Fatigue of inspiratory muscles and their synergic behavior

Abstract

The time (tlim) required to produce inspiratory muscle fatigue was measured in five normal subjects breathing at functional residual capacity (FRC) against a 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.m/min (Wcrit). In three of those subjects the same experiment was conducted at an end-expiratory volume of FRC + one-half inspiratory capacity (1/2IC). 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 + 1/2IC (Pm'max), tlim was shorter than FRC. Pmcrit decreased to 30% Pm'max and Wcrit to 2.6 kg.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.