Intrinsic sympathomimetic activity (ISA) of pindolol in patients with sinus node dysfunction

The effects of pindolol treatment on mean heart rate (MHR) and the number of paced heart beats were investigated in a group of 12 patients, with frequency programmable pacemakers implanted to control symptomatic bradyarrhythmias resulting from sinus node dysfunction. In a randomized double blind cross over experiment the patients were treated with pindolol (5 mg t.i.d.) or with placebo. Before the study the pacemaker was programmed to its lowest frequency in the demand mode. In the pretreatment period and after 2 weeks treatment with either pindolol or placebo a 24-hour ambulatory ECG was recorded. MHR during the night (24.00-8.00) was slightly but significantly increased by pindolol treatment by 6.0% when compared to placebo (P less than 0.05). There was no difference in the daytime MHR during either treatment period. During pindolol treatment the number of paced beats was reduced in 8/12 patients and in 4 of these the reduction exceeded 98%. In 2 patients the number of paced beats increased. The results indicate that in some patients with sinus node dysfunction pindolol can effectively reduce the number of paced beats thereby preserving the atrial contribution to pump function.