Relation between myocardial oxygen consumption and myocardial blood volume: A study using myocardial contrast echocardiography

Myocardial blood volume (MBV) is the volume of blood residing in myocardial vessels, 90% of which is in capillaries. MBV can be measured in vivo using myocardial contrast echocardiography (MCE). It has been shown that when increases in coronary blood flow (CBF) are not associated with increase in myocardial oxygen consumption (MVO₂), MBV does not increase. We hypothesized that MBV would increase when increases in CBF are associated with an increase in MVO₂. The atrioventricular node was ablated in 18 dogs and dual-chamber pacing was instituted. In group 1 dogs (n = 9), heart rate was altered from 50 to 150 bpm^-1 in increments of 20 bpm^-1 in random order. In group 2 dogs (n = 9), heart rate was kept constant, and dobutamine was infused at doses of 5, 10, 20, 30, and 40 μg/kg^-1/min^-1. During each intervention, hemodynamic parameters and MVO₂ were measured, and MCE was performed. MVO₂ increased more (P < .01) with inotropic compared with chronotropic stimulation, resulting in a parallel increase in CBF. MBV fraction and MCE-derived myocardial blood flow increased significantly with increases in MVO₂ (P < .05 and P < .001, respectively) when dobutamine was infused, but remained unchanged when heart rate alone was increased. We conclude that when MVO₂ is increased substantially, the resulting increase in CBF and MCE-derived myocardial blood flow is mediated, in part, by an increase in MBV. Thus, capillary recruitment plays an important role in the physiologic regulation of CBF. Lack of increase in MBV during dobutamine stress may indicate the presence of coronary stenosis or microvascular disease.