Water vapor calibration errors in some capnometers: respiratory conventions misunderstood by manufacturers?
Incorrect calibration has been included in several recently introduced CO2 analyzers. They display a value of "PCO2" internally calculated as FCO2 X Pb rather than FCO2(Pb - 47) where Pb is barometric pressure. This conceptual error appears to have been introduced because new sampling catheter material has become available that effectively removes water vapor before samples reach the sample cell. This seems to have led some manufacturers to assume, incorrectly, that the 47 mmHg factor used to compute PCO2 in patients would no longer be needed. Users can test whether this error is present in an instrument by testing the effect of wet versus dry gases, and make appropriate corrections if the errors are present. Manufacturers should promptly correct this error in all instruments sold previously.
Severinghaus
J W
JW
Department of Anesthesia, University of California, San Francisco 94143-0542.
D002138:Calibration; D002245:Carbon Dioxide / Q000032:analysis*; D006801:Humans; D010313:Partial Pressure; D012119:Respiration; D014835:Volatilization; D014867:Water
D014867:Water; D002245:Carbon Dioxide