Effect of tetanic myocardial contraction on coronary pressure-flow relationships

Cardiac contraction causes a decrease in coronary flow. Despite many studies, it is still not clear what mechanism or mechanisms are responsible for this flow decrease. The phasic nature of myocardial contraction and the complexities intrinsic to intact heart preparations make it difficult to elucidate the mechanisms. We therefore studied coronary pressure-flow relationships during steady-state (tetanic) contractions in the maximally vasodilated isolated canine interventricular septum to see whether waterfall- type behavior is present. Using ryanodine and electrical stimulation allowed the production of reproducible and reversible tetani. This preparation minimizes the difficulties associated with transmural variations and also the effects of intramyocardial capacitance. Two separate protocols were performed to delineate the pressure-flow relationships in the passive and tetanized states. The first compared diastolic and tetanized pressure-flow relationships. In the second protocol, 2,3-butanedione monoxime was added to obtain an intermediate contractile level, thus allowing the comparison of two contractile states. Both the diastolic and tetanized pressure-flow relationships were curvilinear in the low-pressure range. Linear and nonlinear fits to the data showed that the primary effect of contraction was a shift of the pressure-flow relationships to higher pressures at a given flow. This effect was graded by the level of contractility and was independent of developed stress. Although other mechanisms may also be operative, these results support the presence of waterfall behavior in the coronary vascular bed.