Left ventricular blood flow during aortic pressure reduction in hypertensive dogs

We measured left ventricular blood flow with radioactive microspheres during aortic pressure reduction in 10 open-chest, anesthetized dogs with left ventricular hypertrophy due to chronic hypertension and in 10 matched normotensive dogs. Heart rate and left atrial pressure were held constant, and autonomic reflexes were abolished with ganglionic blockade. Aortic diastolic pressure was lowered from baseline to 90, 75, and 60 mm Hg with an arteriovenous fistula. During aortic pressure reduction, a stepwise decline in the endocardial-to-epicardial flow ratio in hypertrophied hearts from 1.23 +/- 0.04 at baseline to 0.96 +/- 0.09 at a diastolic pressure of 75 mm Hg parallelled that in normal hearts and was not associated with any deterioration in left ventricular performance. However, a further fall in the endocardial-to-epicardial flow ratio to 0.76 +/- 0.10 at a diastolic pressure of 60 mm Hg in hypertrophied hearts exceeded that in normal hearts (0.92 +/- 0.05, p less than 0.05) and was accompanied by evidence of left ventricular isovolumic and end-systolic dysfunction. We conclude that in hearts with pressure-overload left ventricular hypertrophy, aortic pressure reduction causes a transmural blood flow redistribution from subendocardial to subepicardial muscle layers. At moderately low aortic pressures, this redistribution is more pronounced than in normal hearts and is associated with functional evidence of myocardial ischemia.