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 exis t despite normal resting transmural myocardial blood flow (MBF). We hypothe sized that this phenomenon occurs because of diminished endocardial MBF res erve. MBF (measured with radiolabeled microspheres) and wall thickening (WT ) (measured with echocardiography) were assessed in 7 dogs after the develo pment of severe left ventricular dysfunction caused by placement of ameroid constrictors on the left anterior descending (LAD) and left circumflex art eries 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 be d 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-epic ardial MBF ratio (EER) was also normal before bypass (1.5 +/- 0.6) and rema ined unchanged after surgery. The prebypass EER at peak dobutamine dose, ho wever, was markedly diminished in the LAD bed (0.7 +/- 0.3) and improved si gnificantly (1.3 +/- 0.8, P < .01) after surgery. Resting WT in the LAD bed also improved to normal levels (36% +/- 40% 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 NW (12% +/- 60%), a biphasic response to dobutamine, and abnormal EER during r est and dobutamine (0.7 0.3). We conclude that persistent myocardial dysfun ction in the presence of normal resting transmural MBF can occur as a resul t of diminished endocardial MBF reserve, with transmural MBF reserve remain ing normal.