Comparison of physiologic dead space/tidal volume ratio and alveolar-arterial PO2 difference during incremental and constant work exercise.

We prefer 1-min incremental exercise testing on a cycle ergometer rather than constant work studies because of its speed, repeatability, and ease of identification of the anaerobic threshold. Although values such as VO2 and anaerobic threshold from both types of studies are reported to be comparable, we questioned whether VD/VT and AaPO2, which depend on simultaneous arterial blood and mixed expired gas sampling, would also be reliable during incremental testing. In 23 patients we did both a constant work test at below the anaerobic threshold and a 1-min incremental test. At matched VO2 we found no difference between VD/VT and AaPO2 despite anticipated differences in VE, VCO2, and R because of differences in their rates of increase. We conclude that VD/VT and AaPO2, determined during 1-min incremental exercise tests, are equivalent to those from constant work tests and that a separate constant work study is not needed for these measurements if determined below the anaerobic threshold.