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This article is in constant revision – as is the topic discussed in it

Concept of Physiologic Pacing

Thoughtful and meticulous placement of the RV lead is of paramount importance for long-term successful pacing. Gone are the days where passive leads were just floated to the RV apex. With active fixation leads the implanter has more options for more physiologic and safe lead placement.

Right ventricular pacing lead provides critical pacing in situations of life-threatening brady-arrhythmias – hence from a purely rhythm maintenance point of view, the most critical lead of a pacing system is the right ventricular lead. However from a long-term physiologic point of view, ventricular only pacing is not desirable and therefore atrial only pacing, dual chamber systems and biventricular systems evolved over time.
Physiologic Pacing refers to mimicking the natural rhythm of the heart by pacing. Core requirements are maintaining AV synchrony, maintaining Bi-Ventricular Synchrony and minimizing pacing of the ventricles. Maintaining inter-atrial synchrony has not shown any long-term benefit but maintaining inter-ventricular synchrony has shown to positively affect preservation of LV function in the long term as evidenced by cardiac resynchronization therapy in patients with heart failure. Minimizing RV only ventricular pacing also helps preserve LV function in the long-term. All of these matter as long term RV only pacing has been proven to reduce LV systolic function. AV synchrony is maintained by co-ordination of atrial and ventricular pacing.

RV apical pacing produces a left bundle branch type conduction pattern in the ventricles and leads to delayed activation of LV compared to RV. When a similar LBBB occurs in native conduction system in patient with pre-existing LV failure, it was shown that LV systolic function improves by pacing both ventricles together as done in CRT – therefore the assumption is that by creating an LBBB activation pattern by RV apical pacing, we are causing an iatrogenic dyssynchrony which leads to LV dysfunction

Therefore to make it physiologic (i.e. prevent dyssynchrony) – we need to ensure both ventricles contract together. The most “physiologic” way to achieve this is by passing the paced electrical impulse along the native ventricular conduction system. In the ideal world, we should be able to pace the proximal ventricular conduction system and pass the current down the native conduction fibers like natural rhythm – but pacing the proximal conduction system (aka His bundle pacing) has technical issues as it is covered by a insulating capsule which makes it difficult to capture and requires a learning curve. One also needs to consider new disease occurring in the conduction system later on.
Update : 2019. Direct his bundle pacing is gaining traction and now is considered a viable option to mimic physiologic pacing. There reports of His bundle pacing being as good as Biventricular pacing when there is a proximal LBBB with heart failure. However his bundle pacing requires a steep learning curve and still needs a significant effort. Out of specialized indications (e.g. LBBB with heart failure and inability to do BiV pacing) are good candidates for direct his bundle pacing. But for vast majority of bradycardia pacing, standard RV septal pacing would be sufficient given the advantages that it gives over apical pacing.

RV Septal Pacing

With the development of active fixation leads, “fixing” the lead to any place in the cardiac chambers became a possibility and this led to experiments with pacing in the interventricular myocardial septum thus mimicking physiological activation of ventricles (i.e. simultaneous activation of RV and LV ) in the hope of reducing pacing related LV dysfucntion. This is much more practical than His bundle pacing. The practical method of septal pacing is discussed in the next section

Another option is to pace both ventricles using different leads at the same time. At the moment this is reserved for cardiac re-synchronization therapy in patients with pre-existing stage C LV failure as the implantation of the LV pacing lead has its own plethora of issues. Please remember that in this discussion we are dealing with patients with normal LV function and our aim is to prevent LV failure in the very long term and not to improve pre-existing LV failure. Standard pacing in patient with pre-existing LV failure has a complex decision making process and if there is a anticipation of more than 40% requirement of ventricular pacing, the current consensus is to implant a BiV device.

The author’s  default preferred method is septal pacing and is described here. However in highly selected situations, apical pacing is preferred over septal pacing

How about evidence for physiologic pacing ? Sadly the state of evidence for long term benefits of septal pacing is equivocal – even in 2019 – because all the attention is going towards direct his bundle pacing

But –  It’s a pleasure to see a narrow complex paced rhythm rather than a wide complex paced rhythm. Narrower the QRS, happier the me !

Personally, the biggest benefit I see from septal pacing is the relative safety and ease of lead extraction. More and more young patients are undergoing device implantations and the indications for lead extraction in later years is rising. Lead extraction it self is seeing major progress but undoubtedly a septal lead is less of a risk than a apical lead to extract

The second reason – a properly placed septal lead has virtually no risk of leading to a tamponade. I have seen many cases of pericaridal effusions following apical placement of leads and septal pacing is definitely safe in this aspect

Is septal pacing safe from a lead performance point of view ? – Yes, and there is evidence for it : Right ventricular outflow tract septal pacing versus apical pacing: A prospective, randomized, single-blind 5-years follow-up study of ventricular lead performance and safety. But as all septal implanters have experienced, active fixation leads can fall from the septum and when they do they can lead to bad consequences. Therefore when pacing for life threatening bradycardia in a patient whose life hangs on the lead, lead stability takes precedence over potential long-term gains from physiologic pacing. Hence in such situations low septal or apical pacing is preferred.