Peter R. Brink

Biological pacemaker implanted in canine left bundle branch provides ventricular escape rhythms that have physiologically acceptable rates.

Background—We hypothesized that administration of the HCN2 gene to the left bundle-branch (LBB) system of intact dogs would provide pacemaker function in the physiological range of heart rates. Methods and Results—An adenoviral construct incorporating HCN2 and green fluorescent protein (GFP) as a marker was injected via catheter under fluoroscopic control into the posterior division of the LBB. Controls were injected with an adenoviral construct of GFP alone or saline. Animals were monitored electrocardiographically for up to 7 days after surgery, at which time they were anesthetized and subjected to vagal stimulation to permit emergence of escape pacemakers. Hearts were then removed and injection sites visually identified and removed for microelectrode study of action potentials, patch clamp studies of pacemaker current, and/or immunohistochemical studies of HCN2. For 48 hours postoperatively, 7 of 7 animals subjected to 24-hour ECG monitoring showed multiple ventricular premature depolarizations and/or ventricular tachycardia attributable to injection-induced injury. Thereafter, sinus rhythm prevailed. During vagal stimulation, HCN2-injected dogs showed rhythms originating from the left ventricle, the rate of which was significantly more rapid than in the controls. Excised posterior divisions of the LBB from HCN2-injected animals manifested automatic rates significantly greater than the controls. Isolated tissues showed immunohistochemical and biophysical evidence of overexpressed HCN2. Conclusions—A gene-therapy approach for induction of biological pacemaker activity within the LBB system provides ventricular escape rhythms that have physiologically acceptable rates. Long-term stability and feasibility of the approach remain to be tested.