Normal exercise capacity in patients with severe left ventricular dysfunction: compensatory mechanisms.

About one-third of patients who have severe left ventricular dysfunction can achieve normal levels of exercise. To elucidate the mechanisms that permit this to occur, we studied six patients with severe left ventricular dysfunction (average left ventricular ejection fraction 17 +/- 2.5% mean +/- SEM) who achieved nearly normal levels of exercise tolerance (greater than 11 minutes of treadmill exercise, Sheffield protocol). All patients had normal pulmonary function at rest and during exercise. Hemodynamics were measured at rest and during supine and upright exercise. The major mechanisms of the preserved exercise capacity in these patients were chronotropic competence, ability to tolerate elevated wedge pressures (33 +/- 3 mm Hg) without dyspnea, ventricular dilation, and increased levels of plasma norepinephrine at rest and during exercise. Also, whereas peripheral vascular resistance was unchanged during supine exercise, it decreased by 50% during similar levels of upright exercise. As a consequence, increases in cardiac output from rest to exercise were greater during upright than supine exercise (100% vs 50%, respectively) (p less than 0.05), and pulmonary wedge pressures were lower during upright than supine exercise (21 +/- 5 mm Hg vs 33 +/- 3 mm Hg). Thus, multiple mechanisms permit some patients with severe left ventricular dysfunction to achieve normal levels of exercise. These studies emphasize that left ventricular function must be assessed by direct means rather than inferring function of the left ventricle from the results of an exercise tolerance test.