Regional myocardial blood flow during exercise in dogs with chronic left ventricular hypertrophy.

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Abstract
The response of myocardial blood flow to exercise was compared in normal dogs and in dogs with left ventricular hypertrophy (LVH) produced by banding the ascending aorta at 6-9 wk of age. Blood flow was measured with 15 .mu.m microspheres after the animals with LVH reached adulthood when left ventricular:body weight ratios were .apprx. 80% > normal. During resting conditions left ventricular systolic pressure was 202 .+-. 18 mm Hg in the dogs with LVH and 119 .+-. 6 mm Hg in the normal dogs (P < 0.01). Three levels of treadmill exercise which increased heart rates to 190, 230 and 260 beats/min resulted in progressive increases in left ventricular systolic pressure to a maximum of 343 .+-. 18 mm Hg in the dogs with LVH compared to 165 .+-. 10 mm Hg in the control dogs (P < 0.01). Unlike normal dogs which showed a significant transmural perfusion gradient favoring the subendocardium at rest [mean subendocardial:subepicardial ratio (endo:epi) = 1.25 .+-. 0.07], subendocardial flow did not significantly exceed subepicardial flow in the animals with LVH (mean endo:epi = 1.10 .+-. 0.08; P > 0.05 between normal and LVH). Myocardial blood flow increased as a direct linear function of heart rate during exercise in both groups of dogs. Exercise decreased the mean endo:epi ratio in normal dogs (mean endo:epi = 1.10 .+-. 0.08 during heavy exercise; P < 0.01) and in the animals with LVH (mean endo:epi = 0.94 .+-. 0.03; P < 0.05); the endo:epi ratios remained consistently less in the LVH dogs than in the normal animals (P < 0.05). The relative reduction of subendocardial flow in dogs with LVH was most apparent in the posterior papillary muscle region where the endo:epi ratio fell significantly below unity during heavy exercise (endo:epi = 0.79 .+-. 0.02; P < 0.01). Relative blood flow to the subendocardium of the left ventricle is significantly less than normal at rest and during exercise in dogs with LVH produced by supravalvular aortic stenosis.