Role of the autonomic nervous system in the genesis of first and second degree atrio-ventricular nodal block

Thirty-four patients with a prolonged A-H interval (group I)and 26 with A-V nodal Wenckebach block (group II) were studiedin the basal state and after autonomic blockade (propranolol0.2mg kg^–1 and atropine 0.04 mg kg^–1 in order toassess the role of autonomic system in A-V nodal conductiondisturbances. In group I, the A-H intervals did not change significantlyafter autonomic blockade, whereas pacing cycle length for Wenckebachblock, effective and functional refractory periods of the A-Vnode decreased significantly (P<005). In the 22 patientswith organic heart disease these variables did not change significantlyafter autonomic blockade, whereas in the 12 without underlyingheart disease, they decreased in all cases (P< 0001). Inthe former, the variables of intrinsic A-V nodal conductionwere normal in only 6% of patients, whereas in the latter theywere normal in 66%. Also in group II, the intrinsic A-H intervalswere normal in only 6% of patients with cardiac disease butwere normal in 63% without underlying heart disease. These datasuggest that in the patients with first and second degree A-Vnodal block and organic heart disease, the conduction disturbanceis predominantly related to intrinsic involvement of A-V node,whereas in the subjects without underlying heart disease theA-V nodal blocks appear mainly related to autonomic alterations.