[Pathogenesis of tachycardia in hyperthyroidism. Value of Holter monitoring and the use of a beta-blocker].

The mechanisms of tachycardia in hyperthyroidism were investigated by means of Holter recordings of heart rate in 45 patients, 33 of whom had sinus rhythm and were left untreated. In the remaining 12 patients, recordings were taken after 3 days of treatment with either propranolol (120 mg/day; 6 patients) or pindolol (15 mg/day; 6 patients). Propranolol is a beta-blocker devoid of intrinsic sympathetic activity whereas pindolol possesses such activity. Changes in heart rate under the influence of each of these drugs were compared with those observed in 96 controls similarly treated. The difference in baseline heart rare between day and night was significantly higher (p less than 0.01) in patients with hyperthyroidism (17 +/- 1 QRS/min) than in controls (13 +/- 1 QRS/min). Day and night heart rates were increased by pindolol, the increase in night heart rate being significantly greater (p less than 0.05) in patients with hyperthyroidism (23.4 +/- 4.9%) than in controls (11.6 +/- 2.6%). These results suggest that sinus tachycardia in hyperthyroidism is related to an increase in the number of myocardial beta-adrenoceptors. They also indicate that thyrotoxicosis should not be treated with beta-blockers possessing intrinsic sympathetic activity.