Monitoring of PCO2 by skin surface sensors.

The methodology of cutaneous PCO2 measurement and the results of the first clinical studies performed with a newly developed sensor are reported. The in vitro data of the sensor are: sensitivity 55 mV/decade; response time tau 90% = 50 s; drift less than 10% per 24 h; no measurable interference by oxygen and anesthetic gases. At a sensor temperature of 44 degrees C, the correlation between arterial PCO2 and cutaneous PCO2 is significant, with correlation coefficients above 0.95 both in case of neonates and adult patients. The cutaneous PCO2 is, however, consistently higher than arterial PCO2, and a correction of the cutaneous PCO2 value needs to be performed. At 44 degrees C, the time lag between changes of cutaneous PCO2 and arterial PCO2 is about 2-3 min as estimated from capnometric measurements. At lower sensor temperatures, the correlation between arterial and cutaneous PCO2 is still good; however, the physiological response time of the cutaneous PCO2 measurement becomes prohibitively long.

Electrochemical sensors for invasive and non-invasive monitoring of blood gases.

The need for continuous monitoring of blood gases in critically ill patients has become more and more evident during the past years. Electrochemical sensors appear to be most appropriate for such measurements because of their simplicity, ease of use and low cost. Intravascular sensors and skin surface sensors for pO2 and pCO2 are available today in different degrees of technical maturity. In the case of intravascular sensors, the use of surface heparinisation techniques may lead to considerable functional improvements. In the case of skin surface sensors, a reliable method for simultaneous measurement of cutaneous blood flow is needed. Laser doppler flowmetry appears to be a promising method for this purpose.

Interference of anesthetic gases at skin surface sensors for oxygen and carbon dioxide.

Several variables may account for the response of electrochemical skin surface PO2 sensors to anesthetic gases: cathode material and size, pH of the electrolyte and membrane material. These variables cannot be chosen arbitrarily and their influence has been tested with two types of sensors. In one type (LSC), a large size cathode (mm range) and a membrane with low permeability for oxygen such as mono-axially oriented polyethylene is used. The other type (MC) contains one or more microcathodes (micron range) and a membrane which is highly permeable for oxygen such as Teflon PTFE. With the LSC sensor, the N2O interference current is smaller than 5% of the air current when the sensor is polarized at --600 mV. The interference current with 2% halothane is smaller than 3% of the air current. With the MC sensor, the N2O interference may be up to 40% of the current in air when the sensor is polarized at --800 mV. The magnitude of this interference depends considerably on the silver deposition on the platinum cathode. At --600 mV the N2O interference is negligible. However, at this polarization voltage, the sensor is not operated within the limiting current plateau of oxygen. The interference current with 2% halothane may be up to 30% of the current in air. With both types of sensors there was no measurable interference by 2% enflurane. The authors conclude that to reduce the interference of anesthetic gases at skin surface sensors for oxygen to a reasonable level, it is necessary to use a membrane with low permeability for oxygen and a polarization voltage of approximately --600 mV. These two conditions can be fulfilled optimally only with a sensor design in which a large size cathode is used. At Stowe-Severinghaus type skin surface sensors for PCO2, there is no measurable interference by N2O, halothane or enflurane.

An electrochemical sensor for continuous intravascular oxygen monitoring.

The intravascular PO2 sensor described in this paper essentialal routine use. The small dimensions of the sensor permit its introduction into the vessel through a conventional guiding catheter size 4 F or larger. With 5 F catheters, simultaneous blood sampling and/or blood pressure measurement is possible. The accuracy and stability of the sensor permit continuous PO2 measurement to be performed over several days.

Principle and characteristics of the ROCHE tissue pH electrode.

A miniaturized probe for continuous measurement of tissue (interstitial fluid) pH is described. The electrode design follows closely the concept described by Stamm and co-workers. The pH sensitive tip has a length of 1 mm and is made of a Li-Ba-Si glass of low electrical resistivity. The reference electrode with a capillary type liquid junction is an integrated part of the electrode assembly. This paper summarizes the important design features of the electrode and describes its in vitro characteristics.

Reliability of cutaneous oxygen measurement by skin sensors with large-size cathodes.

The oxygen partial pressure may be measured cutaneously by directly heated PO2 sensors attached to the surface of the skin. The use of a large-size cathods (diameter 4 mm) allows one to obtain an average PO2 value over a sufficiently large skin area. The permeability of the membrane for oxygen has to be kept low to prevent a disturbance of the oxygen profile in the cutaneous tissue as a result of the oxygen consumption of the sensor. When using of 6 micrometer Mylar membrane, the current can be limited by the diffusion of oxygen through the membrane and is a measure of the arterialized cutaneous PO2. One effect of using membrane of low permeability is that the response time of the sensor is long (tau 90% = ca 45 sec). The reliability of correctly reproducing cPO2 variations by sensors having different response times may be evaluated by determining the transfer function of the membrane which allows the calculation of the frequency characteristics for various membranes. The cutaneous tissue layer also acts as a filter for rapid PO2 fluctuations. The characteristic of this filter is essentially similar to that of a "slow" membrane such as Myler 6 micrometer. The information gained by using a "rapid" membrane such as Telfon 13 micrometer is negligible. In 490 comparative measurements in newborns performed with sensors heated to 44 degree C, the correlation between arterial PO2 and cutaneously measured PO2 is significant (correlation coefficient = 0.934) and the regression line does not deviate significantly from the identity line.

A fast stabliization technique for oxygen sensors with low cathode current densities.

After application of the polarization voltage to a newly prepared oxygen sensor, an initial stabilization of several minutes to a few hours is required. The electrochemical phenomena accounting for this stabilization process are discussed. By far the slowest reaction is the change of the oxidation state of the noble metal cathode after application of the polarization voltage. During this process, reduction of surface oxides at the cathode causes a transient current. The contribution of this reaction to the total current is proportionately higher in the case of oxygen sensors with low cathode current densities (large cathode, membrane of low oxygen permeability) than in the case of sensors with microcathodes. For the first type of sensor, a technique has been developed which allows acceleration of this surface reaction by passing a high constant current across the cathode. With this technique, the stabilization time can be considerably shortened in a reproducible manner, and the sensor is ready to be used within 15 to 30 min of its preparation. This time is comparable to the stabilization time needed for sensors with microcathodes.

Skin sensors for continuous oxygen monitoring of newborns.

Since the introduction of the technique of cutaneous1 pO2 measurement by directly heated oxygen sensors in 1972, the clinical applications and limitations of this new method have been extensively investigated. The method has proven to be of particular value in monitoring of high risk newborns as it affords the possibility of continuously monitoring clinically significant changes in the oxygenation state of the newborn. In this paper, methodological criteria for the assessment of the reliability of cutaneous pO2 monitoring are discussed. Particular consideration is given to the oxygen and temperature profiles in the vicinity of the skin sensor and to the response time of the sensor. In view of the fact that the cutaneous pO2 reflects the oxygen partial pressure at the level of arterialized cutaneous tissue, the method has limitations if it is used as an indirect determinant of arterial pO2.

An instrument for the rapid determination of L-lactate in biological fluids.

An analyzer for lactate has been developed based upon an electrochemical-enzymatic method. It allows the determination of lactate within 2 to 3 minutes after sample collection. The instrument is capable of performing up to 20 measurements per hour. A minimum sample volume of 50 mul is needed. Except for the injection of the sample, all steps of the measurement are automated. The instrument is easy to operate and can also be used by nontechnical staff.