Changes in myocardial blood volume with graded coronary stenosis

Abstract

Vasodilation of microvessels distal to a stenosis results in an increase in myocardial blood volume (MBV). The purpose of this study was to examine the changes in MBV induced by graded coronary artery stenoses by using myocardial contrast echocardiography (MCE). Accordingly, 21 dogs underwent progressive stenosis of a coronary artery in a random order, the severity of which was judged by the pressure distal to it. Total myocardial blood flow (MBF) to the bed distal to the artery (both anterograde and collateral) was measured by injection of radiolabeled microspheres into the left atrium. In seven dogs, anterograde and total MBF were measured at each stenosis stage by injection of different microspheres into the left atrium and directly into the coronary artery, respectively. MBV was calculated by dividing MBF by the mean transit rate of microbubbles injected directly into the coronary artery during MCE. The perfusion bed size of the artery was also measured by MCE. Our major findings are as follows: 1) there is a nonlinear increase in MBV with increasing degrees of coronary stenosis until the coronary stenosis becomes critical; 2) at moderate levels of coronary stenosis, MBV remains constant despite ongoing autoregulation because of reduction in the size of the perfusion bed supplied by the stenotic vessel; and 3) after exhaustion of autoregulation, a decrease in MBV is noted with increasing levels of stenosis. We conclude that assessment of MBV provides insights into myocardial perfusion distal to a coronary stenosis above and beyond that provided by the measurement of MBF alone.

Relationship of apparent myocardial T2 and oxygenation: Towards quantification of myocardial oxygen extraction fraction

“…After allowance was made for the difference between arterial (0.4) and capillary Hct (0.3) in the dogs (28), the resting value of global MBV was measured as 0.064 mL/g, or 6.4%. This value is consistent with MBV measured with Chromium-51-labelled red blood cells in anesthetized dogs (29) and results of myocardial contrast echocardiographic techniques (30).…”

Assessment of myocardial oxygen extraction fraction and perfusion reserve with BOLD imaging in a canine model with coronary artery stenosis

“…The increase of myocardial T 2 in response to hyperemia was significantly lower in the abnormally perfused territory than in normal myocar- dium. In contrast to severe stenosis (7), our dog model with moderate coronary artery stenosis showed little regional difference in the resting T 2 , which means that there is no regional difference in resting MBF (confirmed by the microsphere data), and the post-stenosis changes in MBV involving capillary recruitment were negligible with regard to this degree of stenosis (36,37). By the approximation of MPR, we observed that in remote myocardial regions the average values of MPR varied from two to four, while in the hypoperfused regions distal to stenosis a deficit MPR was consistently observed ranging from one to two.…”

Quantification of Myocardial Blood Flow With Ultrasound-Induced Destruction of Microbubbles Administered as a Constant Venous Infusion

“…2,21 If the sizes of the regions of interest and the ultrasound beam elevation are known, MBF can be calculated in absolute terms. Let us assume that a 1-cm 2 rectangular region of interest is placed over the myocardium and that the ultrasound beam elevation is 0.5 cm; therefore, the volume of tissue being examined is 0.5 cm 3 .…”

“…A screw occluder was placed around the vessel to create coronary stenoses of various severities. 2 The severity of a stenosis was judged by the gradient between the central aortic and distal coronary pressure. Three stages were used: baseline and two different degrees of stenoses, which were not flow limiting at rest.…”

“…Taken together, the results of these studies indicate that direct coronary injection of adenosine increases MBF by dilating microvessels which form a small portion of the MBV or are not present within the myocardia. Thus, by determining both CBV (measurement of the mean transit rate of microbubbles after an intracoronary bolus injection 1,2 ) and MBV (measurement of microvascular CSA during an intravenous infusion of microbubbles), MCE could be used to separate changes in capillary dimension or density from those of larger microvessels.…”