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Cardiovascular responses during static exercise. Studies in patients with complete heart block and dual chamber
T Alexander1, D B Friedman, B D Levine
1Harry S. Moss Heart Center, Department of Internal Medicine, University of Texas Southwestern Medical Center, Dallas.
Background:
During static exercise in normal subjects, the mean arterial pressure increases as a result of an increase in heart rate and thereby cardiac output with no significant change in stroke volume or systemic vascular resistance. We hypothesized that if one component of the blood pressure response to static exercise, ie, heart rate, were fixed, plasticity of the neural control mechanisms during exercise would allow for preservation of the blood pressure response by alternative mechanisms.
Methods And Results:
Thirteen patients 20 to 68 years old with structurally normal hearts, complete heart block, and dual chamber pacemakers performed static exercise during three conditions: (1) normal dual chamber sensing and pacing mode, (2) heart rate fixed at the resting value obtained in the DDD mode of 78 +/- 4 beats per minute, and (3) heart rate fixed at the peak value obtained during exercise in the DDD mode of 94 +/- 4 beats per minute. Heart rate, blood pressure, and cardiac output were measured and stroke volume and systemic vascular resistance were calculated at rest and at 1 and 5 minutes during static one-leg extension at 20% of maximal voluntary contraction. The mean arterial pressures at rest and at 5 minutes were higher when the heart rate was fixed at the faster peak exercise heart rate. In the DDD mode, heart rate increased by 16 beats per minute and cardiac output by 1.1 L/min, with a resultant 25 mm Hg increase in mean arterial pressure at 5 minutes with no change in the stroke volume or systemic vascular resistance. In both fixed heart rate pacing modes, mean arterial pressure increased by 24 mm Hg when the heart rate was fixed at the resting heart rate and by 25 mm Hg when the heart rate was fixed at the faster peak exercise heart rate pacing modes associated with an increase in stroke volume, with similar increases in cardiac output. During static exercise there was no change in systemic vascular resistance from the resting value in any pacing mode.
Conclusions:
When heart rate is fixed in the presence of normal left ventricular function, the mean arterial pressure increases normally during static exercise because of an increase in stroke volume with no change in the systemic vascular resistance.