INTERMITTENT MEASURING OF THE PARTIAL PRESSURE OF AN ANALYTE IN THE SKIN TISSUE

Abstract

It is commonly known within the art of cutaneous/transcutaneous blood gas monitoring to warm up the skin of the patient to allow carbon dioxide and oxygen to diffuse easily through the skin. This is especially the case for transcutaneous partial pressure monitoring of oxygen. Heating the skin to 43 C. to 45 C. over several hours or days may cause damage to the skin. In order to avoid or minimize the risk of these damage, it is proposed to monitor the blood gases at a lower temperature with a cutaneous sensor, and intermittently warm up the skin to a temperature of 42 C. or more for a short duration to monitor the transcutaneous partial pressure of oxygen, before lowering the temperature to the lower set point.

Claims

1. A system for monitoring a transcutaneously measured partial pressure of oxygen at a measuring site of a patient, comprising; a monitor for displaying the measured partial pressure of oxygen; a sensor for measuring the transcutaneous partial pressure of oxygen comprising a heating element for heating the skin of the patient at the measuring site; a communication element between the sensor and the monitor; a controller for controlling the temperature of the heating element, wherein the controller is further adapted for cycling the temperature between two different temperatures; a first time interval during which the measuring site has a temperature close to body core temperature; and a second time interval during which the measuring site has a temperature wherein the skin of the patient is permeable to oxygen in an outward direction from the tissue or blood of the patient to the surroundings, wherein the transcutaneous partial pressure of oxygen is measured at the second time interval, and the first time interval is longer than the second time interval.

2. The system according to claim 1, wherein the first temperature is in the range of 35 C. to 42 C., and the second temperature is at least 41 C.

3. The system according to claim 1, wherein the measured partial pressure of oxygen is displayed at the monitor at all times, and during the first time interval the displayed value is the last partial pressure of oxygen measured at the previous second time interval.

4. The system according to claim 1, wherein the system is further continuously monitoring the a partial pressure of carbon dioxide (CO.sub.2).

5. The system according to claim 1, wherein the system for monitoring the transcutaneously measured partial pressure of oxygen is further adapted for allowing to allow the operator to adjust the time of the first and the second time intervals.

6. A sensor for transcutaneously measuring a partial pressure of oxygen at a measuring site of a patient, comprising; a heating element, for heating the measuring site; a control unit for controlling the temperature of the measuring site, processing of the sensor signals, and controlling communication between the sensor and a monitor; said control unit comprising an analogue to digital converter; the control unit further being adapted to control the heating element to cycle the temperature at a measuring site of a patient between two different temperatures by: adjusting and maintaining the temperature at the measuring site to a temperature close to body core temperature in a first time interval; and increasing and maintaining the temperature at the measuring site to a temperature wherein the skin of the patient is permeable to oxygen in an outward direction from the tissue or blood of the patient to the surroundings in a second time interval, wherein the first time interval is longer than the second time interval, and the sequence of first and second time intervals is repeatable.

7. The sensor according to claim 6, wherein the temperature at the measuring site in the first time interval is maintained in the range of 35 C. to 42 C., and in the second time interval is maintained at at least 41 C.

8. A computer program for controlling an oxygen sensor for measuring a transcutaneous partial pressure of oxygen, wherein the computer program is adapted to control a heating element of the oxygen sensor to: cycle the temperature at a measuring site of a patient between two different temperatures; adjust and maintain the temperature at the measuring site to a temperature close to body core temperature in a first time interval, and; increase and maintain the temperature at the measuring site to a temperature wherein the skin of the patient is permeable to oxygen in an outward direction from the tissue or blood of the patient to the surroundings in a second time interval, wherein the first time interval is longer than the second time interval, and the sequence of first and second time intervals is repeatable.

9. The computer program product according to claim 8, wherein the second time interval comprises at least 10 minutes of heating the skin of the patient, which comprises a lipid structure, and wherein said heating dissolves said lipid structure, followed by 5 minutes of measuring the oxygen partial pressure.

10. The computer program product according to claim 9, wherein the first time interval is at least 20 minutes.

11. The computer program according to claim 8 wherein the computer program product is adapted to evaluate whether there is a stable measurement of the partial pressure of oxygen before the measured partial pressure of oxygen is communicated to the operator.

12. A method for monitoring the transcutaneous partial pressure of oxygen (tcpO.sub.2) at a measuring site of a patient, comprising the steps of: cycling the temperature at the measuring site between two different temperatures; adjusting and maintaining the temperature at the measuring site to a temperature close to body core temperature in a first time interval, and; increasing and maintaining the temperature at the measuring site to a temperature wherein the skin of the patient is permeable to oxygen in an outward direction from the tissue or blood of the patient to the surroundings in a second time interval, and; measuring the partial pressure of oxygen in the second time interval, wherein the first time interval is longer than the second time interval.

13. The method according to claim 12, further comprising measuring a partial pressure of CO.sub.2 continuously.

14. The method according to claim 13, wherein the measuring of the partial pressure of CO.sub.2 takes the cycling temperature into account when calculating the partial pressure of CO.sub.2.

15. The method according to claim 12 wherein partial pressure of oxygen, measured during the previous second time interval, is displayed at the monitor during the following first time interval.

16. The method according to claim 12 wherein the temperature at the measuring site in the first time interval is in the range of 35 C. to 42 C., and the temperature at the measuring site during the second time interval is at least 41 C.

Description

[0044] FIG. 1 shows a sensor according to the invention

[0045] FIG. 2 shows two graphs of the O.sub.2 measurement, and the heating interval respectively.

[0046] FIG. 3 shows two graphs of the transcutaneous partial oxygen pressure measured.

[0047] FIG. 1 shows a blood gas sensor for transcutaneous or cutaneous measurements of blood gases. The sensor head consist of a circular plastics housing 2 with a neck like attachment 3 through which the connecting cables 4, for transferring analogue or digital signals to the monitor, are led. A glass pH electrode 5 is located in the central axis of the sensor. It comprises a glass stem onto whose front end a pH-sensitive glass layer 6 is fused. An internal reference electrode with a platinum lead wire 7 fused into the glass is located inside the glass cylinder. The pH electrode 5 is embedded in a silver block 8 whose surface is covered with a chloride layer. The surface of the silver block thus forms an Ag/AgCl electrode which acts as reference electrode for the pH measurement. An electrolyte solution whose pH will be measured is located on a porous hydrophilic spacer, covered with a gas permeable hydrophobic membrane. To protect the membrane from mechanical damage, it is covered with a metal diaphragm. This diaphragm has in the center an aperture through which the carbon dioxide gas to be measured is able to diffuse into the electrolyte solution at the site of the pH-sensitive glass layer. The spacer, the membrane, and the metal diaphragm are attached to the sensor housing 2 by means of a clamping ring 14. The silver block 8 additionally has the function of a heating element. A heating wire 15 is coiled around it and heats it to the temperature of up to 45 C.

[0048] The sensor may further include a control block, which is not shown in the drawing, for control and processing of the signals measured by the sensor, and distribute signals to or from the monitor.

[0049] FIG. 2 shows two graphs. One indicating the sensor temperature as a function of time and below a diagram of the oxygen level measured over time. The first period of time indicated by the vertical dashed line and also by the non-scattered arrow below the diagrams represent the warm-up phase, where it is usually not advisable to measure, since the measurements may not be reliable. After the initial warm-up phase, the sensor heating element regulates the temperature with predetermined intervals, whereby the sensor temperature and hence the skin temperature fluctuates. When the temperature is low, the oxygen levels measured are less reliable, than when the temperature is high. Thus the transcutaneous partial pressure of oxygen is measured at the time, when the temperature of the skin is sufficient to ensure a reliable measurement. The curved line shows the transcutaneous partial pressure of oxygen over time. The line is solid when the temperature is sufficient for reliable measures and dotted, when the temperature is such that the measures cannot be relied upon directly. Under normal circumstances, the skin will be subjected to the higher temperature for at least 10 minutes, to warm up the capillary bed sufficiently, where after the oxygen tension may be measured for e.g. 5 minutes. The heating element is now turned off, allowing the temperature to drop back to the lower temperature, where it is kept for e.g. 30 minutes before warming up again to get a new set of oxygen partial pressure measurements at the higher temperature, starting a new cycle.

[0050] FIG. 3 shows another embodiment, where the transcutaneous partial pressure of oxygen is still shown at the instances with a low skin temperature. The level shown is in this case extrapolated from the previous one or more levels measured. The dashed line is the actual oxygen level measured and the solid line is a combination of the levels measured, when the temperature is sufficient, and the extrapolated level in between. The care taker is hereby made aware, that the shown level is not the actual current oxygen transcutaneous partial pressure, but a value extracted from the previously measured levels.

[0051] To further explain the invention, an example of the use of the proposed method, system and device will be given in the following. A premature infant is intubated and mechanically ventilated at a neonate intensive-care unit (NICU). To monitor the neonate's respiratory function, the NICU nurse applies a blood gas monitor to the neonate. The monitor includes sensors for measuring the transcutaneous partial pressure of oxygen and carbon dioxide. The sensor is mounted in a fixation ring, fixing the sensor to the skin with a contact gel between the skin and the sensor interface to create a closed measuring chamber between the skin and the sensor. Wires transfer data between the sensor and the monitor. The NICU nurse would like to receive information about both oxygen and carbon dioxide transcutaneous partial pressure. To minimize the risk of injury, the nurse chooses the default program for monitoring oxygen tension, wherein the skin tissue is heated to 43 C. for 10 minutes, where after the temperature is maintained at 43 C. for another 5 minutes while monitoring the oxygen level, followed by 30 minutes where the temperature is lowered to 41 C., where after the programs starts over. The nurse is well aware, that the most reliable measures of the oxygen tension is received only during the 5 minutes of measuring at 43 C., but is also notified of this on the monitoring screen. However to reduce the risk of injuries due to the higher temperature, she accepts this compromise.

[0052] After 12 hours, the NICU doctor and nurse looks at the data collected over the last 12 hours. From the data it appears that the situation, although still critical, is stable. Hence they decide to increase the time where the skin is heated to the lower temperature of 41 C. to 60 minutes, to further reduce the risk of injuring the skin. The nurse now programs the monitor accordingly. The trend within the field of cutaneous/transcutaneous blood gas sensors is generally to decrease the size of both sensors and monitors. A preferred site for measuring the transcutaneous partial pressure of blood gases is the earlobe, since the skin at the earlobe is very thin. Since the earlobe often has a small surface area, the sensor size is important. Furthermore, measuring the transcutaneous partial pressure of oxygen and carbon dioxide is often used on preterm neonates. Here the size of the sensor is even more important.

[0053] The invention claims a first and a second time interval of a time cycle. Despite the wording of a first and a second time interval, the skilled person will understand that the invention covers both scenarios where the cycle is started with the first time interval, and where the cycle is started with the second time interval.

[0054] The proposed system and method may be used for any sensor type for measuring blood gases e.g. electro chemical, optical or other types.