EXERCISE-INDUCED SUBENDOCARDIAL DYSFUNCTION IN DOGS WITH LEFT-VENTRICULAR HYPERTROPHY

The effects of treadmill exercise on regional myocardial blood flow and function were examined in 10 adult, conscious dogs with left ventricular hypertrophy (LVH) induced by aortic banding in puppies, which resulted in a left ventricular (LV) weight/body weight ratio of 8.5 ± 0.3. Data were compared with results from eight control dogs with an LV weight/body weight ratio of 4.9 ± 0.2. At rest, LV systolic and end-diastolic pressures were significantly greater (p < 0.01), and mean arterial pressure was significantly less (p < 0.05) in LVH dogs. Mean myocardial blood flow (control dogs, 0.98 ± 0.11 ml/min/g; LVH dogs, 1.16 ± 0.06 ml/min/g) and the transmural blood flow distribution at baseline, as assessed by endocardial/epicardial blood flow ratio (control, 1.35 ± 0.12; LVH, 1.21 ± 0.09), were similar in the two groups, during exercise to a target heart rate (240 beats/min), LVH dogs demonstrated greater (p < 0.01) increases in LV systolic and end-diastolic pressures. In control dogs, as expected, exercise augmented velocity of circumferential fiber shortening (16 ± 9%) and shortening fraction (15 ± 5%), but in LVH dogs, exercise reduced the velocity of circumferential fiber shortening (-14 ± 6%) and shortening fraction (-17 ± 5%). Exercise also increased full wall thickening (35 ± 5%), subendocardial wall thickening (66 ± 10%), and subepicardial wall thickening (44 ± 9%) in control dogs. In LVH dogs, exercise increased subepicardial wall thickening (31 ± 9%) and reduced subendocardial wall thickening (-40 ± 7%); full wall thickening did not change (-11 ± 9%). This was associated with a fall in endocardial/epicardial flow ratio to 0.72 ± 0.05 (p < 0.01) in LVH dogs. The subendocardial dysfunction persisted late into recovery, at a time when the transmural blood flow distribution had returned to baseline; this occurrence suggested myocardial stunning. Thus, in dogs with LVH, selective subendocardial hypoperfusion and profound selective depression in subendocardial wall thickening are observed during exercise. The subendocardial dysfunction persisted into recovery despite resolution of the perfusion abnormality.