Mechanism of myocardial dysfunction in the Presence of chronic coronary stenosis and Normal resting myocardial blood flow: Clinical implications

In chronic coronary artery disease, resting myocardial dysfunction can exist despite normal resting transmural myocardial blood flow (MBF). We hypothesized that this phenomenon occurs because of diminished endocardial MBF reserve. MBF (measured with radiolabeled microspheres) and wall thickening (WT) (measured with echocardiography) were assessed in 7 dogs after the development of severe left ventricular dysfunction caused by placement of ameroid constrictors on the left anterior descending (LAD) and left circumflex arteries and 3 weeks after selective bypass surgery to the LAD. Before surgery, the mean transmural MBF at rest and at peak dobutamine dose in the LAD bed were 1.1 +/- 0.5 and 3.0 +/- 1.5 mL/min per gram, respectively, and were not significantly changed after LAD bypass. The resting endocardial-to-epicardial MBF ratio (EER) was also normal before bypass (1.5 +/- 0.6) and remained unchanged after surgery. The prebypass EER at peak dobutamine dose, however, was markedly diminished in the LAD bed (0.7 +/- 0.3) and improved significantly (1.3 +/- 0.8, P.01) after surgery. Resting WT in the LAD bed also improved to normal levels (36% +/- 4% versus 13% +/- 6%, P =.0001) and no longer demonstrated a biphasic response to dobutamine. In comparison, the nonbypassed left circumflex bed continued to show reduced resting WT (12% +/- 6%), a biphasic response to dobutamine, and abnormal EER during rest and dobutamine (0.7 +/- 0.3). We conclude that persistent myocardial dysfunction in the presence of normal resting transmural MBF can occur as a result of diminished endocardial MBF reserve, with transmural MBF reserve remaining normal.