Decrease in coronary blood flow reserve during hyperlipidemia is secondaryto an increase in blood viscosity

Background-During maximal hyperemia, capillaries provide the greatest resis tance to flow. A major determinant of capillary resistance is viscosity. We , therefore, hypothesized that abnormal coronary blood flow (CBF) reserve o bserved during hyperlipidemia is secondary to increased blood viscosity and not abnormal coronary vasomotion. Methods and Results-Maximal hyperemia was induced in 9 dogs using adenosine . Serum triglyceride levels were increased by incremental doses of Intralip id. A good correlation was noted between serum triglyceride levels and bloo d viscosity (r=0.82). Neither total coronary blood volume nor myocardial bl ood volume changed with increasing serum triglyceride levels, indicating la ck of vasomotion. Myocardial vascular resistance (MVR) increased with incre asing triglyceride levels (r=0.84), while hyperemic myocardial blood flow ( MBF) decreased (r=-0.64). The decrease in hyperemic MBF was associated with a decrease in blood velocity (r=-0.56). These findings were confirmed with direct intravital microscopic observations in the mice cremaster muscle. Conclusions-Increasing lipid levels in a fully dilated normal coronary bed causes no change in large or small vessel dimensions. Instead, the increase in blood viscosity causes capillary resistance to rise, which attenuates h yperemic CBF. Therefore, the abnormal CBF reserve associated with hyperlipi demia is due to increase blood viscosity and not abnormal vascular function .