Atrial Pacemaker

RV pacing provides life sustaining rhythm support and is the most critical lead of a pacing system. However more and more patients are getting paced for non-life threatening yet highly symptomatic sinus node dysfunction. Therefore functional pacing and maintenance of physiologic cardiac activation is a prime concern when pacing for sinus dysfunction. To enable such, atrial pacing has evolved to be an integral component of pacing for sinus node dysfunction.

As long-term RV pacing has been proven to be deleterious, it became unwise to pace the RV when the problem is in the sinus node. Therefore initial attempts to avoid RV pacing  was to pace only the atrium provided the AV nodal function was acceptable. This was the AAI mode. [ Atrial demand pacing]. Subsequent development of advanced sensor technology enabled physiologic rate adaptation mimicking natural sinus node activity and AAI became AAIR [ Rate adaptive Atrial Demand Pacing]. Atrial only pacing was possible only if the AV node was normal – otherwise had to pace the ventricle. On the west side of the Atlantic, the skepticism for atrial only pacing was high and this accelerated the development of dual chamber pacing systems which would pace both chambers based on the underlying pathology and rhythm.

Dual chamber systems introduced an important benefit – maintenance of AV synchrony. This enabled AV synchronized pacing for even AV nodal disease (e.g. complete heart block) in addition to sinus node dysfunction. The biggest perceived advantage of dual chamber pacing in sinus disease was that if the patient developed AV nodal disease, there would be synchronized pacing for ventricles too. Dual chamber pacing became very popular in North America and was the mode of choice for pacing for a wide variety of brady-arrhythmias except permanent slow AF. However the Europeans were skeptical of dual chamber pacing and until DANPACE shed light to the matter, atrial only pacing remained quite popular in Europe. DANPACE provided evidence base for justification of dual chamber pacing for sinus node dysfunction – for clinico-pathological reasons and economic reasons

Update 2022.  The newly released 2021 ESC pacing guideline does away with AAI pacing in preference to DDD. However the 2018 AHA guideline still has a Class 1 recommendation for AAI pacing in patients with pure sinus node dysfunction.

Regardless of the above, dual chamber systems are still very expensive compared to single chamber systems (and there is a unresolved issue of risk of heart failure with pure sinus node dysfunction). Therefore in cost-constrained developing country environments such as ours, we still attempt AAIR pacing if feasible. Usually these patients are young patients with only sinus node dysfunction and have had a non – invasive workup to assess AV nodal function before pacing AAI. An intra-operative testing of AV nodal function determines the final mode of pacing (our protocol is given in the section on Atrial Lead Implantation)

With regard to the site of pacing, right atrial appendage (RAA) is the location of choice – mainly because its an easily accessible and stable place. We avoid the lateral atrial wall because of phrenic nerve capture (and rare “it has happened” risk of bleeding), The inter-atrial septum is only very rarely used for pacing because almost always, a good site in the RAA can be found – even is situations where the RA appendage might have been partially amputated by the cardiac surgeons

Firstly, the lead has to be parked in the RA. If the system is going to be only AAIR then, the existing sheath can be used to insert the lead as described here for ventricular pacing.  If it is a dual chamber system, an additional sheath has be in place as described here via an additional guide-wire as described here.

If the pacing is for pure sinus node dysfunction, AAIR pacing can be considered (although dual chamber pacing is preferred). To pace in AAIR, one has to be absolutely sure that there is no current AV nodal disease. We assess this in two ways – Non invasively with Holter and Exercise testing and invasively during implantation of device. If holter and exercise ECG shows only evidence of sinus node dysfunction, we plan for an AAIR system and do intra-operative reconfirmation of AV nodal function. The intra-operative method is described below.

AV Nodal Wenckebach Point : AV node has decremental conducting properties i.e. at progressively faster sinus rates, the conduction time over the AV node increases (with manifest PR prolongation) and after a certain rate, it begins to block the conduction of certain P waves. This occurs in a pattern where the PR interval progressively lengthens and suddenly fails to conduct a P wave. Soon after the blocked P the subsequent conducted P wave has a shorter PR interval than the last conducted P wave thus implying that the AV node recovered during the non -conducted P wave. This is similar to type 1 heart block (Wenckebach type) and the heart rate at which the first dropped beat occurred is termed the Wenckbach point (WBP)

AV nodal conduction is highly dependent on autonomic function. Increased vagal tone decreases conduction velocity and sympathetic stimulation increases conduction velocity. Although Wenchbach phenomenon occurs at fast heart rates, the phenomenon is often seen in healthy young physically fit adults during sleep as their vagal tone is high during sleep. AV nodal conduction is also influenced by drugs such as some calcium channel blockers, digoxin, beta blockers etc.