SHORT-TERM VARIABILITY OF BLOOD-PRESSURE AND HEART-RATE IN HYPERTHYROIDISM

The effect of hyperthyroidism on the short-term variability of blood p ressure and heart rate was evaluated in 12 untreated hyperthyroid pati ents during thyrotoxicosis and after a 6 1/2- month treatment designed to achieve a stable euthyroid state. Beat-by-beat finger blood pressu re was measured with a Finapres device. The pulse interval, from which pulse rate was derived, was obtained from the blood pressure signal. Due to the significant change in heart rhythm associated with thyrotox icosis, both pulse interval (taken as a surrogate of heart period) and pulse rate (taken as a surrogate of heart rate) were computed. Power spectral analysis showed a reduction in the overall heart period varia bility in the supine position in the hyperthyroid compared to the euth yroid state. This effect was observed in the low-frequency (0.005-0.06 8 Hz), midfrequency (0.068-0.127 Hz) and high-frequency (respiratory) domains as well, with a significant reduction of the modulus of these bands of 31%, 35% and 47%, respectively. The heart rate spectral modul us also exhibited a reduction of the high-frequency component (31%) in the supine position in the hyperthyroid subjects. These changes in he art rhythmicity corroborate a vagal deficit in hyperthyroidism. In add ition, blood pressure spectral power exhibited a significant deficit i n the orthostatism-induced mid-frequency systolic blood pressure rise in the hyperthyroid state (64%) compared with the euthyroid state. Thi s observation may reflect a reduced vascular sympathetic activation wi th standing. The resulting vasodilatation could well contribute to nor malize blood pressure in thyrotoxicosis in which cardiac output is inc reased. Clin Auton Res 8:181-186 (C) 1998 Lippincott-Raven Publishers.