A comparison of the end-tidal CO 2 measured by portable capnometer and the arterial P CO 2 in spontaneously breathing patients

Abstract

An end-tidal CO 2 (ETCO 2) monitor (capnometer) is used most often as a noninvasive substitute for P aCO 2 in anesthesia, anesthetic recovery and intensive care. However, the utility and accuracy of the portable capnometer in spontaneously breathing patients with or without chronic pulmonary diseases has received little recognition. To determine the utility of the portable capnometer in general wards and in in-home care, we examined the correlation between ETCO 2 measured by a portable capnometer and simultaneous P aCO 2 measured in 41 spontaneously breathing patients. TV-ETCO 2 (ETCO 2 measured by tidal volume maneuver) was lower than P aCO 2 by an average of 9.0 mmHg and VC-ETCO 2 (ETCO 2 measured by vital capacity maneuver) was lower than P aCO 2 by an average of 0.5 mmHg. The mean difference between P aCO 2 and VC-ETCO 2 was not statistically significant. Regression analysis showed a close correlation between VC-ETCO 2 and P aCO 2 ( r = 0.91, P < 0.0001). Thus, VC-ETCO 2 was highly correlated with P aCO 2. Furthermore, a close correlation between VC-ETCO 2 and P aCO 2 was also observed in patients with compromised pulmonary function ( r = 0.88, P < 0.0001 in patients with below 70% of FEV 1.0%; r = 0.89, P < 0.0001 in patients with below 80% of %VC). Our studies show that VC-ETCO 2 measured by the portable capnometer gives a reliable point estimate of P aCO 2, and can be useful to evaluate the respiratory condition of spontaneously breathing patients in general wards and in in-home care.