Intramyocardial Blood Volume

We have measured the effects of barium-induced cardiac contraction on the intramyocardial blood volume content of all vessels and have independently measured the blood volume content of vessels with a diameter greater than 100 ,um in rat myocardium. Measurements of total intramyocardial blood volume were made by using [1251]albumin as a plasma marker and technetium-99m as a red blood cell marker. In one group of rats (n=8), diastolic arrest was induced by an intravenous injection of KCI; in a second group (n=8), systolic arrest was induced by an intravenous injection of BaCI2. In both groups, the hearts were frozen in situ immediately after heart arrest while aortic pressure was decaying from its former physiological level. The left ventricular free wall was sectioned transmurally in a cryomicrotome, and the blood volume within each tissue sample was calculated from its radioactivity. The volume of blood in vessels larger than 100 ,um was independently estimated from the exposed crosssectional area of these vessels in photographs of the frozen tissue taken during tissue sectioning in the cryomicrotome. Total intramyocardial blood volume was found to decrease by about 42% from 8.6+1.3 ml/100 g (mean±SEM) in the KCI group to 5.0±0.7 ml/100 g in the BaCI2 group (p<0.01). Total volume was greater in the epicardial than in the endocardial layers of both groups (p<0.05). The volume of blood contained in vessels larger than 100 ,um decreased from 1.2+0.3 ml/100 g in the KCI group to 0.3±0.1 ml/100 g in the BaCI2 group (p<0.05). The volume of blood contained in vessels smaller than 100 ,im, calculated by subtracting large-vessel volume from total volume, decreased from 7.4 ml/100 g in the KCI group to 4.6 ml/100 g in the BaCI2 group. These results show that barium-induced cardiac contraction strongly affects the volume of blood in the smaller coronary blood vessels. Although differences exist between barium contraction and a normal systolic contraction, the results suggest that during the normal cardiac cycle the small vessels play an important role in the determination of phasic coronary blood flow. (Circulation Research 1991;68:217-225)