End-tidal CO2 responses and ventilatory correlates during voluntary hyperventilation: a repeated-measures pilot study

Background Controlled voluntary hyperventilation is widely used to study the ventilatory control of end-tidal CO 2 (PetCO 2 ), but the breath-by-breath determinants of PetCO 2 during an externally paced maneuver remain incompletely characterized. This repeated-measures pilot study aimed to describe PetCO 2 changes and to quantify associations with ventilatory timing and expired gas output during paced voluntary hyperventilation. Methods Thirty healthy males (age range 19.2–39.8) performed a metronome-paced voluntary hyperventilation maneuver at 30 breaths/min intended to last up to 3 min. Breath-by-breath data were recorded with a COSMED K5 (COSMED, Rome, Italy). After excluding breaths with implausible respiratory frequency, 2,298 breaths from 30 participants were analyzed with a linear mixed model (LMM) using a cubic B-spline for elapsed time, participant-specific random intercepts and random slopes, and fixed effects of minute ventilation (VE), expired CO 2 output (VCO 2 ), and expiratory time (Te). A generalized estimating equation (GEE) with AR(1) working correlation was a sensitivity analysis, and participant-level cluster-bootstrap CIs provided the primary inferential intervals. Results Participant-level mean PetCO 2 during the maneuver was 17.1 ± 1.8 mmHg (range 13.3–21.2 mmHg) and declined over time. In the primary LMM, VE β = −0.144, VCO 2 β = 0.009, and Te β = 4.18. Because residual diagnostics showed non-normality, heteroscedasticity, and strong serial autocorrelation (GEE AR(1) alpha approximately 0.79 for the spline model and 0.86 for the linear-time model), cluster-bootstrap CIs are reported as the primary inferential intervals: VE −0.174 to −0.110, VCO2 0.005 to 0.012, Te 2.330 to 6.164, and a predicted PetCO2 change from 0 to 180 s of −8.58 mmHg (95% CI −10.30 to −6.92 mmHg). Marginal R 2 = 0.775 and conditional R 2 = 0.828; leave-one-participant-out cross-validation yielded R 2 = 0.737 and RMSE = 2.52 mmHg. Conclusions PetCO 2 during paced hyperventilation was associated with elapsed time, VE, VCO 2 , and Te. Because VE and VCO 2 are algebraically coupled, and VCO 2 and PetCO 2 also share information from the expired-CO 2 signal, they should not be interpreted as independent physiological determinants; the principal contribution is the within-participant expiratory-time association. The findings are exploratory and require confirmation in clinical populations.