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Autonomic control of heart rate during dynamic exercise in human hyperthyroidism
B C Maciel1, L Gallo, J A Marin Neto
1Department of Internal Medicine, School of Medicine of Ribeirao Preto, University of Sao Paulo, Brazil.
Clinical Science (London, England : 1979)
|August 1, 1988
Summary
Thyrotoxicosis alters heart rate regulation during exercise, with hyperthyroid patients showing reduced initial heart rate increases but greater sympathetic response later. Treatment normalizes this chronotropic response.
Area of Science:
- Cardiology
- Endocrinology
- Autonomic Nervous System Physiology
Background:
- Thyrotoxicosis, a condition of excess thyroid hormones, affects cardiovascular function.
- The autonomic nervous system, comprising sympathetic and parasympathetic branches, regulates heart rate.
- Understanding autonomic control of heart rate during exercise in thyrotoxicosis is crucial for patient management.
Purpose of the Study:
- To investigate the distinct roles of sympathetic and parasympathetic systems in regulating heart rate during dynamic exercise in women with thyrotoxicosis.
- To compare autonomic responses between hyperthyroid patients, normal individuals, and hyperthyroid patients after treatment.
Main Methods:
- Evaluated heart rate responses to graded dynamic exercise (5-75 W) on a bicycle ergometer in nine thyrotoxicosis patients and seven controls.
- Administered propranolol for sympathetic blockade to assess its effect on heart rate.
- Re-evaluated six hyperthyroid patients after treatment with propylthiouracil.
Main Results:
- While total heart rate increase was similar, hyperthyroid patients exhibited blunted initial heart rate rise (parasympathetic-dominant phase) but exaggerated later rise (sympathetic-dominant phase) compared to controls.
- Beta-adrenergic blockade (propranolol) reduced tachycardia more significantly in hyperthyroid patients at lower workloads (15, 50 W) than in controls (50, 75 W).
- Treated hyperthyroid patients demonstrated a chronotropic response pattern closer to that of normal subjects.
Conclusions:
- Thyrotoxicosis significantly alters the balance of autonomic nervous system control over heart rate during dynamic exercise.
- The sympathetic system appears relatively overactive, and the parasympathetic system relatively underactive in hyperthyroid individuals during exercise.
- Clinical compensation of thyrotoxicosis restores a more normalized autonomic regulation of heart rate during physical exertion.