Cutaneous monitoring of systemic PCO2 on patients in the respiratory intensive care unit being weaned from the ventilator.

A procedure for measuring the partial pressure of CO2 in equilibrium with the epidermis was developed at Hewlett--Packard Laboratories. It consists of determining by infra-red absorption techniques the concentration of CO2 inside a small (50 microliter) chamber applied hermetically over and around an epidermal window (2.25 cm2) stripped of its stratum corneum or horny layer. We have applied the procedure to 25 patients in the Respiratory Intensive Care Unit at Stanford Hospital. Only patients scheduled for weaning from the ventilator were selected for the study. The epidermal windows were on the medial aspect of the forearm and were monitored for 3--6 h. Arterial blood samples were periodically withdrawn from the catheterized radial artery and were analysed by conventional means for subsequent comparison with the cutaneous measurements which were recorded in real time at the rate of one a minute. The range of arterial PCO2 values that were measured varied from 3.33 to 9.30 kPa (25 to 70 mmHg) and correlated well with the corresponding cutaneous PCO2 values that were typically higher than the arterial values by 0.7 kPa (5.2 mmHg) with a standard deviation of 0.2 kPa (1.5 mmHg). Some recordings of cutaneous PCO2 are shown and discussed.