Peak Cardiac Output Determined Using Inert Gas Rebreathing: A Comparison of Two Exercise Protocols

Abstract

ABSTRACTPeak cardiac output (Q˙peak) can be measured noninvasively using inert gas rebreathing (IGR). There is no consensus on the optimal protocol to measureQ˙peakusing IGR, which requires a rebreathing period of ~10 s as close to “maximal” exercise as possible.PurposeThis study aimed to compareQ˙peakelicited by a constant load protocol (Q˙CL) and an incremental step protocol (Q˙step).MethodsA noninferiority randomized crossover trial was used to compareQ˙peakbetween protocols using a noninferiority margin of 0.5 L·min−1. Participants (n= 34 (19 female, 15 male); 25 ± 5 yr) performed two baseline V̇O2peaktests to determine peak heart rate (HRpeak) and peak work rate (Wpeak). Participants then performed theQ˙CLandQ˙stepprotocols each on two separate occasions with the order of the four visits randomized.Q˙peakwas measured using IGR (Innocor; COSMED, Rome, Italy). TheQ˙CLprotocol involved a V̇O2peaktest followed 10 min later by cycling at 90%Wpeak, with IGR initiated after 2 min.Q˙stepinvolved an incremental step test with IGR initiated when the participant’s HR reached 5 bpm below their HRpeak. The firstQ˙CLandQ˙steptests were compared for noninferiority, and the second series of tests was used to measure repeatability (typical error (TE)).ResultsTheQ˙CLprotocol was noninferior toQ˙step(Q˙CL= 17.1 ± 3.2,Q˙step= 16.8 ± 3.1 L·min−1; 95% confidence intervals, −0.16 to 0.72 L·min−1). The baseline V̇O2peak(3.13 ± 0.83 L·min−1) was achieved duringQ˙CL(3.12 ± 0.72,P= 0.87) andQ˙step(3.12 ± 0.80,P= 0.82). The TE values forQ˙peakwere 6.6% and 8.3% forQ˙CLandQ˙step, respectively.ConclusionsTheQ˙CLprotocol was noninferior toQ˙stepand may be more convenient because of the reduced time commitment to perform the measurement.