Diaphragmatic Response Is Influenced by Previous Muscle Activity

OBJECTIVE: Previous muscle activity can alter muscle contractility and lead to strength underestimation or overestimation in functional measurements. The objective of this study was to evaluate changes in the maximum pressure produced by the diaphragm after different series of spontaneous near-to-maximal isometric contractions. METHODS: Duplicate studies were performed on 6 dogs with a mean (SD) weight of 26 (7) kg. The supramaximal response of the diaphragm was achieved by simultaneous supramaximal stimulation of both phrenic nerves, both under basal conditions and after series of 5, 10, and 20 spontaneous inspiratory efforts against the occluded airway, performed before and after spinal anesthesia (which eliminates the ventilatory contribution of the intercostal muscles). The response was measured using the twitch gastric pressure (Pga) and twitch esophageal pressure (Pes) and by muscle shortening (sonomicrometry). RESULTS: The short series or 5 inspiratory efforts and, in particular, the medium series of 10 efforts produced potentiation of the contractile response, with a rise in the Pga from 3.2 (0.4) ern H2O to 3.7 (0.3) cm H2O, and from 3.5 (0.3) cut H2O to 3.9 (0.3) cm H2O, respectively (P=.05 in both eases). The potentiation was somewhat greater after subarachnoid anesthesia (an increase in the Pga of 21% after the medium series of 10 efforts with anesthesia vs 11% without anesthesia). However, the long series of 20 efforts produced a fall in the response, with a decrease in the Pga from 3.2 (0.4) cm H2O to 2.5 (0.3) cm H2O (P<.05), probably due to fatigue overcoming the effect of potentiation. CONCLUSIONS: Previous effort affects the contractile capacity of the diaphragm and it is difficult to predict the predominance of fatigue or potentiation in the response. This factor must be taken into account when determining the maximum respiratory pressures in daily clinical practice.