Myocardial tissue recruitment in the dog as determined by double tracer dilution method

The amount of tissue perfused, as determined from the difference in volume of distribution between a diffusible indicator (¹²⁵I antipyrine) and an intravascular indicator (¹³¹I albumin) was measured at different values of coronary flow, perfusion pressure, and vasomotor tone in the working left ventricle of an open chest dog. Coronary pressure and flow were regulated independently from the systemic circulation and coronary vasomotor tone was reduced by dipyridamole. At each flow vasomotor tone was assessed by using as a reference the maximal vasodilation induced by arrest of flow. Measured tissue space was considered to be related to the capillary surface area available for tracer diffusion and therefore to the number of perfused capillaries per volume of muscle. A relationship between coronary blood flow and tissue volume was observed. It was found to be independent of vasomotor tone. Vasodilation was found to increase available exchanging capillary surface at a constant perfusion pressure.