Relationship of changes in diaphragmatic muscle blood flow to muscle contractile activity

The effect of increases in diaphragmatic muscle contractile activity on diaphragm blood flow remains unclear. The present study examined the effect of electrically induced isometric diaphragmatic muscle contractions on diaphragmatic blood flow. Studies were performed on diaphragmatic muscle strips prepared in anesthetized mechanically ventilated dogs. Diaphragmatic contractile activity was quantitated as the tension-time index (TTI) (i.e., the product of tension magnitude and duration). Blood flow to the strip (Q̇di) was measured from the volume of the phrenic venous effluent using a drop counter. The separate effects on Q̇di of 30-s periods of continuous and rhythmic contractions were examined. Q̇di increased with increases in TTI and peaked at a TTI of 20-30% of maximum after which Q̇di fell progressively with further increases in TTI. At levels of TTI > 30%, the pattern of muscle contraction significantly affected blood flow. Q̇di was significantly lower during activity and the postcontraction hyperemia significantly greater at a given TTI when contractions were continuous than when contractions were intermittent. Above a TTI of 30%, Q̇di during contraction decreased linearly with increases in duty cycle and curvilinearly with increases in tension. We conclude that during isometric diaphragmatic contractions, diaphragmatic blood flow may become mechanically impeded, and the magnitude of the impediment in blood flow depends on the pattern of diaphragmatic contractions. With increases in contractile activity above a critical level, changes in duty cycle exert progressively greater effects on diaphragmatic blood flow than changes in muscle tension.