Volume acceleration as an index of respiratory drive during exercise

In order to evaluate the applicability of volume acceleration (A(I)) at the onset of inspiration as an index of neuromuscular output, CO(2) rebreathing in six healthy subjects and incremental-load exercise in eight healthy subjects was performed while measuring A(I) and mouth occlusion pressure (P(0.1)). During CO(2) rebreathing, A(I) increased linearly with end-tidal CO(2) partial pressure and P(0.1). During incremental-load exercise, P(0.1) and A(I) increased exponentially with minute ventilation and mean inspiratory flow, and A(I) increased linearly with P(0.1). Dyspnoea sensation at rest and exercise with or without the circuit system in eight healthy subjects was examined. Dyspnoea sensation increased markedly with the circuit system in some subjects. Incremental-load exercise was carried out by 13 healthy subjects and 21 patients with chronic obstructive pulmonary disease (COPD) to evaluate the difference in A(I) as respiratory drive between the two groups in the absence of a respiratory circuit. In patients with COPD, A(I) responses to minute ventilation, mean inspiratory flow and carbon dioxide output (VCO(2)) were greater than those in healthy subjects. In patients with COPD, the A(I) response to VCO(2) was greater in those with a lower FEV(1.0) (forced expiratory volume in 1.0 s), but the ventilatory response to VCO(2) was lower in those with a lower FEV(1. 0). These data suggest that A(I) reflects neuromuscular output during CO(2) rebreathing and incremental-load exercise under conditions where mechanical properties of the respiratory system are expected to be involved. During exercise, flow increased markedly, and the influence of the resistance of the respiratory circuit also increased. Therefore the use of A(I) has the advantage of less resistance (no respiratory circuit) and less additional respiratory effort, in comparison with the use of P(0.1), especially in patients with COPD.