Measuring the efficiency of pulmonary gas exchange using expired gas instead of arterial blood: comparing the "ideal" Po of Riley with end-tidal Po.

When using a new noninvasive method for measuring the efficiency of pulmonary gas exchange, a key measurement is the oxygen deficit, defined as the difference between the end-tidal alveolar Po and the calculated arterial Po. The end-tidal Po is measured using a rapid gas analyzer, and the arterial Po is derived from pulse oximetry after allowing for the effect of the Pco on the oxygen affinity of hemoglobin. In the present report we show that the values of end-tidal Po and Pco are highly reproducible, providing a solid foundation for the measurement of the oxygen deficit. We compare the oxygen deficit with the classical ideal alveolar-arterial Po difference (A-aDO) as originally proposed by Riley, and now extensively used in clinical practice. This assumes Riley's criteria for ideal alveolar gas, namely no ventilation-perfusion inequality, the same Pco as arterial blood, and the same respiratory exchange ratio as the whole lung. It transpires that, in normal subjects, the end-tidal Po is essentially the same as the ideal value. This conclusion is consistent with the very small oxygen deficit that we have reported in young normal subjects, the significantly higher values seen in older normal subjects, and the much larger values in patients with lung disease. We conclude that this noninvasive measurement of the efficiency of pulmonary exchange is identical in many respects to that based on the ideal alveolar Po, but that it is easier to obtain.