Transcutaneous Measurement of Pao2at 37° with a Triple Electrode System

Abstract

Listen

Translate article

A disadvantage of transcutaneous measurement is the necessity of heating the skin to a temperature of approximately 44dC. This renders application of a single electrode at a single site impossible over a prolonged period of more than approx. 2 h without risking burns. This problem has generally been circumvented by changing the position of a single electrode from time to time. The drawback of this procedure is (besides the additional work which may be a real problem for routine application in the hospital) that skin thickness and skin metabolism may vary considerably between two places of application (except, maybe, in newborns). The heated area may be well limited, and construction simplified by directly heating the cathode. This enables incorporation of several electrode systems (cathode and anode) into a single transducer. In such a manner intermittent operation is possible without changing the position of the measuring device and with measuring sites very close to each other. In addition such an arrangement allows utilization of the fact that hyperaemization remains for a certain time after discontinuation of heating the skin. It is well known that this effect decreases when heating is repeated intermittently; however, we found that it can be improved considerably by initiating an intermittent heating cycle with a heating period of approximately 15 min at 44dC. Three electrode systems with ring-shaped cathodes have therefore been combined in a single measuring electrode with a diameter of 18 mm without and 28 mm with a membrane holding ring. The design of the single cell is based on a principle described earlier (Kimmich & Kreuzer, 1969). The heat necessary for hyperaemization is produced by passing a current from one edge of the cathode, a 3 μm thick platinum foil, to the other and led to the skin surface by a metal heat-conducting cone. Such an arrangement has well-defined heating zones and small temperature differences between thermistors and skin surface. The heating cycle (preceded by a 15 min warming up to 44dC) consists of a short heating period at 44dC, a measuring period at 37dC, and a recovery period at 31dC, and is shifted by 120d from electrode system to electrode system, so that the Po2 may be measured continuously at 37dC by intermittently sampling one of the electrodes. When calibrated according to the formula tcPo2 = Pao2 - PQo2 where PQo2 is the Po2 drop across the living layer of the epidermis due to cell metabolism and assumed constant, the oxygen electrode should theoretically measure absolute Pao2 as long as peripheral blood flow is such that P-co2 = Pao2.