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

PURPOSE: This study aimed to compare Q˙peak elicited by a constant load protocol ( Q˙CL ) and an incremental step protocol ( Q˙step ). METHODS: A noninferiority randomized crossover trial was used to compare Q˙peak between protocols using a noninferiority margin of 0.5 L·min -1 . Participants ( n = 34 (19 female, 15 male); 25 ± 5 yr) performed two baseline V̇O 2peak tests to determine peak heart rate (HR peak ) and peak work rate ( Wpeak ). Participants then performed the Q˙CL and Q˙step protocols each on two separate occasions with the order of the four visits randomized. Q˙peak was measured using IGR (Innocor; COSMED, Rome, Italy). The Q˙CL protocol involved a V̇O 2peak test followed 10 min later by cycling at 90% Wpeak , with IGR initiated after 2 min. Q˙step involved an incremental step test with IGR initiated when the participant's HR reached 5 bpm below their HR peak . The first Q˙CL and Q˙step tests were compared for noninferiority, and the second series of tests was used to measure repeatability (typical error (TE)). RESULTS: The Q˙CL protocol was noninferior to Q˙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 V̇O 2peak (3.13 ± 0.83 L·min -1 ) was achieved during Q˙CL (3.12 ± 0.72, P = 0.87) and Q˙step (3.12 ± 0.80, P = 0.82). The TE values for Q˙peak were 6.6% and 8.3% for Q˙CL and Q˙step , respectively. CONCLUSIONS: The Q˙CL protocol was noninferior to Q˙step and may be more convenient because of the reduced time commitment to perform the measurement.