Chronic Chagas' disease: possible mechanism of sinus bradycardia

To determine the cause of sinus bradycardia in patients with chronic Chagas' disease by studying their sinus node function with pharmacological agents.

Cohort study.

Referral tertiary care hospital in Cordoba, Argentina.

Thirty-seven chagasic patients (19 females and 18 males, mean age 41 +/- 12 years) were divided into three groups: group 1, 14 patients with positive serology for Chagas' disease but no evidence of cardiac involvement; group 2, 14 patients with Chagas' disease and electrocardiographic abnormalities but without signs or symptoms of congestive heart failure or cardiomegaly; and group 3, nine patients with Chagas' disease and evidence of cardiomegaly and congestive heart failure. Six healthy volunteers with no evidence of heart disease were used as controls.

All subjects received 0.04 mg/kg intravenous atropine sulphate followed 3 mins later by 0.2 mg/kg intravenous propranolol slowly injected. The resultant heart rate obtained with this total autonomic blockade (TAB) was considered to be the intrinsic heart rate observed (IHRo).

The mean increase of heart rate after atropine was +68 beats/min in controls, +45 beats/min in group 1, +45 beats/min in group 2 and +32 beats/min in group 3. The change from the basal heart rate to the IHRo after TAB with atropine and propranolol was +29 beats/min in controls, +17 beats/min in group 1, +17 beats/min in group 2 and +5 beats/min in group 3.

The observed response to atropine in patients with Chagas' disease suggests abnormality in the innervation of the sinus node. The difference found in the IHRo after TAB indicates involvement of the automaticity of the sinus node. These findings may explain the bradycardia reported in this disease.