4.2. Technical Considerations
Fluoroscopy is inaccurate for localizing lead positions compared with other imaging methods, such as cardiac computed tomography (CT). Indeed, in a series of 59 heart failure patients undergoing CRT, the agreement for the left ventricular lead position was observed in 35% and for the right ventricular lead position in only 22% of the patients with fluoroscopy, respectively.12 The agreement within and between observers was much better and more accurate for cardiac CT (kappa 0.87 and 0.85 for both left and right ventricular lead position, respectively). Further, another technical limitation is the implantation tools for HBP. Although several companies in the pharmaceutical industry have created pacing leads for His Bundle, the most commonly used HBP lead is the 69 cm Select Secure™ 3830 (Medtronic), a non-stylet-driven active fixation lead.13 This lead can be delivered to the His bundle region using either the specially-designed non-deflectable His delivery sheath (C315 43 cm; Medtronic) or a deflectable sheath (C304 69 cm; Medtronic). A characteristic of this 4.1 Fr (French gauge) bipolar lead is that the 1.8 mm exposed-helical-screw design forms part of the tip electrode, with 9 mm tip to ring spacing, allowing easier detection and thus pacing of His Bundle fibers. It has unipolar mapping capabilities and minuscule size, reducing flow stresses and valvular motion effects, thus giving maximum stability in that demanding area.14 This lead has the best features after 50 years of trying pacing His Bundle using primarily fluoroscopic guidance and afterward electrical mapping for its placement.
As a technique, it does not have any critical complications for the patient, apart from those related to the vein puncture, as with conventional pacemakers implantation. However, there are two main troubles: the possibility of an increase in pacing threshold, which is observed at about 10% of the patients, leading to shorter battery-life duration, as well as the higher rate of lead revisions in an actual percentage ranging from 6.7 - 8.9 %, due to loss of capture or increased threshold. In order to avoid the above two concerns, a ’backup’ right ventricular lead can be used. The use of a ’backup’ lead is recommended in HBP in cases of the inexperience of the implanter, acceptable threshold achieved at the upper limit, at about 2V/1ms, possible schedule of an AV node ablation shortly, but also in cases of high-degree or infranodal block, especially in entirely pacemaker-dependent patients.1 Undoubtedly, an extra third lead for pacing has the advantage of safety, especially in cases of loss of capture, and it has better sensing compared with the HBP lead. However, more transvenous hardware for pacing (three versus two leads) increases the procedure’s cost, along with the risk of complications, both during implantation (ventricular perforation or vein thrombosis) and after, such as lead damage or risk of endocarditis. From all the anatomical and technical limitations mentioned, which may lead to a possible increase in pacing threshold and the need for lead revisions, arises the third consideration, which is also restrictive of HBP widespread application.