent signal loss may not put people off keeping it inside the tube. The site of the exectrode would be too low for a good respiration trace, but would be suitable for an ECG reading.

Micro-miniature sensors can be connected for detecting important physiological variables such as ECG, respiration, temperature, pH, redox and blood gases. pH This could be measured using an antimony/antimony oxide electrode or an iridium/iridium oxide electrode or any other pH sensitive material such as a pH sensitive polymer. The electrode could be placed anywhere on the NG tube as required. One suitable way of manufacturing this type of electrode would be to sputter it onto the outside of the tube, making contact with a conductor brought through the wall of the tube. This type of electrode would also be suitable for detecting ECG. Redox - This could be monitored using a suitable electrode material such as platinum or gold. The electrode could be made the same way as the ECG/respiration electrode as herein before described. This electrode would also be suitable for detecting ECG and respiration.

Temperature - Temperature could be detected using, for example, a bead thermistor. These can be produced in sizes of less than 1 mm in diameter. The thermistor could be embedded either in the side wall of the naso- gastric tube or mounted at the stomach end of the tube. The monitoring of the temperature could be made using a thermistor ink as the sensor.

Pressure - This could be measured using balloon techniques, miniature strain gauges or other suitable pressure sensors.

Blood Gases - Blood gas measurements such as pC02 and p02 are often monitored as part of a baby monitoring unit and these could, with suitable sensors, be incorporated in the combined naso-gastric feeding tube and electrode of the present invention.

Other means of sensor connection can be employed in conjunction with the apparatus of the present invention. Instead of using an electrically conducting wire to connect the electrode to the connection at the luer end of the feed tube, any other conductive material could be used e.g. conductive ink or graphite loaded silicone rubber or PVC. Alternatively, a fibre-optic cable could be embedded in the wall of the tube or passed through the lumen of the tube. The fibre-optic cable would have a suitable sensor at the measurement site. One such construction is a fibre-optic cable the end of which is coated with a membrane sensitive to, for example, pH or enzyme activity.

Other types of measurement such as measuring the pressure of the bladder can be made using a conventional catheter used for this purpose using the techniques outlined above.

Claims

CLAIMS :

1. A combined naso-gastric feeding tube and electrode comprising a tube with a coupling at one end and an electroconductive material extending from the region of the coupling to adjacent the other end of the tube.

2. A combined naso-gastric feeding tube and electrode as claimed in claim 1, wherein an electrical connector is removably connected to the coupling to make electric contact with the electroconductive material.

3. A combined naso-gastric feeding tube and electrode as claimed in claim 2, wherein the electrical connector is a tubular plug of surgical grade stainless steel having two electric leads electrically connected thereto, the other end of each lead having a pin connector to connect the electrode to a monitor.

4. A combined naso-gastric feeding tube and electrode as claimed in any preceding claim, wherein the coupling is a luer.

5. A combined naso-gastric feeding tube and electrode as claimed in claim 4, wherein the electroconductive material terminates at the luer end of the tube as a conductive strip on the outside of the luer fitting.

6. A combined naso-gastric feeding tube and electrode as claimed in claim 4, wherein the electroconductive material is one or more conductors embedded into the luer, a ring-shaped connector with corresponding conductive strips on the inside would fit to the luer and make contact with the conductors.

7. A combined naso-gastric feeding tube and electrode as claimed in any preceding claim, wherein the electroconductive material is a metal wire.

8. A combined naso-gastric feeding tube and electrode as claimed in claim 7, wherein the metal wire is stainless steel.

9. A combined naso-gastric feeding tube and electrode as claimed in claim 7, wherein the metal wire is titanium.

10. A combined naso-gastric feeding tube and electrode as claimed in any of claims 1 to 5, wherein the electroconductive material is a non-toxic strip embedded within the wall of the feeding tube, made of a conductive polymer.

11. A combined naso-gastric feeding tube and electrode as claimed in any of claims 1 to 5, wherein the electroconductive material is a cotton wick running down the feed tube to take ECG readings.

12. A combined naso-gastric feeding tube and electrode as claimed in any preceding claim including a bead thermistor embedded in the wall of the naso-gastric feed tube to monitor temperature.

13. A combined naso-gastric feed tube and electrode as claimed in any of claims 1 to 11, wherein a bead thermistor is mounted at the stomach end of the feed tube to monitor temperature.

14. A combined naso-gastric feeding tube and electrode as claimed in any of claims 1 to 5, wherein the electroconductive material is a conductive ink.

15. A combined naso-gastric feeding tube and electrode as claimed in any of claims 1 to 5, wherein the sensor is a thermistor ink.

16. A method of manufacturing a combined feeding tube and electrode as claimed in any preceding claim, comprising the steps of making a lower incision and an upper incision in a tube of plastics material having a coupling at one end, inserting a wire through the lumen of the tube, threading the wire through the tube at the upper incision and back into the tube through the lower incision until it re-enters the tube.

17. A method as claimed in claim 16, wherein the wire is cleaned to remove any residue and oiled with an edible oil to ease the passage into the tube.

18. A method as claimed in claim 16 or 17, wherein a stiffer wire is attached to the final wire before introducing it into the coupling of the tube.

19. A method as claimed in any of claims 16 to 18, wherein the incisions are made between 11 cm and 20 cm from the stomach end of the tube, the distance between the upper and lower incisions being between 0.1 cm and 6 cm.

20. A method as claimed in any of claims 16 to 19, wherein the incisions are repaired with PVC or silicone rubber and the sensing part of the electrode is also coated with PVC or silicone rubber, which is then scraped away to expose the electrode.

21. A method as claimed in any of claims 16 to 20, wherein the wire is wound in a spiral around the outside of the tube between the incisions.

22. A method as claimed in any of claims 16 to 21, wherein the wire is made of surgical grade 316 stainless steel.

23. A method as claimed in claim 21 or 22, wherein the spiral of wire around the tube is two to four complete turns.

24. A method as claimed in any of claims 16 to 23, comprising making a removable connector from a metal rod for insertion in the coupling, soldering two leads to the connector, and fitting a standard straight-pin connector to the respective end of each lead to facilitate connection of the combined naso-gastric feed tube and electrode to a monitoring machine.

25. A method as claimed in any of claims 16 to 24, wherein the coupling is a luer.

26. A medico-surgical tube including an electrically- conductive wire extending along its length, wherein the wire is wrapped around the outside of the tube in a region close to the patient end of the tube to form an integral electrode.

27. A medico-surgical tube as claimed in Claim 26, wherein the exposed surface of the wire at the electrode is covered by a conductive polymer.

28. A method of feeding and monitoring an electro¬ cardiogram (ECG) with a medico-surgical tube as claimed in Claim 26.