Parasympathetic nervous system in patients with Graves' disease determined by R-R interval variations on electrocardiogram

Little is known about an interrelationship between thyroid dysfunction and parasympathetic nervous system. R-R interval variations on electrocardiogram (ECG) have been considered to be reliable indicator reflecting abnormalities of parasympathetic nervous system. We have attempted to apply this technique in patients with hyperthyroidism. Studies were conducted in 60 healthy subjects and 57 patients with Graves' disease. R-R interval variations were expressed as coefficient of variation on 100 heart rates at the time of resting (CVq) and deep respiration (CVd). A negative correlation between R-R interval variations (CVq, CVd) and ages was observed in healthy subjects. CVq was significantly lower in untreated Graves' disease than in antithyroid drug-treated Graves' disease and control subjects. A similar result also was obtained in CVd. Decreased CVq in untreated patients with Graves' disease was restored by administration of beta blockades, propranolol and metoprolol, but not by administration of alpha blockade, bunazosine. The present investigation suggests that there are hypofunctions in parasympathetic nervous system associated with beta (especially beta one) effects in patients with hyperthyroidism due to Graves' disease.