isoproterenol as compared to exercise despite greater increment in cardiac output obtained during exertion.Although isoproterenol and exercise both induced an increase in cardiac output and a decrease in systemic vascular resistance, the magnitude of changes in those parameters was different in the two states. The greater increase in cardiac output with exercise can be explained on the bases of metabolic vasodilation in the active muscles decreasing systemic resistance while constriction of the systemic veins and the pumping action of the skeletal muscles increases the venous return to the heart.7' 8 In addition, the potent vasodilator effect of isoproterenol may have contributed to the lower values of systemic arterial pressure obtained with this agent.The increase in ventricular systolic pressures obtained with mild exercise in our patients was most likely related to an increase in preload as well as an increase in myocardial contractility.Our data in patients with pulmonary stenosis are in general agreement with the observations of Neal et al. regarding the increase in peak valvular gradient with isoproterenol out of proportion with the increase in cardiac output as compared to exercise. However, despite the differences in mechanisms causing increased semilunar gradients, the effects of exercise and isoproterenol were fairly consistent in patients with pulmonary or aortic valve stenosis at similar increments of heart rate. Individual values fit well to the regression line in both groups of patients ( figs. 2 and 3). Thus, the linear regression equations generated from these data can be used to estimate gradients of the stenotic lesion under exercise conditions at an equivalent heart rate.We believe that exercise studies remain the method of choice for the complete evaluation of patients with aortic or pulmonary stenosis. However, isoproterenol infusion and application of these regression equations constitute a practical and useful alternative when exercise studies cannot be carried out. References

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