Alveolar Oxygen And Carbon Dioxide Tensions During Pregnancy, Measured With A Novel Technique\

Abstract

Alveolar O2 and CO2 tensions (PAO2 and PACO2) and alveolar-to-arterial oxygen difference (P(A-a)O2) during pregnancy have been calculated with mathematical formulas that require arterial blood taking and are based on the assumption that alveolar and arterial CO2 tensions are equal (P(A-a)CO2=0). The aim of this study was to investigate the changes of alveolar gas tensions during pregnancy, using a novel non-invasive technique. PAO2, P(A-a)O2, PACO2 and P(A-a)CO2 were measured in 10 healthy volunteers during the 3rd trimester of pregnancy and when not pregnant. During pregnancy, the PAO2, P(A-a)O2, PACO2 and P(Aa)CO2 values (in mmHg with SD) were: 101.67 (8.931), 0.78 (7.848), 26.05 (4.670), -4.86 (3.257). When not pregnant the respective values were: 95.75 (8.976), 3.56 (6.047), 35.28 (1.618), -2.63 (1.532). P(A-a)CO2 was different to zero and its absolute value was increased during pregnancy (p=0.01). We detected a change in P(A-a)CO2 during pregnancy suggesting an increase of the shunt, possibly caused by the elevation of the diaphragm and the collapse of alveoli at the lung bases. The changes in PAO2 and PACO2 can be attributed to the hyperventilation of pregnancy and are comparable to the literature.