Flow into ischemic myocardium and across coronary collateral vessels is modulated by a waterfall mechanism.

If a coronary artery is ligated and the distal end cannulated, blood flows retrograde from the cannula when vented to the atmosphere. By varying the height of the outflow tubing, and thereby changing the outflow pressure, pressure-flow relationships can be constructed. We used this technique in eight dogs to assess the characteristics of blood flow into ischemic myocardium. Above a back pressure of 10 mm Hg, increasing back pressure resulted in a decrease of retrograde blood flow. However, below a back pressure of about 10 mm Hg (10.7 +/- 2.7 mm Hg), alterations in back pressure did not result in changes in retrograde blood flow (back pressure-independent region). The transition at 10 mm Hg is interpreted as the critical waterfall pressure in ischemic myocardium. In another group of eight dogs, the ischemic bed was completely embolized with 25-micron sized microspheres to prevent RBF from entering the tissue as back pressure was raised. Pressure-flow relationships performed in this group revealed a back pressure-independent region that extended to approximately 20 mm Hg (23.0 +/- 2.5 mm Hg). This behavior of the pressure-flow relationship is consistent with a waterfall phenomenon on the collateral vessels. To the extent that collateral vessels in the dog are mainly epicardial in location, the findings suggest that extravascular pressures of 20 mm Hg can occur in the more superficial layers of the heart. In addition, the waterfall on the collaterals indicates that this mechanism can operate on nonvenous vessels. Our results suggest separate waterfall phenomena operating on the collateral vessels (20 mm Hg) and on the vessels in the ischemic myocardium (10 mm Hg).