VENTILATORY CONTROL IN EXERCISE

Abstract

The control of the exercise hyperpnoea in humans remains to be convincingly resolved. Thus, while close matching of ventilation (VE) to pulmonary CO2 clearance (VCO2) ensures stability of arterial PCO2 (PaCO2), there is a lack of consensus regarding the complement of reflex and centrogenic drives that normally effects this regulation. Some investigators have proposed that respiratory memory (longterm modulation) might be involved in the control process (e.g. Martin & Mitchell J Physiol 470:601–17, 1993), while others find no evidence of this (e.g. Moosavi et al J Appl Physiol 92:1159–68, 2002). PURPOSE To examine the characteristics of the exercise hyperpnoea in subjects with essentially no established exercise memory. METHODS Nine healthy habitually-inactive adults (NEx) who had never ridden a cycle, and 9 age-, sex-, and activity-matched controls (C) who regularly had, performed constant-load cycle-ergometry (6 min) at 3 consecutive work rates (WRs) (10, 40, 70W), subsequently confirmed to be below the lactate threshold. No exercise familiarisation was given. Pulmonary gas exchange and ventilatory variables were measured breath-by-breath (mass spectrometry, turbinometry). Mean alveolar PCO2 (PACO2), a noninvasive estimator of PaCOa, was estimated from reconstruction of the intra-breath PACO2 profile (DuBois et al J Appl Physiol 4:535–48, 1952). RESULTS The responses of the NEx group were not significantly different from those of C. Across each exercise transition (i.e. 10W–40W; 40W–70W), VE was linearly related to VCO2, with a slope averaging 26.1 ± 2.6 and 26.8 ± 2.5, respectively, for the NEx group, and 25.1 ± 3.1 and 25.8 ± 3.3, respectively, for C. Steady-state mean PACO2 was unchanged across WRs (NEx: 10W = 35.3 ± 2.6 mmHg (SD); 40W = 37.2 ± 2.5 mmHg; 70W = 37.3 ± 3.5 mmHg; C: 10W = 35.3 ± 2.7 mmHg; 40W = 37.7 ± 3.6mmHg; 70W = 37.7 ± 4.0 mmHg. CONCLUSION These findings provide no support for a learned exercise response being obligatory for ‘normal’ ventilatory control during moderate-intensity cycle-ergometer exercise.