In order to evaluate the applicability of volume acceleration (AI) at the onset of inspiration as an index of neuromuscular output, CO2 rebreathing in six healthy subjects and incremental-load exercise in eight healthy subjects was performed while measuring AI and mouth occlusion pressure (P0.1). During CO2 rebreathing, AI increased linearly with end-tidal CO2 partial pressure and P0.1. During incremental-load exercise, P0.1 and AI increased exponentially with minute ventilation and mean inspiratory flow, and AI increased linearly with P0.1. 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 AI as respiratory drive between the two groups in the absence of a respiratory circuit. In patients with COPD, AI responses to minute ventilation, mean inspiratory flow and carbon dioxide output (co2) were greater than those in healthy subjects. In patients with COPD, the AI response to co2 was greater in those with a lower FEV1.0 (forced expiratory volume in 1.0 s), but the ventilatory response to co2 was lower in those with a lower FEV1.0. These data suggest that AI reflects neuromuscular output during CO2 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 AI has the advantage of less resistance (no respiratory circuit) and less additional respiratory effort, in comparison with the use of P0.1, especially in patients with COPD.

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