By Ali Al-Sayegh
Cardiac resynchronisation therapy with defibrillator (CRT-D) reduces major morbidity and decreases mortality, including sudden cardiac death. CRT also progressively reverses cardiac remodelling, which may help to improve long-term clinical outcome. Until recently, most devices on the market had a bipolar pacing configuration of left ventricular (LV) leads. When comparing the worst and best LV pacing configurations for an LV lead, a bipolar configuration has significant haemodynamic benefits (cardiac output [CO], myocardial performance index and mitral regurgitation) over a unipolar lead.
Despite the benefits of CRT, implanting such devices can be time-consuming and challenging, requiring careful selection of the pacing site of the LV leads in order to optimise therapy. Phrenic nerve stimulation (PNS) occurs in 37% of patients at CRT implantation or follow-up (Biffi M et al. Circ Arrhythm Electrophysiol 2009) and is, along with a high LV pacing threshold, which occurs in 10–20% of patients (Gurevitz O et al. Pacing Clin Electrophysiol 2005), cited as a common cause for abandoning an ideal lead pacing site or having to perform revisional surgery. Many cases of PNS can be resolved by reprogramming the pacing lead configuration (using alternative vectors), but sometimes to the detriment of CO.
Additionally, approximately 30% of patients do not respond to CRT. The reasons are multiple but commonly include a non-optimal LV lead position and loss of LV capture. The LV lead can be repositioned for improved pacing, but this requires corrective surgery and is associated with difficulties due to variations in patient anatomy and lack of a stable, optimal site.
A recent innovation, the Promote Quadra CRT-D and the Quartet LV lead (St Jude Medical), have been designed to help overcome these problems. It is the only CRT-D system on the market with four poles and 10 pacing vectors, which, theoretically, offers more options to avoid PNS and attain the best CO. Having four poles makes a larger pacing area of myocardium available (up to 5cm) due to the distance between the first and last poles. This should make implanting the device in the most stable position easier, and give physicians greater control to make adjustments electronically to optimise CO and avoid complications like PNS while reducing the need for surgical revisions.
This quadripolar system is also more efficient than its predecessors. Among existing CRT-D devices, this one has the highest stored (45J) and delivered (40J) energy, which, therefore, enables it to overcome high defibrillation thresholds in markedly dilated hearts.
But do the theoretical advantages translate into clinical advantages? Our team tested the Promote Quadra CRT-D combined with the Quartet LV lead at the Kuwait Heart Center in a small trial of two men and two women (aged 64±5 years) with sinus rhythm, left bundle branch block and NYHA class III/IV heart failure. The study assessed the feasibility and safety of implantation as well as the CO achieved. Results showed that there were no complications, less need to reposition leads due to PNS (two cases of PNS in two vectors) or loss of capture, all for a procedure time reduced to 1 hour±12 mins. With more vectors, the quadripolar system allowed more options for pacing to achieve a better cardiac performance with maximum aortic flow rates of 1.36, 0.87, 0.68 and 0.92 m/sec, respectively, for the four patients.
Although further larger trials are required for more adequate assessment, it is anticipated that this quadripolar system, the first of a new generation, will afford improved options for CRT. The enhanced control it brings can better enable us to obtain a stable, optimal and reliable LV pacing site.
Ali Al-Sayegh is consultant adult cardiologist at the Chest Diseases Hospital, Kuwait.