BACKGROUND--The measurement of effective alveolar carbon dioxide tension (PA-CO2eff) is still a matter of debate. It has, however, become common practice to use arterial instead of alveolar CO2 tension for computing alveolar oxygen tension (PAO2) and physiological dead space, not only in normal subjects but also in patients. The purpose of this study was to estimate alveolar CO2 tension during spontaneous breathing with a new bedside technique which is simple and non-invasive, and to compare these values with arterial CO2 tension measured in normal subjects and patients with chronic airways obstruction. METHODS--The subjects breathed quietly through the equipment assembly (mouthpiece, monitoring ring, Fleisch transducer head) connected to a pneumotachograph and a fast response infrared CO2 analyser. The method is a computerised calculation of the volume weighted effective alveolar CO2 tension obtained from the simultaneously recorded expiratory flow and CO2 concentration versus time curves. An arterial blood sample was taken to measure PaCO2 for comparison during the study. RESULTS--The results showed a mean difference (PACO2eff-PaCO2) of -0.205 kPa in 20 normal subjects and -0.460 kPa in 46 patients. The 95% confidence interval of the bias was -0.029 to -0.379 kPa in normal subjects and -0.213 to -0.707 kPa in patients. The limits of agreement between PACO2eff and PaCO2 were 0.526 to -0.935 in normal subjects and 1.170 to -2.088 in patients. CONCLUSIONS--The volume weighted effective alveolar PCO2 in normal subjects and patients with chronic airways obstruction is lower than the arterial PCO2 and is recommended as a better estimate in the classical equations for estimating dead space and intrapulmonary shunt.
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