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

Peak cardiac output (Q˙ peak) can be measured noninvasively using inert gas rebreathing (IGR). There is no consensus on the optimal protocol to measure Q˙ peak using IGR, which requires a rebreathing period of ~10 s as close to "maximal" exercise as possible. 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 V̇O2peak tests to determine peak heart rate (HRpeak) and peak work rate (W peak). 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 V̇O2peak test followed 10 min later by cycling at 90% W peak, 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 HRpeak. 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 V̇O2peak (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. [ABSTRACT FROM AUTHOR]