Last update:
10-Jun-2005

OBJECTIVE The aim of this study was to investigate pharmacologically the chronotrope reserve of patients with asymptomatic sinus bradycardia. Specifically, lower than normal sinus responses to intravenous (IV) infusion of atropine (<90 beats/min) and those following IV infusion of isoproterenol were evaluated. Thus, the requirement or not for pacemaker implantation among subjects showing asymptomatic sinus bradycardia was determined.

METHOD The study included 100 subjects aged 18-70 years presenting with permanent asymptomatic sinus bradycardia, but no apparent cardiopathy. The following protocol was applied in each case: (a) Administration of atropine (2 mg IV) and recording of the corresponding sinus rate. (b) Subsequent IV infusion of isoproterenol (0.24 mg/dL) with an initial dose of 2.4 μg/min and progressive steps of 1.2 μg/min, with a dose range from 2.4 μg/min to 7.2 μg/min. The maximal sinus rate was recorded at every step lasting 2-3 min.

RESULTS Applying this protocol it was possible to classify the population into groups A, B and C depending on the responses to atropine and isoproterenol: Group A-control: Normal response to atropine (sinus rate >=90 beats/min) and isoproterenol (final sinus rate 135.7±7 beats/min). Group B: Response to atropine and isoproterenol in the ranges of 81.2±5.3 beats/min and 126.7±6 beats/min, respectively. Group C: Response to atropine and isoproterenol in the ranges of 74.4±4.9 beats/min and of 99.2±11 beats/min, respectively. Statistically, the results from group A and B did not differ significantly (p=0.11). However differences in response between groups A and C, and between groups B and C showed considerable statistically significant differences (p<0.0000... and p=0.000003, respectively).

CONCLUSIONS Among the cases with an insufficient response to administration of atropine, isoproterenol has the advantage of identifying a group of patients with a good response (where the implantation of a pacemaker is not required) and a category of patients with insufficient reserve in whom the precautionary implantation of pacemaker is judged necessary.

Key words: Atropine, Isoproterenol, Sick sinus syndrome, Sinus node.