Synchronous activation of the right and left ventricles requires a normally functioning sinoatrial (SA) node, atrioventricular (AV) node and ventricular conduction system. When AV conduction has been interrupted by congenital heart block or acquired damage to the AV node, impulses arising from the SA node do not reach the ventricular myocardium. Standard therapy involves the placement of a permanent pacing system. In young children, such a system consists of either placing a single epicardial pacing lead on the right ventricular (RV) surface or separate epicardial pacing leads placed on the right atrium (RA) and RV. The former is commonly referred to as single chamber pacing and the latter as dual chamber pacing \[AV sequential, single-site (SS) ventricular pacing\]. The decision regarding single versus dual chamber lead placement is heavily influenced by operator convenience in the setting of a limited sternotomy. The main functions of the pacing system are sensing spontaneous activity and delivering pacing stimuli when appropriate. The two major advantages of dual chamber pacing are maintenance of physiologic heart rates and restoration of AV synchrony. The disadvantage of this system, however, is that the intrinsic synchronous depolarization of the ventricles is interrupted by initial paced ventricular activation in the RV rather than via the specialized His-Purkinje system; thus desynchronizing ventricular electrical activation. In addition, recent studies have demonstrated that despite early institution of cardiac pacing, some infants and children with AV block (AVB) develop late-onset left ventricular (LV) dilated cardiomyopathy over a follow-up period of 10 years.(1) The underlying mechanisms for this cardiomyopathy are not clear. A novel method of pacing (biventricular pacing, BiV) has been used in adult patients with left bundle branch block (LBBB, which results in dysynchronous ventricular activation) and congestive heart failure (CHF). Biventricular pacing in adults involves transvenous endocardial pacing leads placed in the RA, RV apex and LV via the coronary sinus. This results in "resynchronization" of ventricular activation, presumably by restoring a "normal" ventricular activation sequence, and has been associated with decreased CHF symptoms and improved ventricular function.

Children with acquired or congenital AVB, on the other hand, typically have a narrow QRS; owing to a midline ventricular escape rhythm and synchronous ventricular activation. The typical modes of pacing (VVI or DDD, single-site ventricular) in these children result in interventricular dyssynchrony, as described above. Recent studies in adult populations (DAVID, AAIR vs. DDDR in SSS, and MOde Selection trials)(2-4) demonstrated the detrimental effects of standard single-site ventricular pacing. Unfortunately, children with acquired and/or congenital AVB do not have the option of pacing or no pacing, and are destined to potentially decades of being 100% ventricular paced. This further emphasizes the importance of optimizing pacing strategies in this young, vulnerable population.

Specific Aim 1 (SA1): To evaluate the safety and feasibility of biventricular epicardial pacing in children.