A novel method for determining ventilatory and gas exchange dynamics during exercise: the “chirp” waveform

Quantitating exercise ventilatory and gas exchange dynamics affords insights into physiological control processes and cardiopulmonary dysfunction. We designed a novel waveform, the chirp waveform, to efficiently extract moderate-intensity exercise response dynamics. In the chirp waveform, work rate fluctuates sinusoidally with constant amplitude as sinusoidal period decreases progressively from ∼8.5 to 1.4 min over 30 min of cycle ergometry. We hypothesized that response dynamics of pulmonary ventilation (V̇e) and gas exchange [oxygen uptake (V̇o₂) and carbon dioxide output (V̇co₂)] extracted from chirp waveform are similar to those obtained from stepwise transitions. Thirty-one participants [14 young healthy, 7 older healthy, and 10 patients with chronic obstructive pulmonary disease (COPD)] exercised on three occasions. Participants first performed ramp-incremental exercise for gas exchange threshold (GET) determination. In randomized order, the next two visits involved either chirp or stepwise waveforms. Work rate amplitude (20 W to ∼95% GET work rate) and exercise duration (30 min) were the same for both waveforms. A first-order linear transfer function with a single system gain (<i>G</i>) and time constant (τ) characterized response dynamics. Agreement between model parameters extracted from chirp and stepwise waveforms was established using Bland-Altman analysis and Rothery's concordance coefficient (RCC). V̇e, V̇o₂, and V̇co₂ <i>G</i>s showed no systematic bias (<i>P</i> > 0.178) and moderate-to-good agreement (RCC > 0.772, <i>P</i> < 0.01) between waveforms. Similarly, no systematic bias (<i>P</i> = 0.815) and good agreement (RCC = 0.837, <i>P</i> < 0.001) was found for τV̇o₂. Despite moderate agreement for τV̇co₂ (RCC = 0.794, <i>P</i> < 0.001) and τV̇e (RCC = 0.722, <i>P</i> = 0.083), chirp τ was less [-6.9(11.7) s and -12.2(22.5) s, respectively]. We conclude that the chirp waveform is a promising method for measuring exercise response dynamics and investigating physiological control mechanisms.<b>NEW & NOTEWORTHY</b> We investigated the ability of a novel waveform to extract exercise ventilatory and gas exchange dynamics. In the chirp waveform, work rate fluctuates sinusoidally with constant amplitude as sinusoidal period decreases progressively over 30 min of exercise. In a study of 31 healthy individuals and patients with COPD, comparison of exercise dynamics derived from chirp to those from stepwise waveforms suggests that the chirp waveform is a promising method for derivation of exercise response dynamics.