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1 Centre for Human Drug Research and 2 Department of General Internal Medicine, Leiden University Medical Centre, 2333 CL Leiden; and 3 Department of Psychiatry, Erasmus University, 3015 GD Rotterdam, The Netherlands
We assessed
sympathovagal balance in thyrotoxicosis. Fourteen patients with
Graves' hyperthyroidism were studied before and after 7 days of
treatment with propranolol (40 mg 3 times a day) and in the euthyroid
state. Data were compared with those obtained in a group of age-, sex-,
and weight-matched controls. Autonomic inputs to the heart were
assessed by power spectral analysis of heart rate variability. Systemic
exposure to sympathetic neurohormones was estimated on the basis of
24-h urinary catecholamine excretion. The spectral power in the
high-frequency domain was considerably reduced in hyperthyroid
patients, indicating diminished vagal inputs to the heart. Increased
heart rate and mid-frequency/high-frequency power ratio in the presence
of reduced total spectral power and increased urinary catecholamine
excretion strongly suggest enhanced sympathetic inputs in
thyrotoxicosis. All abnormal features of autonomic balance were
completely restored to normal in the euthyroid state.
-Adrenoceptor
antagonism reduced heart rate in hyperthyroid patients but did not
significantly affect heart rate variability or catecholamine excretion.
This is in keeping with the concept of a joint disruption of
sympathetic and vagal inputs to the heart underlying changes in heart
rate variability. Thus thyrotoxicosis is characterized by profound
sympathovagal imbalance, brought about by increased sympathetic
activity in the presence of diminished vagal tone.
heart rate variability; catecholamines
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