Respiratory muscle function during hypercapnic and isocapnic hyperpnea

To investigate the relative patterns of respiratory timing, respiratory muscle recruitment and diaphragm contractility during progressive hypercapnic and voluntary isocapnic hyperpnea we measured respired volumes, rib cage (Vrc) and abdominal (Vab) volumes, end-tidal PCO (PsrCOz), and pleural (Ppl), abdominal (Pab) and transdiaphragmatic pressures (Pdi (= Pab - Ppl)) in three trained subjects. Throughout, ΔPdi with 0.1ms bilateral phrenic nerve stimulation (PdiT) was measured at end-expiration and endinspiration. During isocapnic hyperpnea tidal volume (V _r) and respiratory frequency (f) were voluntarily matched to those achieved during normoxic hypercapnic hyperpnea at equivalent levels of ventilation (w) PetCOz increased from 38±2 to 61 ±1 mmHg (mean±SD) during hypercapnic hyperpnea, and was maintained at eucapnic levels (36±1 mmHg) during isocapnic hyperpnea. w progressively increased from 8±2 to 62±2 I.min ' and 10±4 to 63±3 I.min ^-1 during hyper- and isocapnic hyperpnea, respectively. Despite matching only Vi and / between the two conditions, at equivalent VE similar changes were also observed between inspiratory and expiratory time, and end-inspiratory and end-expiratory Vrc, Vab, Ppl, Pab, Pdi, and PdiT. PdiT measured at usual FRC before and after hypercapnic hyperpnea was also similar. These findings demonstrate equivalent respiratory muscle responses to hypercapnic and isocapnic hyperpnea, without evidence of CO ₂- rinduced depression of diaphragm contractility. This suggests that, when proprioceptive inputs (V _r and f) are matched, the output of the respiratory pattern generator is similar regardless of whether the stimulus to breathe is chemosensory or volitional in origin.