The effects of stretch on coronary pressure-flow relations are not well understood. To examine the role of wall stretch per se on coronary hemodynamics, we studied arterially perfused isolated canine interventricular septa in a noncontracting state with vasodilated vessels. We compared the hemodynamic parameters of zero-flow pressure and resistance during passive stretching in the circumferential and the base-to-apex directions alone as well as during simultaneous biaxial stretching in both directions. Even in the unloaded state the zero-flow pressure was positive. Any type of stretching significantly increased the zero-flow pressure and the resistance from their unloaded values. The pressure-flow responses also showed directional dependence. When stretches with matched strains or stresses in each direction were applied sequentially, the resistance increases corresponded to the direction of higher stress. Conversely, the zero-flow pressure response increase corresponded to the direction of greater strain. However, neither response correlated with a measure of global tissue stiffness. Thus there is a complex and tight mechanical interaction between the vessels and the surrounding tissue. These interactions, but not the tissue stiffness, are important determinants of coronary pressure-flow responses during stretch.
A small but clearly discernible portion of the shift in tetanised pressure-flow relationships is attributable to the Gregg effect. Similar conclusions pertained when quadratic regressions were fitted to the pressure-flow data.
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