Effect of alterations in muscle fiber length on diaphragm blood flow

A variety of studies have examined the responses of diaphragmatic blood flow (Qdi) to rhythmic pleiometric (i.e., shortening) and isometric contractions. The effect of changes in diaphragm fiber length on Qdi are, however, unknown. The present study examined the effect of changes in diaphragm fiber length on Qdi and the effect of changes alterations in length on the response of Qdi to increases in diaphragm contractile activity. Studies were performed on 21 anesthetized mechanically ventilated dogs in which a strip of costal diaphragm was developed in situ. The strip was immobilized in a rigid metal frame that permitted precise adjustment of muscle length. Strip blood flow was assessed with a drop counter attached to a catheter in the branch of the phrenic vein draining the strip. Strips were electrically stimulated via intramuscular electrodes, and the isometric tension developed was measured with a force transducer. Fiber length was expressed as a percentage of the length at which active isometric tension was maximum (L(o)). With the diaphragm at rest, steady-state blood flow fell by 59 ± 6% (SE) (P < 0.001) as fiber length was increased from 92 to 107% L(o). Blood flow also varied as a function of length when muscles contracted rhythmically (15 contractions/min, duty cycle 50%) to generate isometric tensions equal to 20 and 80% of maximum. As fiber length increased from 92 to 106% L(o), Qdi fell by 36 ± 6% (SE) when tension was 80% of maximum (P < 0.01) and by 38 ± 6% (SE) when tension was 20% of maximum (P < 0.05). Passive muscle tension increased with muscle lengthening and Qdi was inversely proportional to passive tension [r = 0.92 ± 0.02 (SE)]. We conclude that changes in diaphragm fiber length alter Qdi at a given level of muscle activity. We speculate that the effect of diaphragm length on Qdi is mediated, at least in part, by the mechanical effect of passive tension on vascular caliber.