Development of respiratory muscle contractile fatigue in the course of hyperpnoea.

To assess the development of inspiratory and expiratory muscle fatigue during normocapnic hyperpnoea, we studied fourteen healthy men performing 8min hyperpnoea, 6min pause, 8min hyperpnoea, etc., until task failure. Twitch transdiaphragmatic (P(di,tw)) and gastric (P(ga,tw)) pressures were measured during cervical and thoracic magnetic nerve stimulation, before hyperpnoea, after every 8min of hyperpnoea, and at task failure (i.e., at 25.3+/-4.7min). P(di,tw) decreased during the first 16min (-28+/-7%, p<0.001) and P(ga,tw) during the first 8min (-20+/-7%, p<0.001) of hyperpnoea without further change until task failure. During inspiration, the pressure-time-product of oesophageal pressure (PTP(oes)) increased relative to PTP(di) during the first 16min (+11+/-21%, p<0.05). Similarly, during expiration, PTP(oes) increased relative to PTP(ga) during the first 8min (+10+/-16%, p<0.05). Also, blood lactate concentration and respiratory sensations significantly increased during the first 8min (+1.0+/-0.5mmoll(-1), p<0.001) and 16min (breathlessness +1.6+/-1.8 points, respiratory effort +5.9+/-2.2points, p<0.001), respectively. We conclude that, during hyperpnoea, contractile fatigue of the diaphragm and abdominal muscles develops long before task failure and may trigger an increased recruitment of rib cage muscles.