Ventricular preload alters intravascular and extravascular resistances of coronary collaterals.

Coronary collateral blood flow is determined by both the collateral vessel resistance and a waterfall mechanism. The aim of this study was to determine which of these two mechanisms predominates during alteration of ventricular preload. The left anterior descending coronary artery of 12 anesthetized dogs was cannulated, and the distal vasculature was completely embolized with 25-micron diameter microspheres. Retrograde blood flow (RBF) was collected when the cannula was opened to the atmosphere, and the outflow tubing height was adjusted to provide a variable back pressure. RBF is back pressure-dependent at higher back pressures, and the slope in this region of the constructed pressure-flow relationship determines the collateral conductance. The transition point between the back pressure-dependent and a back pressure-independent region indicates a waterfall pressure impinging on the collateral vessels. At a left ventricular diastolic pressure of 9.3 mmHg, mean RBF, collateral conductance, and the collateral waterfall pressure were 7.3 ml/min, 0.175 ml.min-1.mmHg-1, and 30.1 mmHg, respectively. Corresponding values when preload was reduced to 3.5 mmHg were 9.3 ml/min, 0.186 ml.min-1.mmHg-1, and 23.7 mmHg, all changes being significant. Mean contribution to the overall increase in RBF was 0.5 ml/min for the conductance and 1.2 ml/min for the waterfall mechanism (P less than 0.05), or 29 and 71%, respectively. The results indicate that the extravascular resistance mechanism mediates the collateral flow response to a greater degree than the intravascular resistance during variations in preload. The increase in slope of the conductance portion of the relationship was not accompanied by a concomitant increase in slope of the back pressure-independent region. These data further support a collateral waterfall, and not collateral vessel compliance, as the basis for the back pressure-independent portion of the pressure-flow relationship.