FATIGUE OF INSPIRATORY MUSCLES AND THEIR SYNERGIC BEHAVIOR

The time (tlim) required to produce inspiratory muscle fatigue was measured in 5 normal subjects breathing at functional residual capacity (FRC) against a wide variety of high inspiratory resistive loads. In every breathing test the subjects generated with each inspiration a mouth pressure (Pm) that was a predetermined fraction of maximum Pm (Pmmax). They continued breathing until they were unable to generate this Pm. The Pm/Pmmax that could be generated indefinitely (Pmcrit) was around 60%. The inspiratory power output at that level of breathing was 6.6 kg x min (Wcrit). In three of those subjects the same experiment was conducted at an end-expiratory volume of FRC + one-half inspiratory capacity (1/2 IC). The higher lung volume was actively maintained by the subjects watching end-expiratory transpulmonary pressure on an oscilloscope. For any fraction of the maximum mouth pressure at FRC + 1/2 IC (Pm'max), tlim was shorter than FRC. Pmcrit decreased to 30% Pm'max and Wcrit to 2.6 kg x m/min. Monitoring the abdominal pressure revealed that the contribution of the diaphragm and intercostal accessory muscles alternated in time, possibly postponing the onset of fatigue.