Assessment of cardiac vagal activity in patients with hyperthyroidism

Previous studies suggested that in patients with hyperthyroidism an autonomic imbalance and in particular a lower than normal vagal activity might be present. To verify this hypothesis we have evaluated the respiratory sinus arrhythmia (RSA, a measure of cardiac vagal activity) in ten hyperthyroid patients and in ten normal subjects. RSA was calculated from the power of the spectral component of the heart rate variability in high frequency band (HF–RR) during both spontaneous (supine and passive head-up tilt) and controlled breathing (supine). During controlled breathing the phase relation between heart rate and respiratory has been computed. The hyperthyroid patients showed a higher heart rate in all three conditions (P<0.001) and higher spontaneous respiratory rate in supine position (centered frequency of HF–RR: 0.342±0.015 vs 0.262±0.016 Hz; P<0.001). No difference was found in hyperthyroid patients compared to controls in terms of the HF–RR power in normalized units both during spontaneous breathing (supine, 43±8.3 vs 39.7±6.7%; tilt 18.8±5.9 vs 19.3 vs 4.1%; mean ± SE) and controlled breathing (45.4±7.1 vs 48.9±6.9%). No difference was found also in terms of the phase relationship between the heart rate and the respiratory signals (77.5±32.3 vs 77.5±28.1, degrees). Hyperthyroid patients seem not to have an impaired cardiac vagal activity.