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 method for monitoring a transcutaneous partial pressure of oxygen (tcpO.sub.2) at a measuring site of a patient, comprising: providing a sensor for measuring the tcpO.sub.2, wherein the sensor comprises a heating element for heating skin of the patient; securing the sensor to the skin of the patient, such that the sensor defines the measuring site on the patient; and performing a tcpO.sub.2 measuring cycle repetitively at the measuring site, wherein the tcpO.sub.2 measuring cycle comprises: heating the site to a first temperature in a range of 35° C. to 42° C. during a first time interval, after the first time interval, heating the measuring site to a second temperature to thereby permeabilize the skin to oxygen in an outward direction from tissue or blood of the patient to the sensor, maintaining the second temperature without erythema to the skin of the patient during a second time interval that is shorter than the first time interval, after the second time interval, measuring the tcpO.sub.2 at the site during a third time interval until consistent tcpO.sub.2 measurements are obtained, determining a tcpO.sub.2 level in the patient using the consistent tcpO.sub.2 measurements, reporting the tcpO.sub.2 level in the patient, and allowing the skin at the measuring site to cool before repeating the tcpO.sub.2 measuring cycle.

2. The method according to claim 1, further comprising measuring a partial pressure of CO.sub.2 continuously.

3. The method according to claim 2, wherein the measuring of the partial pressure of CO.sub.2 takes the first and second temperatures into account.

4. The method according to claim 1, wherein the tcpO.sub.2 measured during the third time interval is displayed at a monitor during a following first time interval of a subsequent tcpO.sub.2 measuring cycle.

5. The method according to claim 1, wherein the second temperature is at least 41° C.

6. The method according to claim 1, wherein the second temperature is at least 43° C.

7. The method according to claim 1, wherein the second time interval is at least 10 minutes.

8. The method according to claim 1, wherein the third time interval is 5 minutes.

Description

(1) FIG. 1 shows a sensor according to the invention

(2) FIG. 2 shows two graphs of the O.sub.2 measurement, and the heating interval respectively.

(3) FIG. 3 shows two graphs of the transcutaneous partial oxygen pressure measured.

(4) 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.

(5) 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.

(6) 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.

(7) 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.

(8) 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.

(9) 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.

(10) 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.

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