Power spectral analysis of heart rate in hyperthyroidism

The aim of the present study was to evaluate the impact of hyperthyroidism on the cardiovascular system by separately analyzing the sympathetic and parasympathetic influences on heart rate. Heart rate variability was evaluated by autoregressive power spectral analysis. This method allows a reliable quantification of the low frequency (LF) and high frequency (HF) components of the heart rate power spectral density; these are considered to be under mainly sympathetic and pure parasympathetic control, respectively. In 10 newly diagnosed untreated hyperthyroid patients with Graves' disease, we analyzed power spectral density of heart rate cyclic variations at rest, while lying, and while standing. In addition, heart rate variations during deep breathing, lying and standing, and Valsalva's maneuver were analyzed. The results were compared to those obtained from 10 age-, sex-, and body mass index-matched control subjects. In 8 hyperthyroid patients, the same evaluation was repeated after the induction of stable euthyroidism by methimazole. Heart rate power spectral analysis showed a sharp reduction of HF components in hyperthyroid subjects compared to controls [lying, 13.3 +/- 4.1 vs. 32.0 +/- 5.6 normalized units (NU; P < 0.01); standing, 6.0 +/- 2.7 vs. 15.0 +/- 4.0 NU (P < 0.01); mean +/- SEM]. On the other hand components were comparable in the 2 groups (lying, 64.0 +/- 6.9 vs. 62.0 +/- 6.5 NU; standing, 77.0 +/- 6.5 vs. 78.0 +/- 5.4 NU). Hence, the LF/HF ratio, which is considered an index of sympathovagal balance, was increased in hyperthyroid subjects while both lying (11.3 +/- 4.5 vs. 3.5 +/- 1.1; P < 0.05) and standing (54.0 +/- 12.6 vs. 9.8 +/- 2.6; P < 0.02). This parameter was positively correlated with both T3 (r = 0.61; P < 0.05) and free T4 (r = 0.63; P < 0.05) serum levels. Among traditional cardiovascular autonomic tests, the reflex response of heart rate during lying to standing was significantly lower in hyperthyroid patients than in controls (1.12 +/- 0.03 vs. 1.31 +/- 0.04; P < 0.002). No statistically significant difference in reflex responses between the two groups was found in deep breathing or Valsalva's maneuver. In the 8 patients reexamined after methimazole treatment, we observed complete normalization of altered cardiovascular parameters, with slight predominance of the vagal component compared with controls. These results suggest that thyroid hormone excess may determine reduced parasympathetic activity and, thus, a relative hypersympathetic tone.