Transcutaneous carbon dioxide monitoring

We studied changes in transcutaneous PCO2 in healthy volunteers and patients to establish the accuracy and practicability of a new cutaneous capnometer (HP47201A). In 9 healthy volunteers, as many as 4 experiments each were performed during each laboratory session. After cutaneous electrode stabilization, a gas mixture of CO2 in O2 was rebreathed under "open-loop" conditions allowing a rapid progressive increase in alveolar PCO2. The electrode stabilized on the skin with a time constant of 2.87 +/- 0.72 min. During rebreathing, alveolar PCO2 increased 4.25 +/- 0.63mm Hg/min. The simultaneous rate of increase in transcutaneous PCO2 was 2.91 +/- 0.59 mm Hg of CO2/min. Electrode response was also studied during steady-state conditions after a step increase in alveolar PCO2 of 10 mm Hg more than the resting end-tidal PCO2. The time constant of the skin electrode's response under these conditions was 3.2 +/- 1.9 min. We compared the arterial PCO2 of 25 blood samples with simultaneous transcutaneous PCO2 measurements in hemodynamically and metabolically stable patients. The range of arterial PCO2 values varied between 24 and 98 mm Hg, and the electrode remained stable during each 12-h study. Observed values closely approximated arterial PCO2 measurements, with transcutaneous readings 4 mm Hg more than measured arterial values (r = 0.98). We conclude that the cutaneous capnometer accurately and consistently reflects arterial PCO2 and has delay and response characteristics that may allow its use in clinical monitoring of patients.