Conduction System Pacing with Dr Bill Petrellis

Cardiac Resynchronisation Therapy (CRT)

The advent of CRT at the turn of the century, was a major advancement in the management of heart failure patients with intrinsic LBBB. This technique involves positioning one lead within the right ventricle (RV) and another to pace the left ventricle (LV) via a lead advanced into a suitable coronary sinus tributary overlying the lateral left ventricular myocardium. The timing of left and right ventricular pacing can be adjusted to reduce dyssynchrony caused by the pre-existing LBBB. Most commonly, the LV lead is activated before the RV lead by a mere 20 to 40 milliseconds resulting in a narrower paced QRS pattern. The seemingly insignificant advancement of left ventricular activation results in a marked improvement of LV systolic contraction and reduced functional mitral regurgitation in up to 70% of patients. Occasionally, cardiac resynchronisation results in normalisation of the ejection fraction, and these individuals are known as super responders. However, CRT is limited by the coronary sinus anatomy, the presence of LV scar and the inherent potential for phrenic nerve stimulation, a structure which overlies the lateral left ventricle.

Placing an LV lead is often technically challenging, not always successful, and furthermore, 30% of recipients are non-responders despite successful LV lead placement. I recall during the second year of my advanced cardiology training at the Austin Hospital in 2002, a newly minted consultant cardiologist, who had just returned from the United States, requested CRT for an inpatient with heart failure. The experienced senior staff electrophysiologist accepted the challenge to implant his first CRT device. I recall his frustration while we spent several hours at the private hospital on a Saturday morning, trying to position a simple bipolar lead in the coronary sinus using rudimentary sheaths. Today, we have an almost endless catalogue of sheaths, additional tools and quadripolar leads, which have significantly increased the success rate of lead placement and improved RV/LV resynchronisation. Multipolar pacing and AV synchrony algorithms have further added to cardiac mechanical optimisation and perhaps CRT response.

Conduction system pacing is a technique that recruits the native His-Purkinje system producing physiological pacing which results in a narrow paced ventricular QRS complex. It therefore avoids the aforementioned phenomenon of electrical dyssynchrony induced by standard pacing techniques, including CRT. Fast forward to 2004 during the second year of my Electrophysiology and Pacing fellowship at the University of Western Ontario in London, Canada. I quickly became a proficient implanter, spending a day or two each week grinding through my list of 5-7 new implants, upgrades, and generator replacements. But there was one thing I really didn’t look forward to - my scheduled afternoon in the pacing research lab. One of my supervising professors in London was researching the viability of His bundle pacing and comparing its haemodynamics (including cardiac output) to conventional RV apical pacing. The His bundle is a collection of heart muscle cells specialised for electrical conduction. It transmits electrical impulses from the atrioventricular node (AVN) to the bundle branches, causing the ventricles to contract. Hours were spent testing His bundle pacing on consenting pacemaker recipients, positioning, and fixing the lead, pacing at different rates and measuring heamodynamics, only to remove it and leave the final permanent lead at the RV apex. At the time, I did not appreciate the significance of his research, nor did I find it remotely interesting, for shame…. then there was radio silence for over a decade.

Conduction System Pacing 

An enthusiastic explosion for conduction system pacing occurred around 2015, which was building over the preceding 5 years. #don’t-dis-the His (https://twitter.com/hashtag/ dontdisthehis?lang=en) was its catch cry. Imagine, cardiac pacing which produces a normal paced QRS complex and the ability to correct intrinsic LBBB. Yes, you read correctly, pacing the His bundle above the site of LBBB often corrected it to produce a narrow paced QRS. Fantastic! I was invited to attend a His bundle pacing course at Geelong in July 2017.

We observed several cases and attended lectures outlining the technique. I was excited to return to Sydney and offer it to my patients, but my initial enthusiasm quickly waned. I was discouraged that the “landing site” for the His bundle lead was rather small making it a tricky implant. I also noted that the lead dislodgement rate was much higher than conventional pacing, requiring lead repositioning with its inherent high risk of infection, not to mention patient inconvenience. More importantly was the observation of increasing His capture threshold over time and the potential for non-capture, an unacceptable outcome for pace dependant patients with complete heart block. Another frequent observation was that loss of His bundle capture resulted in capture of the septal myocardium, so the benefi t of physiological pacing was lost, despite all efforts.

Pacemaker follow up is also time consuming as there is a necessity to confi rm His bundle capture using a 12 lead ECG, not to mention issues arising from lead over- and under sensing. My conclusion was that although His bundle pacing is the most elegant solution in theory, it is not so in practice. In November 2019, I had the opportunity to visit Fuwai Hospital, National Cardiovascular Centre, in Beijing China. This impressive true “centre of excellence” is focussed solely on the management of cardiovascular disease. The facility boasts 700 beds, 12 operating rooms, 2 hybrid operating rooms and 10 interventional catheterization rooms. On offer was an excellent training program for left bundle area pacing (LBAP), an alternative method for conduction system pacing. LBAP involves advancing a lead just beyond the tricuspid valve, positioned perpendicular to the basal right interventricular septum and then vigorously advanced into the septum aiming to penetrate the myocardium to reach the left bundle branch (see fi gures A and B). The landing site is larger than the His region allowing for a simpler procedure.

The resultant lead is position is stable and lead parameters exhibit excellent long term sensing and low capture threshold. Furthermore, follow up is no different to a standard pacemaker and physiological pacing is effectively achieved. I excitedly returned home only to suffer the worst “plane fl u” I had ever experienced which almost knocked me off my feet for the next 2 weeks. The world has since changed, but at the time, I took some OTC meds, avoided intubation and “soldiered on”.