LONG-TERM BRADYCARDIA BY ELECTRICAL PACING - A NEW METHOD FOR STUDYING HEART-RATE REDUCTION

Objective: The aim was to develop a method of inducing bradycardia on a chronic basis by electrical pacing and to study its effect upon myocardial capillarity in pigs. Methods: Farm pigs were instrumented with Medtronic(R) dual chamber telemetric pacemakers and two leads, either both atrial or one atrial and one ventricular. Bradycardia was achieved by linking pacing stimuli to endogenous atrial or ventricular events for the different electrode positions respectively, and monitored postoperatively for 4-5 weeks by implanted telemetric ECG devices. Myocardial capillary supply (lectin staining) and myocyte cross sectional areas were then estimated in sections of the left ventricle. Results: Heart rates were reduced by 40-50 beats.min(-1) at operation from a resting rate of 110(SEM 3) beats.min(-1) in eight animals and bradycardia maintained upon recovery from anaesthesia for up to five weeks, with no obvious limitations for the animals. After this time, heart rates measured under anaesthesia were found to remain low for up to 2 h after pacing had been switched off. There was no evidence of myocyte hypertrophy, yet mean capillary density.mm(-2) was significantly increased by pacing, from 1470(50) to 1734(82), p<0.02. Conclusions: It is possible to produce primary heart rate reduction in the pig by a novel means of pacing on a chronic basis, leading to increased myocardial capillary supply without myocyte hypertrophy. This method can provide a basis for future investigations of the beneficial effects of heart rate reduction in the vascularly compromised heart.