Transmural differences in myocardial blood flow and in coronary dilatory capacity in hemodiluted conscious dogs.

In 16 conscious resting dogs regional myocardial blood flow and the local coronary dilatory capacity were studied with the particle distribution technique during isovolemic hemodilution (hct = 13%). Postischemic peak coronary hyperemia following release of temporary circumflex coronary artery occlusion was used for quantification of regional coronary dilatory capacity. In hemodilution (arterial blood oxygen content less than one third of normal) left ventricular blood flow (LVBF) was 460 +/- 36 ml/100 g - min, subendocardial/subepicardial flow amounted to 1.3 +/- 0.1. During postischemic peak hyperemia LVBF increased by 33% up to 606 +/- 63 ml/100 g - min. This 33% increase in LVBF was distributed mainly to the subepicardial layer, while in the subendocardial layer there was no significant flow increase. It is concluded that the increase in heart rate and systolic coronary vascular compression in addition to the lowered arterial oxygen content lead to exhaustion of the dilatory reserve in the subendocardium during hemodilution. Therefore the remaining overall dilatory capacity is without functional significance.