EFFECT OF FATIGUE ON MAXIMAL INSPIRATORY PRESSURE-FLOW CAPACITY

The inspiratory muscles can be fatigued by repetitive contractions characterized by high force (inspiratory resistive loads) or high velocities of shortening (hyperpnea). The effects of fatigue induced by inspiratory resistive loaded breathing (pressure tasks) or by eucapnic hyperpnea (flow tasks) on maximal inspiratory pressure-flow capacity and rib cage and diaphragm strength were examined in five healthy adult subjects. Tasks consisted of sustaining an assigned breathing frequency, duty cycle, and either a "pressure-time product" of esophageal pressure (for the pressure tasks) or peak inspiratory flow rate (for the flow tasks). Esophageal pressure was measured during maximal inspiratory efforts against a closed glottis (Pes(max)), maximal transdiaphragmatic pressure was measured during open-glottis expulsive maneuvers (Pdi(max)), and maximal inspiratory flow (Vi(max)) was measured during maximal inspiratory efforts with no added external resistance before and after fatiguing pressure and flow tasks. The reduction in Pes(max) with pressure fatigue (-25 +/- 7%) was significantly greater than the change in Pes(max) with flow fatigue (-8 +/- 8%, P < 0.01). In contrast, the reductions in Pdi(max) (-11 +/- 8%) and Vi(max) (-16 +/- 3%) with flow fatigue were greater than the changes in Pdi(max) (-0.6 +/- 4%, P < 0.05) or Vi(max) (-3 +/- 4%, P < 0.05) with pressure fatigue. We conclude that respiratory muscle performance is dependent not only on the presence of fatigue but whether fatigue was induced by pressure tasks or flow tasks. The specific impairment of Pes(max) and not of Pdi(max) or flow with pressure fatigue may reflect selective fatigue of the rib cage muscles. In contrast, flow fatigue had a more pronounced effect on diaphragm performance and consequent flow generation.