[Clinidine-induced bradycardia: studies of the effect on the human sinus node (author's transl)].

Heart rate, atrioventricular conduction, sinus-node recovery time and effectiveness of carotid-sinus massage on heart rate were measured before and every five minutes after intravenous injection of 0.15 mg clonidine (Catapresan) in 11 patients (averaged age 60 years) with symptomatic bradycardia (dizzy spells, syncope, palpitations). Th results indicated the following significant (P less than 0.05) mean maximal effects: heart rate fell by 12% (from 59 to 52/min), capacity of atrioventricular conduction by 9% (from 132 to 121/min), while maximal sinus-node recovery time ("over-drive suppression") doubled (from 1704 to 3562 ms), heart rates of 120, 150 and 200/min being used for diagnostic atrial overdrive stimulation at each five-minute period after clonidine. During clonidine administration three patients developed a so-called hypersensitive carotid sinus reflex, in two patients the effectiveness of carotid sinus massage decreased, in three if increased, while in three it remained normal. It is concluded that (1) during bradycardia, initiated or increased by sympatholytic drugs, tests should be undertaken to exclude latent sinus-node syndrome and (or) hypertensive carotid-sinus reflex; (2) clonidine should not be used in patients with signs of sinus-node dysfunction.