Technical Field

The present disclosure relates to gastric tubes that are inserted into a person's gastric system via their nose or mouth and in particular to nasogastric (NG) and orogastric (OG) tubes.

Background

Nasogastric (NG) tubes are tubes, typically of silicone or polyurethane, about 3 - 5mm in external diameter, with one or more internal lumina and several outlet apertures at their distal end.

NG tubes are used primarily for enteral feeding of patients who are unable or unwilling to feed normally. They are also used for a variety of other therapeutic and diagnostic purposes including gastric decompression and aspiration of gastric content. Millions of NG tubes are used each year.

NG tubes are generally placed by inserting into a nostril and blindly navigating through the nasal cavity to the stomach via the oesophagus. The nature of this process results in a very high proportion of NG tubes being mal-positioned in the pulmonary system, not reaching the stomach due to the tube curling up in the oesophagus or throat or insufficiently inserted into the stomach such that not all of the apertures of the NG tube are located in the stomach. Around 20% of NG tubes are misplaced in adults and around 40% in children.

A number of tests are used by clinicians to detect misplacement including air insufflation (blowing air into the NG tube whilst listening for bubbling using a stethoscope), checking the pH of aspirate with pH indicator strips, and 'feeling' whether the NGT is in the more closed space of the oesophagus. Taking an X-ray to confirm placement is considered to provide the best indication of correct placement and is used as a matter of routine in at least some hospitals for adults. However, none of these procedures are conclusive. Even use of an x-ray is prone to misinterpretation.

The result of this lack of accuracy is that up to 2% of NG tubes are not detected as being misplaced. Real consequences from misplaced, and particularly undetected misplaced NGTs include:

• Reflux and aspiration pneumonia due to NG tube misplacement in oesophagus.

• Pneumothorax and death due to malposition in the pulmonary system. There were 32 deaths out of 95 cases of harm in the UK between 2011 and 2016 due to misplaced NG tubes.

Attempts to increase the accuracy of NG tube placement confirmation (including the use of x-rays) are costly, require specialist operators, are not available in all settings, time consuming, delay feeding and/or have other patient impacts such as radiation exposure.

Similar problems exist for orogastric (OG) tubes that are inserted via the mouth rather than the nose of the patient. However, OG tubes are used less frequently than NG tubes. There is an ongoing need for correct positioning of gastric tubes within a patient to address the adverse consequences of mal-positioned or misplaced gastric tubes.

Summary of the Disclosure

The present disclosure provides a system for monitoring of the location of gastric tubes, particularly nasogastric or orogastric tubes during positioning within a patient.

According to one embodiment, the present disclosure provides a gastric tube assembly for detecting the location of a gastric tube within a patient during positioning comprising:

a gastric tube having a proximal end and a distal end;

at least one lumen extending between the proximal end and the distal end wherein the lumen comprises an inlet at the proximal end and one or more lumen outlets at the distal end adapted to deliver or remove a substance to or from the patient's stomach; and

a position sensor assembly wherein the position sensor assembly comprises:

(a) a first sensor configured to sense whether the distal end of the tube has entered a patient's oesophagus by detecting an absence of air flow associated with respiration; and

(b) a second sensor configured to sense whether the distal end of the tube has entered the patient's stomach such that the lumen outlets are positioned within the stomach.

According to one embodiment, the present disclosure provides a gastric tube assembly comprising: a nasogastric or orogastric tube having distal and proximal ends and at least one lumen extending between the proximal and distal ends, each lumen having an inlet at the proximal end and at least one outlet at the distal end; a gas flow sensor incorporated with the tube and located towards the distal end of the tube; and a pH sensor located incorporated with the tube and located towards the distal end of the tube.

According to another embodiment, the present disclosure provides a gastric tube assembly comprising a nasogastric or orogastric tube having distal and proximal ends and at least one lumen extending between the proximal and distal ends, each lumen having an inlet at the proximal end and at least one outlet at the distal end; a gas flow sensor incorporated with the tube and located towards the distal end of the tube; and an ion selective sensor incorporated with the tube and located towards the distal end of the tube.

According to another embodiment, the present disclosure provides a position sensor assembly that is connected to a distal end of a nasogastric or orogastric tube, the position sensor assembly comprising:

a body that connects to the tube; and

a gas flow sensor incorporated with the body. According to another embodiment, the present disclosure provides a gastric tube system comprising a gastric tube assembly according to any one of the above embodiments and a position indicator that is configured to receive a signal from the sensors of the gastric tube assembly and produce an output based on the signal to communicate the position of the gastric tube to a user.

According to another embodiment, the present disclosure provides a method for monitoring of the location of a gastric tube within a patient during positioning, wherein the gastric tube has a proximal end and a distal end and at least one lumen extending between the proximal end and the distal end wherein the lumen comprises an inlet at the proximal end and one or more lumen outlets at the distal end adapted to deliver or remove a substance to or from the patient's stomach, wherein the method comprises the steps of:

i. inserting the gastric tube comprising a position sensor assembly via the pharynx of the patient wherein the position sensor assembly is in communication with an external position indicator and comprises:

(a) a first sensor configured to sense whether the distal end of the tube has entered a patient's oesophagus by detecting an absence of air flow associated with respiration; and

(b) a second sensor configured to sense whether the distal end of the tube has entered the patient's stomach such that the lumen outlets are positioned within the stomach; ii. using the first sensor to determine whether the tube has entered the patient's

oesophagus or trachea;

iii. continuing with insertion of the tube if the tube has entered the patient's oesophagus or retracting the tube and repeating step (ii) or steps (i) and (ii);

iv. using the second sensor to determine whether the tube has entered the patient's

stomach such that the lumen outlets are positioned within the stomach and v. preparing the gastric tube for use to deliver or remove the substance if the tube is

correctly positioned within the patient's stomach or continuing with insertion of the tube and repeating step (iv).

Brief Description of the Drawings

Embodiments of the present disclosure will now be described, by way of example only, with reference to the accompanying Figures:

Figure 1A is a perspective view of a nasogastric tube system according to an embodiment of the present disclosure;

Figure IB is a perspective view of the distal end of a nasogastric tube of the system of Figure 1A; Figure 1C is a detailed perspective view of an ion selective sensor incorporated into the nasogastric tube of Figure 1A;

Figure ID is a cross-sectional view through A-A of the nasogastric tube of Figure 1A;

Figure IE is a cross-sectional view through B-B of the nasogastric tube of Figure 1A;

Figure 2A is side and perspective views of a sensor assembly body of the nasogastric tube of Figure 1;

Figure 2B is cross-sectional side and perspective views of the sensor assembly body of Figure 2A; Figure 3A is a circuit diagram of a gas flow sensor of the nasogastric tube system of Figure 1A;

Figure 3B is a circuit diagram of a comparator circuit associated with the gas flow sensor;

Figure 4A is a circuit diagram of an ion selective sensor of the nasogastric tube system of Figure 1A; Figure 4B is a circuit diagram of a comparator circuit associated with the ion selective sensor;

Figure 5A is a schematic view of a position indicator of the nasogastric system of Figure 1A;

Figure 5B is a schematic view of a method of using the nasogastric system;

Figure 6 is a graph of the voltage response of the ion selective sensor when placed in a solution with a different pH;

Figure 7 is a graph showing the relationship between gate output voltage of the ion selective sensor and pH;

Figure 8 is a calibration graph of the gas flow sensor;

Figure 9 illustrates the degrees of freedom for the orientation of the gas flow sensor;

Figure 10 is a graph of the output voltage of the gas flow sensor when rotated at different angles about the x-axis relative to the direction of flow of gas;

Figure 11 is a graph of the output voltage of the gas flow sensor when rotated at different angles about the y-axis relative to the direction of the flow of gas;

Figure 12 is a side view of a sensor assembly body according to another embodiment of the present disclosure and;

Figure 13 is a cross-section of the sensor assembly body of Figure 12.

Detailed Description of Embodiments

In one embodiment a gastric tube assembly comprises a nasogastric or orogastric tube having distal and proximal ends and at least one lumen extending between the proximal and distal ends, each lumen having an inlet at the proximal end and at least one outlet at the distal end; a gas flow sensor incorporated with the tube and located towards the distal end of the tube; and a pH sensor located incorporated with the tube and located towards the distal end of the tube.

In another embodiment a gastric tube assembly comprises a nasogastric or orogastric tube having distal and proximal ends and at least one lumen extending between the proximal and distal ends, each lumen having an inlet at the proximal end and at least one outlet at the distal end; a gas flow sensor incorporated with the tube and located towards the distal end of the tube; and an ion selective sensor incorporated with the tube and located towards the distal end of the tube.

In another embodiment a gastric tube assembly for detecting the location of a gastric tube within a patient during positioning comprises:

a gastric tube having a proximal end and a distal end;

at least one lumen extending between the proximal end and the distal end wherein the lumen comprises an inlet at the proximal end and one or more lumen outlets at the distal end adapted to deliver or remove a substance to or from the patient's stomach; and

a position sensor assembly wherein the position sensor assembly comprises:

(a) a first sensor configured to sense whether the distal end of the tube has entered a patient's oesophagus by detecting an absence of air flow associated with respiration; and

(b) a second sensor configured to sense whether the distal end of the tube has entered the patient's stomach such that the lumen outlets are positioned within the stomach.

In one embodiment the first sensor is located at or proximate the distal end.

In another embodiment the lumen outlets are positioned intermediate the first sensor and the second sensor.

In one embodiment the gastric tube is a nasogastric tube. In another embodiment the gastric tube is an orogastric tube.

In one embodiment the first sensor is a gas flow sensor and the second sensor is an ion selective sensor wherein the ion selective sensor is configured to detect an ion concentration indicative of the stomach. In another embodiment the first sensor is a gas flow sensor and the second sensor is an impedance sensor.

In another embodiment the first sensor is a gas flow sensor and the second sensor is a pH sensor wherein the pH sensor is configured to detect a pH indicative of the stomach indicative of the stomach.

In one embodiment the position sensor assembly is in communication with an external position indicator as further described herein.

In an embodiment, the ion selective sensor is configured to provide a signal based on the concentration of hydronium (H ₃ 0 ⁺ ) or chloride (Cl ) ions.

In an embodiment, the ion selective sensor is a pH sensor.

In an embodiment, the ion selective sensor comprises a primary electrode and a reference electrode. In an embodiment, the primary electrode is ion sensitive field effect transistor or an antimony electrode and the reference electrode is a reference field effect transistor or a Ag/AgCI electrode.

In an embodiment, the ion selective sensor is located between the proximal end of the tube and the outlet(s) of each of the lumen.

In an embodiment, the ion selective sensor is located further from the distal end of the tube than all of the lumen outlets.

In an embodiment, the ion selective sensor is located close to, preferably within 1 - 10mm of the lumen outlet furthest from the distal end of the tube.

In an embodiment, the ion selective sensor is located 1 - 10mm along the tube from the lumen outlet furthest from the distal end of the tube, in a direction away from the distal end of the tube.

In an embodiment, the ion selective sensor is configured with the tube to be exposed, in use, to the external environment of the tube.

In an embodiment, the ion selective sensor is located in a recess in the external surface of the tube. In an embodiment, the gas flow sensor is housed within a body that allows gas to flow past the gas flow sensor but provides a barrier to liquid.

In an embodiment, the body has a plurality of apertures to enable gas to flow past the gas flow sensor.

In an embodiment, the body is connected to the gastric tube.

In an embodiment, the body projects beyond the distal end of the gastric tube.

In an embodiment, the body has a curved leading end.

In an embodiment, the body has a hollow core and the gas flow sensor is held within and spaced from the internal surfaces of the hollow core.

In one embodiment of the present disclosure a position sensor assembly that is connected to a distal end of a nasogastric or orogastric tube comprises a body that connects to the tube and a gas flow sensor incorporated with the body.

In an embodiment, the position sensor assembly also comprises a stylet configured to extend through a lumen of the nasogastric or orogastric tube, the stylet connected to the gas flow sensor.

In an embodiment, the position sensor assembly is configured to enable the stylet to be removed from the nasogastric or orogastric tube with the gas flow sensor whilst leaving the body connected to the tube.

In an embodiment, the position sensor assembly is configured to enable the stylet to be removed from the nasogastric or orogastric tube whilst leaving the gas flow sensor and the body connected to the tube According to another embodiment, the present disclosure provides a gastric tube system comprising a gastric tube assembly according to any one of the above embodiments and a position indicator that is configured to receive a signal from the sensors of the gastric tube assembly and produce an output based on the signal to communicate the position of the gastric tube to a user.

In an embodiment, the system is configured to determine whether all of the lumen outlets have entered a patient's stomach based on a threshold signal, preferably a threshold voltage, from the ion selective sensor. In an embodiment, the ion selective sensor has a voltage output and the system comprises a comparator circuit which is configured to compare the voltage output of the ion selective sensor to a reference voltage to determine whether the ion selective sensor has entered a patient's stomach.

In an embodiment, the reference voltage corresponds to the voltage output of the ion selective sensor when the sensor is in an external environment having a pH of 3 - 5, preferably 3.8 - 4.7, more preferably 4 - 4.5.

In an embodiment, the system is configured to determine whether the distal end of the gastric tube has entered a patient's trachea based on a threshold signal, preferably a threshold voltage, from the gas flow sensor.

In an embodiment, the gas flow sensor has a voltage output and the system comprises a comparator circuit which is configured to compare the voltage output of the gas flow sensor to a reference voltage to determine whether the distal end of the nasogastric tube has entered a patient's trachea. In an embodiment, the reference voltage corresponds to the voltage output of the gas flow sensor when exposed to a gas velocity of 0.64m/s or less, preferably 0.6m/s or less, more preferably 0.5m/s or less, more preferably 0.4m/s or less.

In an embodiment, the position indicator comprises a processor that is configured to process the signals received from each of the sensors to produce the output to indicate the position of the distal end of the nasogastric tube to a user.

In an embodiment, the processor incorporates the comparator circuits for each of the sensors. According to another embodiment, the present disclosure provides a method for monitoring of the location of a gastric tube within a patient during positioning, wherein the gastric tube has a proximal end and a distal end and at least one lumen extending between the proximal end and the distal end wherein the lumen comprises an inlet at the proximal end and one or more lumen outlets at the distal end adapted to deliver or remove a substance to or from the patient's stomach, wherein the method comprises the steps of:

i. inserting the gastric tube comprising a position sensor assembly via the pharynx of the patient wherein the position sensor assembly is in communication with an external position indicator and comprises:

(a) a first sensor configured to sense whether the distal end of the tube has entered a patient's oesophagus by detecting an absence of air flow associated with respiration; and

(b) a second sensor configured to sense whether the distal end of the tube has entered the patient's stomach such that the lumen outlets are positioned within the stomach; ii. using the first sensor to determine whether the tube has entered the patient's

oesophagus or trachea;

iii. continuing with insertion of the tube if the tube has entered the patient's oesophagus or retracting the tube and repeating step (ii) or steps (i) and (ii);

iv. using the second sensor to determine whether the tube has entered the patient's

stomach such that the lumen outlets are positioned within the stomach and v. preparing the gastric tube for use to deliver or remove the substance if the tube is

correctly positioned within the patient's stomach or continuing with insertion of the tube and repeating step (iv).

In an embodiment, the location of the gastric tube is monitored in real time.

Referring now to Figures 1A to IE a nasogastric tube system 1 according to an embodiment of the present disclosure is shown. The system 1 comprises a nasogastric tube 10, a position sensor assembly 11 and a position indicator 12. The nasogastric tube 10 and the position sensor assembly 11 form a nasogastric tube assembly.

It is to be appreciated that although the following description is provided in relation to a nasogastric tube, an orogastric tube system could be provided comprising an orogastric tube and a similar position sensor assembly and a similar position indicator.

The nasogastric tube 10 is elongate between a proximal end 15 and a distal end 16. The distal end 16 is the leading end of the nasogastric tube 10 during insertion of the tube into a patient.

In the illustrated embodiment, the nasogastric tube 10 has a single lumen 17 extending between the proximal and distal ends 15, 16. However, in other embodiments, the nasogastric tube has two or more lumina extending between the proximal and distal ends.

The nasogastric tube 10 has an external diameter of 3 - 5mm. The finer width nasogastric tubes (about 3mm in diameter) have only a single lumen as illustrated in the Figures. The finer width nasogastric tubes have relatively thinner walls which means that they are only capable for use in delivering to the stomach. They generally cannot be used to aspirate (i.e. withdraw) stomach contents because under negative pressure the lumen tends to collapse and occlude the flow path. The wider width nasogastric tubes (about 5mm in diameter) have wider internal lumen(s) and therefore can be used to both deliver to and withdraw from the stomach. The wider nasogastric tubes can have multiple lumina. Typically one lumen is used to deliver nutrition and/or

pharmaceutical preparations and a second, separate lumen is used for venting.

The length of the nasogastric tube 10 for use with an adult patient is 100 - 130cm. Markings (not shown) are provided along at least a portion of the length of the nasogastric tube to indicate to the clinician the length of the tube that has been inserted into the patient. A radiopaque strip (not shown) is also provided along at least a portion of the length of the nasogastric tube to more readily enable identification of the nasogastric tube under X-ray.

The nasogastric tube 10 may be formed from any conventional material used to form nasogastric tubes. Preferably, the nasogastric tube 10 is formed from silicone or polyurethane.

The or each lumen 17 of the nasogastric tube 10 has a proximal opening at the proximal end 15 of the nasogastric tube. The proximal opening is configured to enable flow of material into or out of the nasogastric tube 10 to or from the exterior of the patient. The proximal opening is an opening through the proximal end wall of the nasogastric tube.

At its proximal end 15, the nasogastric tube 10 has a coupling element or feeding port that is configured to be connected to an enteral feeding pump (not shown). The coupling element is in fluid communication with the proximal opening of the or each lumen 17. When engaged with the coupling element, the enteral feeding pump is placed in fluid communication with the or at least one of the lumina 17 of the nasogastric tube. This enables conventional delivery of material to the nasogastric tube lumen and hence to the patient.

The or each lumen 17 has a plurality of distal openings 20 at the distal end 16 of the nasogastric tube 10. Nutrition and/or pharmaceutical materials are delivered to a patient's stomach out of the distal openings 20. In the case of wider nasogastric tubes 10, at least some of the distal openings 20 are used to aspirate from a patient's stomach. The distal openings 20 are distributed around the circumference of the nasogastric tube and along a portion of the length of the nasogastric tube. Multiple distal openings 20 are provided in case one or more of the openings become occluded or blocked during use.

The position sensor assembly 11 comprises a body 30, a gas flow sensor 40 and an ion selective sensor 50. The gas flow sensor 40 is configured to sense whether the distal end 16 of the nasogastric tube 10 has entered a patient's oesophagus by detecting an absence of air flow associated with respiration. Or, in other words, the gas flow sensor 40 detects misplacement of the nasogastric tube in the respiratory system by detecting air flow associated with respiration. The ion selective sensor 50 is configured to sense whether the distal end of the tube has entered the patient's stomach by detecting a concentration of an ion, preferably hydronium (H ₃ 0 ⁺ ) or chloride (Cl ), indicative of the stomach.

In other embodiments, the position sensor assembly 11 does not include the ion selective sensor 50 but does comprise at least the gas flow sensor 40. In some of these embodiments, the position sensor assembly 11 is only capable of detecting the misplacement of the nasogastric tube in the respiratory system and is not capable of detecting whether the nasogastric tube has correctly entered the stomach. In some others of these embodiments, the position sensor assembly comprises an impedance sensor in addition to the gas flow sensor. In these embodiments, the impedance sensor is capable of detecting gastric fluid and thus the correct positioning of the nasogastric tube in the stomach.

Figures 2A - B show the sensor assembly body 30 in detail. The body 30 is configured to encase the gas flow sensor 40 and thereby securely attach the gas flow sensor 40 to the nasogastric tube 10 and isolate the gas flow sensor 40 from the or each lumen 17 of the nasogastric tube. The body also minimises the contact of the gas flow sensor 40 with a patient's body fluids and other material during insertion of the nasogastric tube so as to minimise fouling of the gas flow sensor 40.

The body 30 is connected to and extends from the distal end 16 of the nasogastric tube 10. The body 30 thus forms a tip of the nasogastric tube and leads the nasogastric tube 10 during insertion. Because the gas flow sensor 40 is integrated with the body 30, location of the body at the distal extremity of the nasogastric tube 10 enables the earliest possible detection of respiratory misplacement of the nasogastric tube as will be discussed below.

The body 30 extends between first and second ends 31, 32 with the first end 31 attached to the distal end 16 of the nasogastric tube 10. The second end 32 is curved or domed to provide a smooth lead-in surface for insertion of the nasogastric tube 10. The body 30 has a bullet shape.

The body 30 is formed as a separate component and either joined to a separately formed nasogastric tube or the nasogastric tube is over-moulded to the body 30. In other embodiments, the body and nasogastric tube are unitarily formed in a single process.

The body 30 has a hollow core 33 in which the gas flow sensor 40 is housed. The hollow core 33 is formed at least in part by a cylindrical side wall 34. A barrier 35 is formed between the hollow core 33 and the lumen(s) 17 of the nasogastric tube 10 by the end wall at the distal end of the nasogastric tube and/or first end of the body. However, in other embodiments there is no barrier and the hollow core 33 of the body is open to the lumen(s) 17 of the nasogastric tube 10.

A plurality of apertures 36 are provided along the side wall 34. In the embodiment shown in Figures 2A - B, the apertures are arranged in two arrays on opposed sides of the body. The apertures 36 are configured to selectively enable the passage of gas through the apertures but not liquid. That is, the apertures enable moving gas, and in particular air moving due to respiration, to pass through the apertures and over the gas flow sensor 40 but are also configured to minimise the likelihood that liquids will pass through the apertures. Each of the apertures 36 have a diameter of 0.1 - 2mm, preferably 0.5 - 2mm, preferably 1 - 2mm. The apertures are spaced apart 0.1 - 2mm, preferably 0.5 - 2mm, preferably 1 - 2mm along the length of the body.

Without wishing to be bound by theory, it is thought that the apertures are able to selectively allow the passage of gas whilst limiting the passage of fluid due in part to the surface tension of liquid.

As shown in Figures IB and D, the gas flow sensor 40 is located within the hollow core 33 of the sensor assembly body 30. The sensor 40 is held generally centrally within the hollow core 33, spaced away from the internal surface of the body 30. In the illustrated embodiment, arms 37 project inwardly from opposed internal surfaces of the body 30 to hold the sensor 40 away from the internal surfaces.

The gas flow sensor 40 comprises a thermal mass flow sensor. In other embodiments, the gas flow sensor comprises a pitot tube, a strain gauge, a piezoelectric sensor or a micromechanical flow sensor.

The thermal mass flow sensor exploits heat transfer properties to determine the velocity of a surrounding fluid medium. The thermal mass flow sensor is configured to act as a constant temperature anemometer, seeking to maintain a constant temperature difference between the sensor and the surrounding medium. Flowever, in other embodiments, the thermal mas flow sensor is configured to act as a constant voltage anemometer or as a constant current anemometer.

When configured as the constant temperature anemometer, the relationship between the velocity of the surrounding fluid medium and the voltage output of the sensor is given by:

U = U ₀ x Vl + k x v ⁿ

Equation (1) where:

U = sensor voltage output

U ₀ = heat loss due to free or natural convection

k = Fluid dependent constant

v = Fluid velocity

In Equation (1) above, free convection (U ₀ ) is the movement of heated air caused by the density differences of the air across a temperature gradient. In other words, it is the flow of air that is caused by the heater heating the air itself. In the present disclosure, the thermal mass flow sensor 40 is configured to detect changes in gas velocity, specifically air velocity. The thermal mass flow sensor is thus configured to distinguish between tracheal and oesophageal airflow patterns.

The thermal mass flow sensor comprises a sensor chip 41 having two resistive elements. The first resistive element is a heater 42 of constant resistance and the second resistive element is a temperature variable resistor 43. That is, the second resistive element has a resistance that changes with temperature. Power is supplied to the heater 42, which produces heat that is transferred to other components of the chip 41, in particular the temperature variable resistor 43. Heat is also transferred from the chip to the surrounding medium. The flow rate of the surrounding medium across the chip affects the amount of heat transferred from the chip to the surrounding medium and consequently varies the resistance of the temperature variable resistor.

In an embodiment, a FS5™ thermal mass flow sensor produced by Innovative Sensor Technology 1ST AG is suitable for use in the present disclosure.

Figure 3 shows a circuit diagram illustrating how the thermal mass flow sensor is configured to act as a constant temperature anemometer by combining the sensor with a feedback circuit. This is done through temperature regulation of the heater by varying the power supplied to it. The variable resistor 42 and the heater 43 are combined with other resistors to provide a Wheatstone bridge 44. When the velocity of the surrounding medium is zero, there is a constant temperature difference between the variable resistor and surrounding medium and the Wheatstone bridge is balanced. Both inputs of an op-amp 45 are approximately equal and power is delivered at a constant rate to the heater 42 to maintain the temperature difference.

When the velocity of the surrounding medium is non-zero, heat transfer cools the variable resistor, increasing its resistance and causing an imbalance in the Wheatstone bridge 44. The imbalance leads to increased output from the op-amp 45, causing a transistor 46 to draw more current and thus supply more power to the heater 42. This increases the temperature of the variable resistor to maintain the constant temperature difference and rebalances the circuit due to the resultant decrease in the resistance of the variable resistor.

The voltage output from the top 47 of the Wheatstone bridge 44 gives a measure of the power supplied to the heater. This voltage output is a function of the surrounding medium velocity across the chip in accordance with Equation (1) above.

The gas flow sensor 40 also comprises an associated comparator circuit 48. The comparator circuit 48 takes the output voltage from the thermal mass flow sensor circuit and compares it with a reference voltage 49. That is, the voltage at the top 47 of the Wheatstone bridge 44 is compared to a reference voltage. The reference voltage for the gas flow sensor 40 is set to a voltage output equivalent to a threshold air velocity below 0.64m/s, preferably below 0.6m/s, more preferably below 0.5m/s, more preferably below 0.4m/s. The reference voltage 49 is set to be low enough to obtain a signal when breathing is occurring but not so low that it causes the gas flow sensor to pick up noise. The reference voltage in this regard is dependent on the nature of the gas flow sensor 40 and its associated circuits, as well as the configuration of the body 30 in which the sensor is housed. The normal mean tracheal air speed for an adult breathing normally and at rest is approximately 0.64m/s. By setting the reference voltage to a voltage corresponding to an air velocity below this speed, the gas flow sensor 40 via its comparator circuit 48 will sense air flow associated with respiration.

It is to be appreciated that the gas flow sensor 40 is configured to detect sufficient air speed that is indicative of breathing. The gas flow sensor 40 is intentionally not configured to provide an accurate measurement of respiratory performance. In this respect, the gas flow sensor is configured to filter low level noise by setting the reference voltage at a relatively high level; for example corresponding to at least 50% of the mean tracheal air speed for an adult breathing normally and at rest, preferably at least 60%, more preferably at least 70%, more preferably at least 80%.

The gas flow sensor 40 produces a signal based on the comparison between the output voltage from the thermal mass flow sensor and the reference voltage. The signal is communicated to the position indicator 12 to provide an indication to the user of whether the distal end of the nasogastric tube is (or is not) in the respiratory system. Signal processing techniques may be applied to this signal in order to improve the accuracy of the gas flow sensor 40 in detecting whether the distal end of the nasogastric tube is in the respiratory system. In some embodiments, a moving average filter is applied to the gas flow sensor signal in order to filter noise caused by intermittent and non respiration based non-indicative air flow events such as burping for example. In other

embodiments, a moving average filter is applied to the output voltage before it is compared to the reference voltage to generate the signal to the position indicator. In further embodiments, frequency analysis is applied to the signal produced by the gas flow sensor in order to detect a pattern indicative of breathing.

Referring to Figures IB, C and E, the ion selective sensor 50 is located within the nasogastric tube 10. The ion selective sensor is located in a recess 38 in the external surface of the nasogastric tube 10. The location of the ion selective sensor is such that, in use, the ion selective sensor is exposed to the surrounding environment of the nasogastric tube 10. This enables the ion selective sensor to be brought into contact with and thereby detect the concentration of a particular ion indicative of the stomach. In particular, the ion selective sensor is configured to detect the concentration of hydronium (H ₃ 0 ⁺ ) or chloride (Cl ) ions. This is because the stomach has a uniquely acidic environment due to the presence of hydrochloric acid (HCI). In the illustrated embodiment, the ion selective sensor is a pH sensor that is configured to detect the concentration of H ⁺ ions.

The ion selective sensor 50 is located on the nasogastric tube 10 so that it is further from the distal end 18 of the nasogastric tube than all of the distal openings 20 of the nasogastric tube 10. As a result, when the ion selective sensor detects the presence of acidic material indicative of the stomach, the ion selective sensor and all of the distal openings will be located within the stomach. Advantageously, this minimises the likelihood that some of the nasogastric tube's distal openings are still above the oesophageal sphincter when feeding commences which can lead to reflux and aspiration pneumonia.

The ion selective sensor 50 comprises a primary electrode and a reference electrode. In the illustrated embodiment, the primary electrode is an ion sensitive field effect transistor (hereafter "ISFET") sensor 51. However, in other embodiments, the primary electrode is an antimony electrode. The ISFET 51 has three terminals; a gate 52, a drain 53 and a source 54. The voltage at the gate 52 controls the conductivity of the drain-source channel. An ion sensitive gate insulator influences the relationship between the gate and the drain-source channel. The greater the hydrogen ion concentration in the surrounding medium, the lower the gate voltage required to achieve a given level of drain-source conductivity.

In an embodiment, an ISFET produced by Winsense Co. Ltd is suitable for use in the present disclosure.

In an embodiment, an ISFET produced by Sentron Europe BV is suitable for use in the present disclosure.

Figure 4 shows a circuit 57 configured for use with the ISFET 51. The circuit 57 is configured to hold a constant voltage and current between the drain 53 and the source 54. Associated with the ISFET circuit 57 is a feedback circuit 58 from the source 54 to the gate 52. The feedback circuit 58 is completed through the surrounding medium via a reference electrode 59. The reference electrode 59 is in the form of an Ag/AgCI reference electrode. In other embodiments, the reference electrode is a reference field effect transistor ("REFET") or a pseudo-reference electrode being an electrode with a potential that varies predictably with a change in conditions such as pH. An output 60 of the feedback circuit 58 is the threshold voltage of the ISFET.

In use, voltage is supplied to the ISFET circuit 57 at approximately ±9V. The drain-source channel voltage is biased at approximately 0.3V using voltage reference diodes 61, 62. The drain-source channel current is maintained at approximately 30mA. The source voltage is connected to an inverting input of an op-amp 63, to amplify this voltage at high gain to output to the gate 52. This negative feedback drives the gate voltage down to the minimum voltage required to keep the drain- source channel on. This minimum voltage is the ISFET's threshold voltage. Thus, the gate voltage is always at the threshold voltage provided by the feedback circuit 58, which is dependent on pH.

The gate voltage is taken as an output voltage for the ion selective sensor 50. The output voltage is isolated from the gate and the rest of the circuit 57 by a buffer amplifier 64 in order to prevent the use of the output voltage from disrupting the ion selective sensor's behaviour. Due to the behaviour of the ion-sensitive gate insulator, the threshold voltage of the feedback circuit 58 decreases as pH decreases. Because the gate voltage equals the threshold voltage of the feedback circuit 58, the gate voltage is dependent on the pH of the surrounding medium.

The ion selective sensor 50 comprising the ISFET 51 also comprises an associated comparator circuit 65. The comparator circuit 65 takes the output voltage from the ISFET circuit and compares it with a reference voltage 66. That is, the voltage at the gate 52 of the ISFET 51 is compared to a reference voltage. The reference voltage 66 for the ion selective sensor 50 is set to a gate voltage equivalent to a pH of 3 - 4.5, preferably about 4. The reference voltage in this regard is dependent on the nature of the ISFET 51 and its associated circuits. The pH in a human stomach is generally less than 5 and can be as low as 1. By setting the reference voltage to a voltage corresponding to a pH below the lower bound of the pH of the oesophagus (which should be below the upper bound of the stomach pH), the ion selective sensor 50 via its comparator circuit 65 will sense location within the stomach when the gate output voltage drops below the reference voltage 66.

Each of the gas flow sensor 40 and the ion selective sensor 50 communicate with the position indicator 12 to provide an indication to the user of the position of the distal end 16 of the nasogastric tube 10. In the embodiment shown in Figures 1A - E, each of the sensors 40, 50 communicate with the position indicator 12 via respective wires 70, 71 extending through the nasogastric tube 10. In the illustrated embodiment, the wires 70, 71 are embedded in and extend along the wall of the nasogastric tube 10. However, in another embodiment, one or both of the wires extend through the or one of the lumina 17, which may be specifically dedicated for receiving the wires.

Each of the wires 70, 71 extend from their respective sensors 40, 50 to the proximal end 15 of the nasogastric tube 10. Each of the wires 70, 71 extend through the coupling element that is used to connect to an enteral feeding pump. The position indicator 12 connects to the wires 70, 71 and thus to the sensors 40, 50 by connecting to the coupling element. It is to be appreciated that each wire 70, 71 can be a single wire or a bundle of wires. In another embodiment, the nasogastric tube has a connection element for the wires to be connected to the position indicator that is separate from and additional to the coupling element which connects to an enteral feeding pump.

In further embodiments, the gas flow and pH sensors communicate to the position indicator via a common wire.

In another embodiment, the gas flow and pH sensors communicate to the position indicator wirelessly. The wireless communication can be by Wi-Fi, Bluetooth ^® or radiofrequency.

Figure 5A shows the position indicator 12 schematically. The position indicator is a separate unit from the nasogastric tube. The position indicator is external to the patient during use. The position indicator is in the form of a device that can be hand held and/or mounted to conventional hospital mounting equipment.

The position indicator 12 comprises a receiver 80 that is configured to receive signals from each of the gas flow and ion sensitive sensors. In the above described embodiment in which the sensors communicate to the position indicator physically via the wires 70, 71, the receiver 80 comprises a connector for connecting to the wires 70, 71. In an embodiment, the connector comprises a socket in which a plug connected to the wires on the nasogastric tube is received. In an embodiment, the plug comprises the coupling element or feeding port.

In other embodiments, where the sensors communicate wirelessly with the position indicator wirelessly, the receiver comprises a Wi-Fi, Bluetooth ^® or radiofrequency receiver.

The position indicator 12 also comprises a processor 81. The receiver 80 transmits signals received from the sensors 40, 50 to the processor 81 for processing to produce an output. The processor 81 incorporates the comparator circuits 48, 65 for the respective sensors 40, 50. The processor 81 is also configured to apply the signal processing techniques described above. The output from the processor 81 contains information about the position of the distal end of the nasogastric tube.

The position indicator 12 comprises a user interface 82 for the user of the nasogastric tube system 1. The user interface 82 communicates the position of the nasogastric tube, in particular the distal end of the nasogastric tube, based on signals from the gas flow and ion sensitive sensors 40, 50. The user interface 82 receives the output 83a from processor 81 and displays information about the position of the distal end of the nasogastric tube based on that output.

In an embodiment, the user interface 82 comprises a green and a red light in respect of each sensor. For the gas flow sensor, a green light indicates negligible airflow is detected and a red light indicates that gas flow indicative of respiration is detected. For the ion sensitive sensor, a green light indicates that an ion concentration indicative of the stomach is detected and a red light indicates that an insufficient concentration of ions is detected. Correct stomach placement of the distal end of the nasogastric tube is confirmed by a green light from both sensors.

In another embodiment, as illustrated in Figure 1A, the user interface comprises a three light system. When the first light, which is preferably red, is lit this indicates that the gas flow sensor is detecting air flow and the ion selective sensor is not detecting a sufficient concentration of ions. When the gas flow sensor detects the absence of air flow, but ion selective sensor is no detecting a sufficient concentration of ions, the middle light, which is preferably orange, is lit. The third light, which is preferably green, lights once the ion selective sensor detects a sufficient concentration of ions.

In other embodiments, the user interface comprises a display screen that is configured to communicate the position of the nasogastric tube by displaying relevant symbols. In some embodiments, the position indicator also comprises a speaker to provide audible signals to communicate the position of the nasogastric tube. For example, the position indicator is configured to sound an alarm when the gas flow sensor detects that the distal end of the nasogastric tube is in the respiratory system.

The position indicator 12 also comprises a data storage unit 84 which is configured to receive and store the output 83b from the processor 81. The data storage unit 84 in some embodiments is also configured to store patient related data such as age, weight, gender or medical history for example. The patient related data in some embodiments is input to the data storage unit by the user of the nasogastric system prior to use. The position indicator 12 is also provided with a data port 85 for receiving a connection to extract data from the data storage unit 84. In some embodiments, the data port 85 is a USB port, a flash drive port or other suitable port for connecting a physical data medium. In other embodiments, the data port is a wireless data port such as a WiFi transmitter, a Bluetooth ^® chip or a radiofrequency source.

The position indicator 12 also comprises a power source 86 to provide power for the position indicator components, in particular the receiver 80, the processor 81, the user interface 82 and the data storage unit 84. The power source 86 also powers the position sensor assembly 11 including both the gas flow sensor 40 and the ion selective sensor 50 in the embodiments in which the sensors are connected to the position indicator via wires 70, 71.

The power source 86 preferably comprises one or more batteries. Flowever, in other embodiments, the power source comprises a cable that connects to mains power. The cable is permanently fixed to the position indicator or coupled via a power socket and may retain one or more batteries.

The position indicator 12 comprises a housing 87 that houses the receiver 80, the processor 81, the user interface 82, the data storage unit 84, the data port 85 and the power source 86 in the form of one or more batteries. The housing 87 shields these components of the position indicator 82 from the external environment. The housing 87 comprises a mounting portion (not shown) to enable the position indicator 82 to be mounted to conventional hospital mounting equipment.

Referring now to Figure 5B, a method of using the nasogastric system 1 is illustrated.

• Steps 1 - 3: After determining that a patient requires a nasogastric tube ("NGT"), the

position indicator ("external unit" in Figure 5B) is attained the nasogastric tube of required size is selected. In general, the clinician would decide whether to use a wide or narrow nasogastric tube.

• Step 4: The nasogastric tube 10 integrated with the position sensor assembly 11 is then removed from its sterile packaging and the position sensor assembly 11 connected to the position indicator 12 by plugging the connecting element of the nasogastric tube 10 into the receiver socket 80 of the position indicator 12.

• Step 5: The distal end of the nasogastric tube and/or the sensor assembly body 30

projecting beyond the distal end have a lubricant applied thereto to ease insertion of the nasogastric tube. Typically the lubricant contains an anaesthetic compound such as lidocaine.

• Steps 6 - 8: The clinician selects which nostril to insert the nasogastric tube and commences insertion at a right angle to the nostril to ease passage of the nasogastric tube through the nasal cavity and down the pharynx of the patient. Optionally, the position indicator 12 and the sensors 40, 50 are only switched on once the nasogastric tube has been inserted approximately into the pharynx, which the clinician can estimate using the graduated markings. Alternatively, the position indicators and sensors 40, 50 are switched on before the nasogastric tube is inserted into the nostril.

• Step 9: Once the nasogastric tube has been inserted a sufficient distance so that its distal end, if correctly located, is in the oesophagus and not the trachea, the clinician should consider whether the position indicator 12 is indicating correct location. In some embodiments this involves briefly pausing the process of insertion to enable the gas flow sensor to detect air flow without causing any noise in the sensor signal due to movement of the nasogastric tube. The position indicator 12 via the gas flow sensor may provide this information in less than 5 seconds, preferably less than 2 seconds from pausing the insertion of the nasogastric tube.

• Steps 10 and 11: If, after step 9, the position indicator 12 is showing that the gas flow sensor is detecting a flow of air then this indicates malpositioning of the nasogastric tube in the trachea or another part of the respiratory system. The clinician accordingly withdraws the nasogastric tube partially or wholly and recommences or restarts the insertion process. However, if the position indicator 12 is showing no detected air flow, this indicates that the distal end of the nasogastric tube is in the oesophagus and the clinician proceeds with inserting the nasogastric tube.

• Steps 12 and 13: After continued insertion of the nasogastric tube to a sufficient depth, the distal end of the nasogastric tube should pass the oesophageal sphincter and reach the stomach. At this point, the position indicator 12 should be showing that the ion sensitive sensor should is detecting an ion concentration indicative of the stomach. The clinician can secure the nasogastric tube at its proximal end and disconnect the position indicator from the nasogastric tube. If the position indicator is not showing stomach location of the ion sensitive sensor, this may indicate that the nasogastric tube has curled up in the

oesophagus. In some situations the clinician would withdraw the nasogastric tube partially or wholly and recommences or restarts the insertion process. In other situations, particularly when the patient is expected to potentially have an abnormally high stomach pH, the clinician uses an X-ray to check the location of the distal end of the nasogastric tube.

• Step 14: Once the nasogastric tube has been correctly positioned, it is then connected to an enteral feeding pump in a conventional manner and feeding (or draining) can commence. In some embodiments, the position indicator can be periodically reconnected to the nasogastric tube to check whether the distal end of the distal end of the nasogastric tube is still correctly positioned in the stomach.

In another embodiment not illustrated in the figures, the sensor assembly body that houses the gas flow sensor is integrated with the nasogastric tube such that it forms an extension of the distal end of the nasogastric tube. In this embodiment, the apertures of the sensor assembly body that are configured to enable the flow of gas over the gas flow sensor also provide at least some of the outlet openings of the or at least one of the lumen. The lumen(s) in this embodiment are in fluid communication with the hollow core of the sensor assembly body.

In at least one form of this embodiment, the gas flow sensor wire that connects the gas flow sensor to the position indicator extends through one of the nasogastric tube lumen. The wire and the gas flow sensor connected to that wire are configured to be removable from the nasogastric tube after the nasogastric tube has been inserted into a patient. To achieve this, the gas flow sensor is decoupled from its attachment to the sensor assembly body by pulling, pushing or twisting the wire from its proximal end. In some embodiments, an electrical signal may be applied to the connection between the gas flow sensor and the sensor assembly body to cause the connection to break or deform and thereby enable removal of the gas flow sensor with the wire. In another embodiment, the position sensor assembly comprises a stylet that extends through the or one of the lumen(s) and is connected to the gas flow sensor. Preferably, the stylet is formed from metallic wire. At least a portion of the stylet is electrically conductive to enable power to be supplied to and signals received from the gas flow sensor via the stylet. Advantageously, the stylet also increases the rigidity of the nasogastric tube and thus assists with the insertion process by reducing the likelihood that the nasogastric tube will curl on itself. Preferably, the stylet is removed from the nasogastric tube after the nasogastric tube has been inserted into the patient. In some

embodiments, the stylet is adapted to disconnect from the gas flow sensor so that when the stylet is removed the gas flow sensor remains within the body. In other embodiments, the stylet is permanently connected to the gas flow sensor so that when the stylet is removed from the nasogastric tube it takes the gas flow sensor with it. Advantageously, this means that the sensor does not remain within the patient. Where the gas flow sensor is to be removed with the stylet, it may have no connection to the body but instead sits loosely within the body. The body, however, may have spacers or bumps on its inner surface to ensure that the gas flow sensor remains spaced away from the internal surface of the body.

Figures 12 and 13 show a sensor assembly body 130 according to another embodiment of the present disclosure for receiving the gas flow sensor 40. The body 130 comprises a sensor holding portion 190 in which the gas flow sensor 40 is held and a stem 191 extending form the sensor holding portion. The stem 191 provides a connecting portion for connecting the body 130 to a nasogastric tube. In an embodiment, the nasogastric tube is overmoulded to the stem 191.

The sensor holding portion 190 has a hollow core 133 in which the gas flow sensor is positioned. Figure 13 shows a cross-section of the sensor holding portion 190. Opposed recesses 192 are provided on the inner surface of the sensor holding portion 190. The recesses 192 are formed as channels in projections 193 formed on the inner surface of the sensor holding portion 190. Side portions of the air flow sensor 40 slot into these recesses 192 to position and hold the gas flow sensor 40 within the body 130 in a substantially stable position with respect to the body 130. The stem 191 is also hollow so that a passage 194 is provided through the stem from the or at least one of the lumens of the nasogastric tube to the hollow core 133 of the sensor holding portion. A stylet 195 (as described above) is therefore able to extend through the nasogastric tube and the stem to connect to the gas flow sensor 40 located within the sensor holding portion. The stylet provides a means to convey power to and receive sensing signals from the gas flow sensor. Also, after the nasogastric tube has been inserted, the stylet may be withdrawn from the nasogastric tube, taking the gas flow sensor with it. Advantageously, the recesses 192 in which the side edge portions of the gas flow sensor 40 are received, enable the gas flow sensor 40 to slide out of sensor holding portion 190 when the style is pulled out of the nasogastric tube. The passage 194 in the stem 191 also has to be sufficiently sized to enable the gas flow sensor 40 to pass through.

A plurality of apertures 136 are provided along a side wall 134 of the sensor assembly body 130. The apertures are provided in a number of arrays, the apertures in each array being offset from each adjacent array. The apertures are in 12 arrays of at least 9 apertures in the embodiment disclosed in Figure 12. Some of the arrays share a common aperture at the distal end of the sensor assembly body 130. The apertures are provided along a substantial portion of the sensor assembly body 130. This is to ensure that apertures are both proximal and distal to the sensor to enable flow across the sensor. The diameter of the aperturesis between 0.1 and 0.5mm, preferably less than 0.4mm, more preferably 0.35mm or less.

In a further embodiment not shown in the figures, the position sensor assembly is attachable to a nasogastric (or orogastric) tube. In this embodiment, the position sensor assembly comprises a separate device that can be attached to the nasogastric tube including conventional nasogastric tubes. The position sensor assembly is attached to the distal end of the nasogastric tube by a coupling portion of the sensor assembly body. The coupling portion attaches to the nasogastric tube by gripping, clamping, crimping or piercing the surface of the nasogastric tube. The position sensor assembly comprises a wireless communicator that communicates to the position indicator wirelessly by for example Wi-Fi, Bluetooth ^® or radiofrequency. The wireless communicator is incorporated in the sensor assembly body and is shielded from the external environment. In a variation of this embodiment, the position sensor assembly, when attached to the nasogastric tube, is engaged with wires incorporated into the nasogastric tube to enable the sensors in the sensor assembly to communicate with position indicator by a physical connection.

Examples

Gas Flow Sensor

A circuit as shown in Figure 3 was built incorporating a FS5 thermal mass flow sensor from

Innovative Sensor Technologies ISG AG. The variable resistor was adjusted until the output voltage, when air velocity was zero, was around 4V (hereafter the zero flow voltage). This resulted in the variable resistor being set to 91W.

A testing rig was built which consisted of two computer fans of differing sizes and speeds. Each fan was connected to clear plastic cylindrical piping of average tracheal diameter.

The testing rig provides a testing environment that models some of the in-vivo conditions (air velocity and tracheal diameter). Details of the testing rig are provided in Table 1 below. Table 1: Gas Flow Sensor Testing Rig

A. Calibration

A calibration curve was developed for the gas flow sensor output voltage at a range of air velocities. The speed of the fan was controlled by varying the voltage supplied to it. To determine what air velocities this produced in the pipe a Pitot tube was used. The Pitot tube was placed within the pipe and pressure measurements were taken at a minimum of six fan supply voltages. These pressure measurements were then converted to velocities using Bernouilli's equation. These measurements are set out in Table 2 below.

Table 2: Pitot tube measurements

The airflow sensor was placed within the tube and the sensor output voltage was measured at a range of air velocities. Sensor output voltage was recorded for at least 30 seconds duration, at a frequency of 200Hz. At each air velocity this was repeated multiple times and averaged. The results from these tests are set out in Table 3 below. Figure 8 shows a plot of this data and includes a non- linear least squares curve fit to the sensor output data collected.

Table 3: Gas Flow Sensor calibration measurements

The data collected showed increased sensor output voltage with increased air velocity through a range of physiologically relevant air velocities. At the lowest speed (0.667m/s) the output was well above the zero flow voltage and provided a distinguishable signal over noise. The calibration curve in Figure 8 approximately follows the expected shape in accordance with Equation (1) above.

The disparity between results from different fans (as shown in Figure 8 and in Table 3) relate to the experimental set up including variations in the exact placement of the sensor within the piping.

B. Orientation testing

The testing rig from calibration was then utilised to perform orientation tests. These were performed in order to quantify how the orientation of the sensor relative to airflow affects the sensor output. The sensor in the testing set up has two degrees of freedom. With axes defined as shown in Figure 9 and air flowing along the z-axis (assumed to be laminar flow), the sensor was rotated about the x- and y-axes, by varying 0 _! and 0 ₂ . This is shown in Figure 9B.

The results from these tests are set out in Tables 4 and 5 are plotted in Figures 10 and 11. Table 4: Gas Flow Sensor Voltage Output Rotated in x-axis

Table 5: Gas Flow Sensor Voltage Output Rotated in y-axis

In the orientation tests data recorded at 1.25m/s and 2.98m/s were collected with the low speed fan and the data recorded at 4.62m/s and 7.60m/s were collected with the high speed fan. Sensor output voltages are shown as a change in voltage over the zero flow velocity) voltage.

The orientation tests indicate that the sensor is able to operate throughout the full range of potential orientations.

The relationship between sensor orientation and sensor output voltage recorded at 1.25m/s and 2.98m/s are distinct from those recorded at 4.62m/s and 7.60m/s. This is likely due to variability of testing set up between the low and high speed fans.

pH Sensor

A circuit as shown in Figure 4 was built incorporating an ISFET pH sensor produced by Winsense Co. Ltd. The ISFET and a reference electrode were connected to the PCB circuit and 9V was supplied by a battery.

The output voltage of the sensor was compared with a reference voltage when the sensor was immersed in solutions containing hydrochloric acid (HCI) of different pH in the range of pH 1.2 to 7 to simulate gastric fluid. The buffer solutions also contained sodium chloride (NaCI). The actual pH values of these solutions, as well as pH 4 and 7 buffer solutions, were measured using a Eutech Instruments CyberScan pH 510. The sensor output voltage was measured for each solution. The response from the sensor for each solution was measured at 10Hz. The sensor was immersed in the solution approximately 5 seconds after beginning the recording. Table 6 below contains the results of these tests.

Table 6: pH sensor voltage

The ISFET's response is extremely fast taking about 0.2 seconds to get to a stable value and having minimal signal fluctuation as shown in Figure 6. An approximately linear calibration relationship was produced with a slope of 51mV per unit of pH (see Figure 7).

In the claims which follow and in the preceding description of the invention, except where the context requires otherwise due to express language or necessary implication, the word "comprise" or variations such as "comprises" or "comprising" is used in an inclusive sense, i.e. to specify the presence of the stated features but not to preclude the presence or addition of further features in various embodiments of the invention.