WO2005115234A1

WO2005115234A1 - Neonatal monitor

Info

Publication number: WO2005115234A1
Application number: PCT/GB2005/002076
Authority: WO
Inventors: Christopher Neil Mcleod; A. Barrington Baker; Katie A. Mcleod
Original assignee: Christopher Neil Mcleod; Baker A Barrington; Mcleod Katie A
Priority date: 2004-05-26
Filing date: 2005-05-26
Publication date: 2005-12-08
Also published as: GB0411812D0

Abstract

Method of optimisation of the positioning of an electrode for monitoring ECG or breathing in a patient, such as a neonatal child comprising measuring the circumference of the head of the patient and applying the formula: y = x - b where y = the optimal position to place the proximal electrode within the oesophagus, expressed as the distance of the proximal electrode from the nares or lips of the neonatal child, x = the circumference of the head of the neonatal child in centimetres, b = a value between 18 and 22. Oesophageal feeding tubes comprising position indicators are also provided.

Description

Neonatal Monitor

The invention relates to a method for identifying the optimum position for placing electrodes for monitoring the electrocardiogram (ECG) and/or respiratory movement in a neonatal child, to methods for determining the optimal arrangement of an oesophageal feeding tube comprising such monitoring electrodes, and to oesophageal feeding tabes for use in such methods.

Very immature infants need to be continuously monitored electronically for a period of several weeks. This is done using conducting electrodes attached to the skin on the chest to record an electrocardiogram (ECG) and the electrical impedance across the thorax. The impedance increases during inspiration and decreases during expiration. The main purpose of this monitoring is to detect periods of apnoea (absence of breathing) and bradycardia (decreased heart rate), which are both common and hazardous events in premature babies. The respiratory monitoring is easily disturbed by movement of the infant or wires and changes in electrode-skin contact impedance.

Premature babies have very thin skin, which is therefore fragile. Because the electrodes have to be used over a long period of time they are periodically repositioned, and occasionally adhesive tape is used to maintain contact between the electrodes and the skin. A superficial layer of skin can be removed in the process of repositioning electrodes and this leaves the baby more susceptible to infection.

At birth premature babies do not have the sucking reflexes needed for breast/bottle feeding so, until these have developed, they are fed through a nasogastric/orogastric tube. The oesophagus is an ideal place to monitor from as it is very close to the heart and lungs and also has a good surface for the conduction of electrical signals. A Modified oesophageal Feeding Tube (MoFeT), with electrodes attached to the surface for monitoring, is therefore a solution not only to the problem of skin damage, but also has the advantage of keeping all wires away from the body, allowing access for medical therapy and nursing care and making the baby more accessible for parents. Oesophageal recordings are not disrupted by tube feeding. Research groups have used the oesophagus for monitoring signals from inside the chest but this has not been transferred to routine clinical practice.

We describe a method which can be used easily by nursing staff as a standard procedure to give the best recording position within the oesophagus on babies of different sizes.

The invention provides a method of optimising the optimal position of a proximal electrode and a distal electrode for monitoring ECG and/or breathing in a patient, such as a neonatal child, comprising measuring the circumference of the head of the patient and applying the formula: y = x - b where: y = the optimal position to place the proximal electrode within the oesophagus, expressed as the distance of the proximal electrode from the nares or lips of the neonatal child, x = the circumference of the head of the neonatal child in centimetres, b = a value between 18 and 22, preferably 19 to 21, especially 20.

The invention also provides a method of determining the optimal position to place a monitoring electrode within the oesophagus of a neonatal child or for determining the optimal arrangement of an oesophageal feeding tube comprising such a monitoring electrode, comprising the steps of:

(i) Providing an oesophageal feeding tube, the oesophageal feeding tube comprising a proximal end and a distal end, and further comprises a proximal electrode and a distal electrode for measuring electrocardiogram (ECG) and/or respiratory movement in a neonatal child;

(ii) Measuring the head circumference of the neonatal child; and (iii) Determining the optimal position for the proximal electrode when used in the child by applying the formula: y = x - b where: y = the optimal position to place the proximal electrode within the oesophagus, expressed as the distance of the proximal electrode from the nares or lips of the neonatal child, x = the circumference of the head of the neonatal child in centimetres, b = a value between 18 and 22, preferably 19 to 21, especially 20. 20 is especially preferred as this is easily calculated by nursing staff and gives a very good indication for the positioning of the tube.

The end ranges of the values for b are still expected to give a workable electrode when used in a neonatal child. The value need not be a whole integer. It could be, for example, 19.5 or 19.75.

The position and arrangements can be still further optimised by using the formula: y = 1.0051x - 20.37

Preferably, the neonatal child does not have a deformity of the skull, such as hydrocephalus, as this formula may not apply if the child has an enlarged head.

The same electrodes may be used to measure both ECG and breathing; passively recording ECG and, by driving a non-interfering current, measuring the impedance which indicates there is breathing.

The proximal electrode is the electrode which, in use, is closest to the nose or mouth of the child. The oesophageal tube may be one suitable for insertion through the nose or through the mouth. That is, it may be a nasogastric or orogastric tube. Preferably, the tube comprises an elongated sleeve defining a lumen or cavity. The cavity is used to pass feed to the stomach of the neonatal child when in use. The electrodes, and associated wires for connection to one or more monitoring devices, may be embedded in the wall of the sleeve. Alternatively, the tube may be provided with two or more lumens, one of which may contain the wires attached to the electrodes.

The proximal electrode will be used with a distal electrode placed, typically away from it.

Preferably, there are 3 or more electrodes on the feeding tube. These are typically 3-5 mm. wide and are spaced apart, typically at a distance of approximately 2 cm. Preferably, the distal electrode is 5 cm. from the distal end of the tube. One of the electrodes may be a reference electrode.

Typically, the feeding tubes are 30 cm. long, although other lengths may be used according to nursing practice. For example premature babies in incubators may use a longer tube to allow it to be accessed with greater ease. This may be adjusted depending on the position required for the proximal electrode to produce the optimal readings when in use. The feeding tube is typically 5 FG (1.67 mm.) in diameter.

The tube itself may be made out of any suitable flexible material, such as PNC or polyurethane.

The feeding tube may be used to monitor both ECG and an impedance respiratory signal.

The feeding tube may additionally comprise a balloon on the outside wall for monitoring the pressure and/or sounds within the thorax. The tube may also comprise electrodes for monitoring the diaphragm electromyogram (EMG). Additionally, the feeding tube may comprise electrodes for monitoring the pH in the oesophagus and/or within the stomach. These electrodes and the other sensors referred to are themselves known in the art, although not in the arrangement claimed herein. The claimed method allows the position of the monitoring electrodes to be optimised when the feeding tube is placed within the oesophagus of the neonatal child. It also allows the selection of the length of the optimum feeding tube from a plurality of oesophageal feeding tubes, each tube having the proximal electrode a different distance along the length of the tube from the proximal end of the tube compared with the other feeding tubes. A still further option is to provide a feeding tube which is capable of being cut to an optimal length, and the electrodes connected via suitable connectors to the monitoring device. Such a cut tube is also preferably attachable to a suitable Luer fitting to enable a syringe containing food to be attached to allow food to be inserted through the tube into the stomach of the neonatal child when in use.

A still further variation in the method providing an oesophageal tube comprising a plurality of indicators at the proximal end of the tube, the indicators being indicative of the distance of the proximal electrode from each indicator; and inserting the oesophageal tube into the oesophagus of the neonatal child by the mouth or nasal passage until the optimal position of the proximal electrode is indicated by the position of the indicator in comparison to the nares or lips of a neonatal child. Such an indicator is preferably in the form of a number representing the diameter of the head of the child. This enables the person inserting the tube, such as a nurse, to adjust the amount of insertion simply by measuring the head of a neonatal child, and without having to refer to any tables or having to subtract any figures from the circumference of the head. This considerably eases the use of the feeding tube and reduces the risk of errors.

A still further method of the invention provides a method of positioning a monitoring electrode at an optimal position within a neonatal child comprising:

(i) Measuring the head circumference of the neonatal child;

(ii) Providing an oesophageal feeding tube, the oesophageal feeding tube comprising a proximal end and a distal end, and further comprises a proximal electrode and a distal electrode for measuring electrocardiogram (ECG) and/or respiratory movement in a neonatal child; characterised in that the oesophageal feeding tube additionally comprises towards the proximal end of the feeding tube a plurality of spaced apart indicators along a length of said tube, said indicators indicating a head circumference.

Preferably, the methods additionally comprise a step of inserting the oesophageal feeding tube into the neonatal child.

A still further aspect of the invention provides an oesophageal feeding tube comprising an elongate sleeve defining a lumen, a proximal and a distal end, a proximal and a distal electrode for measuring electrocardiogram (ECG) and/or respiratory movement in a neonatal child; characterised in that the feeding tube additionally comprises towards the proximal end a plurality of spaced apart indicators, each indicator indicating the approximate position of the distance of the proximal electrode according to the measured head circumference.

Preferably, the indicator is indicative of a potential circumference of the head of a child into which the tube may be inserted.

The tube may additionally comprise electrodes for monitoring both the diaphragm electromyogram (EMG) and ECG. The electrode may also comprise a balloon on its outside wall for monitoring the pressure and/or sound within the thorax, such a device may additionally comprise further electrodes for monitoring the diaphragm electromyogram.

Preferably, the device additionally comprises electrodes for monitoring the pH in the oesophagus and/or within the stomach of a neonatal child.

The measurement of the neonatal child's head is around the widest part of the head, including the forehead and the back of the head. The invention will now be described by way of example only, with reference to the following figures:

Figure 1 shows a schematic diagram demonstrating a feeding tube according to the invention.

Figure 2 is a photograph of a prototype feeding tube made in accordance with the invention.

Figure 3 shows a schematic diagram of the electrical flow between two electrodes from the feeding tube.

Figure 4 shows an ECG trace from a feeding tube, made in accordance with the invention, placed at different parts of the oesophagus of a neonatal child.

Figure 5 shows the respiratory trace obtained using a feeding tube, according to the invention, placed in different parts of the oesophagus of a neonatal child.

Figure 6, the top and second traces show ECG recorded from chest surface electrodes and from oesophageal electrodes respectively; trace 3 is respiration recorded from chest surface electrodes; trace 4 (respiration wave 2) is recorded from oesophageal electrodes; trace 5 (respiration wave 3) is recorded by measuring the air temperature at the nares.

Figure 7 shows the ratio of the best position of the electrode from the nose compared with baby's weight at the time of the study.

Figure 8 shows the best position of the electrode from the nose compared with the head circumference. Materials & Methods

A preferred device is shown in Figure 1 and in the photograph in Figure 2. The Modified Feeding Tube (MoFeT) (10) is used to monitor the ECG and chest impedance of neonates. The tube (10) is 30 cm long and 5-FG diameter (1.67 mm), the same dimensions as feeding tubes currently in use. The MoFeT is made of medical grade PNC double-lumen tubing, one lumen for the feeds (12) and another for the wires (14) that lead to the electrodes at the distal end (16, 18, 20 arrowed). The electrodes are formed by painting a band of silver-loaded PNC (Electrodag 915, Acheson Colloids, Plymouth) at the appropriate points on the tube. Each band is between 3 mm and 5 mm wide. Two electrodes are sufficient, but three may also be used (22, 24, 26). The electrodes comprise a proximal electrode (16), a distal electrode (20) and a middle electrode (18). The middle electrode, when present is usually situated midway between the distal and proximal electrodes. When two are used,_, two connecting leads to the monitoring equipment are connected to the distal electrode (20) and one to the proximal electrode (16). The two joined leads are the negative input and the reference. The positive input is connected to the proximal electrode. When 3 electrodes are used, the negative input (22) to the momtoring equipment is connected to the distal electrode (20), the positive input (24) is connected to the proximal electrode (16) and the reference input (26) is connected to the middle electrode (18). The connecting wires are approximately 30 cms long and are terminated with standard ECG connectors (28) to fit all standard monitors. There is a standard Luer tapered attachment (30) at the proximal end of the tube for feeding.

The tubing may commonly be polyurethane to increase the flexibility; the connecting leads may be embedded within the wall, leaving the second lumen (14)- if present- free for pressure or sound measurement. The electrodes may be constructed using a number of other similarly effective methods, for instance, metal bands. For more extensive monitoring further electrodes may be added, increasing the functionality of the MoFeT.

The ECG is detected passively from the electrodes which sense the signal generated by the electrical currents in the heart. The electrical impedance signal is generated by a small high frequency current, passing between two of the electrodes. The voltage needed to drive the current depends on the impedance between the electrodes. Skin electrodes are usually placed laterally so the whole chest contributes to the impedance. Tissue is a much better electrical conductor than air. During inspiration the chest expands with non-conductive air so the impedance rises; during expiration the impedance decreases.

With oesophageal electrodes, the signal is strongest from the spherical volume around the electrodes, the electrodes forming a diameter. More distant impedance changes will register less at the electrodes. Figure 3 demonstrates the way in which current flows between the 2 electrodes, most of it staying close to the oesophagus.

The feature of a baby's size that is important in determining the optimal position of the electrodes is the oesophageal length; this is not measurable. The invention enables head circumference to be used to estimate oesophageal length. Babies often lose weight after birth, but their length may have increased, so weight is a less accurate measurement to be using for comparison of size.

The criteria used to determine the best recording position within the oesophagus are the quality of the ECG and respiratory traces. These change as the electrodes are moved down the oesophagus.

Figure 4 shows the ECG seen at different levels within the oesophagus. The distance is measured from the nose to the proximal electrode. When the proximal electrode is high up in the oesophagus, near the pharynx, (top trace) the ECG is very small. As the electrodes are moved down towards the stomach the ECG becomes larger, and when the electrodes are past the level of the atria the p-wave of the ECG inverts (shown in the 4th ECG here). When the electrodes have moved past the ventricles the whole QRS complex of the ECG wave inverts.

Figure 5 shows the impedance traces recorded from different levels in the oesophagus. The best signals tended to be from high up, near the pharynx, or nearer the diaphragm. There was a large element of cardiac interference when the electrodes were close to the heart. Blood is a very good electrical conductor and the blood volume changes with each cardiac cycle create a cardiac wave on top of the respiratory wave. This can be seen most clearly in the 3rd and 7th traces in this figure.

When there is cardiac interference on the impedance trace the monitor records the heart rate and not the respiratory rate. This is particularly significant during periods of apnoea, as the alarm will fail to sound as the cardiac wave is being wrongly registered as breathing.

In Figure 6 the top and second traces show ECG recorded from chest surface electrodes and from oesophageal electrodes respectively. Trace three is respiration recorded from the chest surface electrodes; trace four (Respiration Wave 2) is recorded from the oesophageal electrodes. Trace 5 (Respiration Wave 3) is recorded by measuring the air temperature at the nares. Figure 6 demonstrates cardiac interference on all 3 respiratory signals during a period of apnoea. This is often a problem in monitoring chest impedance with skin electrodes as much as with oesophageal electrodes. Even the nasal thermistor detects a change in air temperature next to the nose coincident with the cardiac pulse when the airway is open.

In determining the best recording position (BRP) for oesophageal electrodes, as long as an ECG can be detected, the main criterion is lack of cardiac interference. The BRP has been plotted against weight (g) of the baby (Figure 7). The Pearson Product Moment Correlation Coefficient is 0.54, indicating some degree of correlation between weight and the BRP.

The BRP is then plotted against head Circumference (Figure 8). The Pearson Product Moment Correlation Coefficient for head circumference relating to the BRP is 0.72, showing a statistically significant relationship. The formula for the regression line is: y = 1.0051 x - 20.37

If the BRP is rounded up to the nearest whole number (as it is bound to be in clinical practice) it becomes clear that the best recording position, in terms of the distance from the proximal electrode to the nose, is: Best Recording Position = Head Circumference - b (cm) - where b is between 18 and 22, preferably 19 to 21, especially 20 cm. This can be reproduced as a simple table for clinical use.

Practical Method

A Modified Feeding Tube (MoFeT) is made with two electrodes -one at 5 cms from the distal end, the other at 2 cms proximal to that. The MoFeT has surface markings beginning 4 cms above the proximal electrode. The first marking is labelled "24". There are further markings every cm above the first, marked successively "25", "26", "27"....

Alternatively, a MoFeT is made with three electrodes -one at 5 cms from the distal end, the second is 1 cms proximal to that and the third is a further 1 cm proximal to the second. The MoFeT has surface markings beginning (32) 2 cms above the proximal electrode. The first marking is labelled "24". There are further markings every cm above the first, marked successively "25", "26", "27"....

The nurse measures the circumference of the baby's head in cms. He/she then inserts the MoFeT using either the oro-gastric or naso-gastric route according to the standard practice of that department until the mark on the MoFeT corresponding to the head circumference is at the nares or lips respectively. The MoFeT will then be in the optimal position for recording ECG and respiration.

Claims

1. A method of optimising the optimal position of a proximal electrode and a distal electrode for monitoring ECG and/or breathing in a patient, such as a neonatal child, comprising measuring the circumference of the head of the patient and applying the formula:

= x - b where: the optimal position to place the proximal electrode within the oesophagus, expressed as the distance of the proximal electrode from the nares or lips of the neonatal child, the circumference of the head of the neonatal child in centimetres, b = a value between 18 and 22.

2. A method of determining the optimal position to place a monitoring electrode within the oesophagus of a neonatal child, or for determining the optimal arrangement of an oesophageal feeding tube comprising such a monitoring electrode, comprising the steps of:

(ii) Measuring the head circumference of the neonatal child; and

(iii) Determining the optimal position for the proximal electrode when used in the child by applying the formula: x - b where: y = the optimal position to place the proximal electrode within the oesophagus, expressed as the distance of the proximal electrode from the nares or lips of the neonatal child, x = the circumference of the head of the neonatal child in centimetres, b = a value between 18 and 22.

3. A method according to claim 1 or claim 2, wherein the optimal position or arrangement of the electrode is determined by applying the formula: y - 1.0051x - 20.37

4. A method according to claim 2 or claim 3 additionally comprising the steps of selecting the oesophageal feeding tube from a plurality of oesophageal feeding tubes, each tube having a proximal electrode a different distance along the length of the tube from the proximal end of the tube.

5. A method according to claim 2 or claim 3, wherein the oesophageal tube comprises a plurality of indicators at the proximal end of the tube, the indicators being indicative of the distance of the proximal electrode from each indicator; and inserting the oesophageal tube into the oesophagus of the neonatal child via the mouth or a nasal passage until the optimal position of the proximal electrode is indicated by the position of the indicator at the nares or lips of the neonatal child.

6. A method according to claim 2 or claim 3, wherein the oesophageal tube is cut to remove at least a portion of it, so that in use the proximal electrode is optimally positioned.

7. A method of positioning a monitoring electrode at an optimal position within a neonatal child comprising:

(i) Measuring the head circumference of the neonatal child; (ii) Providing an oesophageal feeding tube, the oesophageal feeding tube comprising a proximal end and a distal end, and further comprises a proximal electrode and a distal electrode for measuring electrocardiogram (ECG) and/or respiratory movement in a neonatal child; characterised in that the oesophageal feeding tube additionally comprises towards the proximal end of the feeding tube a plurality of spaced apart indicators along a length of said tube, said indicators each indicating a different head circumference, and the tube is inserted into the child to the appropriate indicator.

8. A method according to any one of claims 2 to 4 or claim 6, additionally comprising the step of inserting the oesophageal feeding tube into the neonatal child.

9. An oesophageal feeding tube comprising an elongate sleeve defining a lumen, a proximal and a distal end, a proximal and a distal electrode for measuring electrocardiogram (ECG) and or respiratory movement in a neonatal child; characterised in that the feeding tube additionally comprises towards the proximal end a plurality of spaced apart indicators indicating the approximate position of the distance of the proximal electrode relative to the indicator.

10. An oesophageal feeding tube according to claim 9, wherein each indicator is indicative of the potential circumference of a neonatal child's head, wherein said tube may be inserted into the mouth or a nasal passage of said neonatal child.

11. An oesophageal feeding tube according to any one of claims 9 to 10, comprising further electrodes for monitoring both an impedance respiratory signal and ECG.

12. An oesophageal feeding tube according to any one of claims 9 to 11, comprising a balloon on its outside wall for monitoring the pressure and/or sounds within the thorax.

13. An oesophageal feeding tube according to any one of claims 9 to 12, comprising further electrodes for monitoring the diaphragm electromyogram (EMG) and a balloon on its outside wall for monitoring the pressure and/or sounds within the thorax.

14. An oesophageal feeding tube according to any one of claims 9 to 13, comprising further electrodes for monitoring the pH in the oesophagus and/or within the stomach.

15. A method of monitoring a neonatal child comprising inserting into the oesophagus of the neonatal child an oesophageal feeding tube according to any one of claims 9 to 14.

16. A method according to claim 15, wherein the tube is inserted into an optimal position, the optimal position having been predetermined by a method according to any one of claims 1 to 3.