Non-invasive measurement of the mean alveolar O2 tension from the oxygen uptake versus tidal volume curve
The classical equations for measuring the mean and the ideal alveolar O2 tension are based on assumptions, which are shown to be invalid. So we thought to develop a new, non-invasive method for measuring the mean alveolar P,O2 within the volume domain (PA,O2(Bohr)). This method is based on the oxygen uptake vs. tidal volume curve (VO2 vs. VT) obtained during tidal breathing of room air and/or air enriched with oxygen. Methods:
PA,O2(Bohr) and the ideal alveolar PO2 (PA,O2(ideal)) were simultaneously measured in 10 healthy subjects and 34 patients suffering from chronic obstructive pulmonary disease (COPD) breathing tidally room air at rest. Additionally, 10 subjects (three healthy subjects and seven COPD patients) were studied while breathing initially room air and subsequently air enriched with oxygen. Results:
According to the results, PA,O2(Bohr) considerably differed from PA,O2(ideal) (P = 0.004). The cause of the difference, at the individual's R, is: (1) the difference between the arterial and Bohr's alveolar CO2 tension, mainly in COPD patients, and (2) the inequality between Bohr's alveolar part of the tidal volume for CO2 and O2. Furthermore, end-tidal gas tension (PET,CO2 and PET,O2) differed from Pa,CO2 and PA,O2(Bohr) respectively. Conclusion:
The deviation of PA,O2(Bohr) from PA,O2(ideal) has a definite impact on Bohr's dead space ratio for O2 and CO2, and on the alveolar–arterial O2 difference. The difference (PA,O2(Bohr) − PA,O2(ideal)) is not related to the pathology of the disease. So, gas exchange within the lungs should be assessed at the subject's R from PA,O2(Bohr) and PA,CO2(Bohr) but not from PA,O2(ideal) nor Pa,CO2.