Alveolar partial pressures of carbon dioxide and oxygen measured by a helium washout technique

Abstract

A non-invasive technique was developed for measuring alveolar carbon dioxide and oxygen tension during tidal breathing. This was achieved by solving the Bohr equations for mean alveolar carbon dioxide and oxygen tensions (P̄ACO2, P̄AO2) from known values of the dead-space:tidal volume ratio measured by helium washout, and from the mixed expired partial pressure of carbon dioxide and oxygen. The derived values of wP̄ACO2 and wP̄AO2 were compared with PaCO2 obtained from arterial gas analysis and P̄AO2 calculated from the ideal air equation. Four normal subjects and 58 patients were studied. Calculated and measured PCO2 values agreed closely with a difference in mean values (wP̄ACO2-PaCO2) of 0.01 kPa; the SD of the differences was 0.7 kPa. The difference in mean values between wP̄AO2 and P̄ AO2 was 0.02 kPa; the SD of the differences was 0.93 kPa. The method is simple and not time consuming, and requires no special cooperation from the patients. It can be applied in the laboratory or at the bedside to any subject breathing tidally. Physiological deadspace:tidal volume ratio, P̄AO2 and P̄ACO2, static lung volumes, respiratory exchange ratio, carbon dioxide production, oxygen uptake, tidal volume, and total ventilation can be measured with acceptable ccuracy and reproducibility in one test. An arterial blood sample is needed initially to provide an independent measure of PaCO2 and for measurement of the alveolar-arterial PO2 difference. Subsequently, PaCO2 can be estimated from wP̄ACO2 sufficiently well for clinical purposes and PaO2 or SaO2 can be monitored by non-invasive methods.