Modification of sinus node function by epicardial laser irradiation in dogs

This study tested the feasibility of neodymium:YAG laser photocoagulation of selected sinus node areas to depress sinus rate responsiveness. In 14 open-chest dogs, origin of the sinus impulse (O point) was electrically mapped from the epicardium before and during isoproterenol infusion. Epicardial laser photocoagulation was applied to the O point observed during isoproterenol infusion and stepwise to remapped new O points until a 30 +/- 5% decrease in heart rate occurred. Long-term effects were assessed by Holter monitoring and electropharmacologic testing preoperatively and up to 10 weeks or 6 months. Mean (+/- SEM) percent decreases were observed at 10 weeks in the following parameters: average 24-hour heart rate, 17.4 +/- 5.0%; maximum heart rate on Holter, 30.5 +/- 3.5%; heart rate during pharmacologic autonomic blockade, 32.7 +/- 3.5%; and maximum heart rate on isoproterenol, 23.1 +/- 4.6% (all p less than 0.01). Curves with pacemaker recovery time plotted against control cycle length remained unchanged. Holter monitoring did not show excessive bradycardic episodes even after administration of propranolol. In three control dogs (sham operation), sinus node function remained unchanged. Histologic study of the irradiated area showed replacement by inflammatory cells, fibrosis, and cartilage formation with surrounding normal cells and occasional cells resembling pacemaker-like cells at the caudal end of the sinoatrial node. This study suggests that 1) map-guided graded laser photocoagulation of sinus node regions showing earliest activation during catecholamine stimulation successfully limits maximum heart rates without causing significant bradycardia, 2) the effects are long lasting, and 3) the remaining pacemaker behaves like the sinus node. Laser modification of sinus node function could become a form of nonpharmacologic heart rate control in patients with coronary artery disease undergoing surgery and in the syndrome of inappropriate sinus tachycardia.