Mechanisms of subendocardial dysfunction in response to exercise in dogs with severe left ventricular hypertrophy

The effects of exercise on regional myocardial blood flow and functions were examined in the presence and absence of β-adrenergic receptor blockade in 10 adult conscious dogs with severe left ventricular (LV) hypertrophy induced by aortic banding in puppies, which increased the LV weight/body weight ratio by 87%. Exercise at the most intense level studied increased LV systolic (+87±8 mm Hg) and end-diastolic (+28±5 mm Hg) pressures, systolic (+85±12 g/cm²) and diastolic (+49±11 g/cm²) wall stresses, and subepicardial wall thickening (+0.18±0.05 mm) but reduced subendocardial wall thickening (-0.45±0.12 mm) and full wall thickening (-0.42±0.13 mm). This was associated with a fall in the subendocardial/subepicardial (endo/epi) blood flow ratio to 0.87±0.06 from 1.24±0.08. Subendocardial dysfunction persisted during recovery, at a time when transmural blood flow distribution returned to baseline, suggesting myocardial stunning. At the least intense level of exercise studied, the endo/epi blood flow ratio did not fall (1.27±0.14), but increases in heart rate (+73±8 beats per minute) and LV systolic (+35±8 g/cm²) and diastolic (+27±3 g/cm²) wall stresses were observed, and subendocardial wall thickening fell significantly (-0.21±0.08 mm, p±0.05). With anticipation of exercise, subendocardial wall thickening was not changed. However, subendocardial dysfunction was even evident after 10 beats, i.e., the first 3 seconds of exercise, at a time when LV pressures and stresses had not increased. After β-adrenergic receptor blockade with propranolol, the most intense level of exercise was associated with lesser increases in systolic and diastolic LV wall stresses, heart rate, and LV dP/dt, and the endo/epi blood flow ratio was no longer reduced below unity (1.17±0.09). In addition, there were no decreases in subendocardial or full wall thickening, and myocardial stunning was no longer observed. Thus, the subendocardial hypoperfusion and depression in subendocardial wall thickening observed during exercise in dogs with LV hypertrophy was prevented by pretreatment with β-adrenergic receptor blockade. Furthermore, the subendocardial dysfunction occurred rapidly, before alterations in LV systolic or diastolic wall stress or an alteration in the endo/epi blood flow ratio. These data suggest that subendocardial dysfunction in response to the stress of exercise 1) is one of the earliest manifestations of altered LV function of the severely hypertrophied ventricle, 2) may be a protective mechanism reducing the requirement for enhanced subendocardial perfusion, and 3) may actually be responsible in part for the reduced endo/epi blood flow ratio.