Comparison of Transcutaneous and Capillary Measurement of PCO2 in Hypercapnic Subjects.

BACKGROUND Measurement of PCO2 is vital in determining effective alveolar ventilation. However, obtaining capillary PCO2 by a skin prick of the earlobe is painful, and nocturnal measurements disturb sleep. End-expiratory measurement of PCO2 is also well established, but there is a low precision in predicting arterial or capillary CO2. The purpose of the study was to evaluate nocturnal measurement of noninvasive, transcutaneous PCO2 (PtcCO2 ) measurement in hypercapnic subjects. METHODS In this prospective study, 31 subjects with chronic hypercapnic failure--in a stable phase of the underlying disease--and a control group of 12 healthy volunteers were included. Transcutaneous measurements were taken by the Tosca sensor (Radiometer, Copenhagen, Denmark) over a period of at least 6 h during the night. A capillary blood gas was measured at midnight and 4:00 am. RESULTS The mean nocturnal capillary PCO2 (PcapCO2 ) of subjects was 50.6 ± 10.2 mm Hg. In the 31 subjects with known hypercapnic respiratory failure, the correlation between PtcCO2 and PcapCO2 at midnight was 0.86 and at 4:00 am r = 0.80. The bias of the hypercapnic subjects was d = + 4.5 with a limit(s) of agreement of 2 SD = 13.0. The process of blood sampling caused no significant change in PtcCO2 . CONCLUSIONS Our study evaluated transcutaneous capnography as a continuous nocturnal measurement in hypercapnic subjects. We found a good agreement between the methods. Because CO2 is not constant in patients with respiratory failure, but instead fluctuates, we would recommend the continuous transcutaneous measurement of PCO2 as our method of choice in the diagnosis of nocturnal hypercapnia.