Impact of periodic breathing on measurement of oxygen uptake and respiratory exchange ratio during cardiopulmonary exercise testing

Metabolic exercise testing is valuable in patients with chronic heart failure (CHF), but periodic breathing may confound the measurements. We aimed to examine the effects of periodic breathing on the measurement of oxygen uptake (VO2) and respiratory exchange ratio (RER). First, we measured the effects of different averaging procedures on peak VO2 and RER values in 122 patients with CHF undergoing cardiopulmonary exercise testing. Secondly, we studied the effects of periodic breathing on VO2 and RER in healthy volunteers performing computer-guided periodic breathing. Thirdly, we used a Fourier analysis to study the effects of periodic breathing on gas exchange measurements. The first part of the study showed that 1min moving window gave a mean peak VO2 of 13.8mlμmin-1μkg-1 for the CHF patients. A 15s window gave significantly higher values. The difference averaged 1.0mlμmin-1μkg-1 (P<0.0001), but varied widely: 41% of subjects showed a difference greater than 1.0mlμmin-1μkg-1. RER values were also higher by an average of 0.09 (P<0.0001); in 20% of subjects the difference was greater than 0.10. In the second part of the study, we found artefactual elevations of peak VO2 (without averaging) of 2.9mlμmin-1μkg-1 (P<0.01) and of peak RER of 0.13 (P<0.001), which were still significant when 30s averaging was applied [Δ(peak VO2) = 1.8mlμmin-1μkg-1, P<0.01; ΔRER = 0.08, P<0.001]. The third, theoretical, part of the study showed that values of carbon dioxide output and VO2 oscillate with different phases and amplitudes, resulting in oscillations in their ratio, RER. Averaging over 15s or 30s can be expected to give only 10% or 36% attenuation respectively. Thus periodic breathing causes variable artefactual elevations of measured peak VO2 and RER, which can be attenuated by using longer averaging periods. Clinical reports and research publications describing peak VO2 in CHF should be accompanied by details of the averaging technique used.