4.3. Programming Considerations
Minimal dimensions of both the structures and the HBP lead cause a variety of expressions of attempted CSP.15 During the implantation process, the pacing lead may achieve exclusive stimulation of the intrinsic conduction system (selective pacing), may stimulate a part of the system and part of adjacent myocardial fibers (non-selective pacing) or the neighboring myocardial fibers only, failing directly stimulate the conduction system (myocardium-only pacing). These combinations may occur for both the His Bundle and the Left Bundle Branch area, presenting nine patterns of pacing: Selective HBP, Non-Selective HBP, Myocardium-Only (Para-hisian) Pacing, Selective Left Bundle Branch Pacing (LBBP), Non-Selective LBBP, Left Ventricular Septal Pacing, Right Ventricular Septal Pacing, Mid-septal capture, and Anodal capture (simultaneous capture of the interventricular septum and left bundle branch from lead’s ring and tip, respectively). Of course, the arisen question is whether Selective pacing is superior to Non-Selective pacing in terms of clinical endpoints. In a study of 350 consecutive patients with successful HBP and ≥20% ventricular pacing burden, Selective and Non-Selective HBP have associated with similar outcomes of death and heart failure hospitalizations together as a primary endpoint. However, when heart failure hospitalizations and all-cause mortality were studied as secondary endpoints separately, Selective pacing appeared to be superior in the latter.16
Selective and Non-Selective pacing can be recognized through the His bundle lead threshold check electrocardiographic parameters during the implantation. These parameters are mainly related to the morphology and duration of the intrinsic QRS. These electrical responses are hard to assess by only the implanter, so an additional familiar with the procedure person needs to estimate these changes, and that’s another reason that makes this procedure demanding.15 In situations with narrow QRS, Selective HBP is easily distinguished with an isoelectric interval (corresponding to the H-V interval) between the pacing spike and QRS onset, whereas in Non-Selective HBP, a ’pseudo-delta’ wave is observed due to the capture of local myocardium, before the onset of paced QRS.17, 18 In situations of Bundle Branch Block (BBB) presence, correction of BBB may be observed during these electrical tests at the pacing threshold.19 Another characteristic of the transition with reduction of pacing output is that Myocardium only capture instead of Selective HBP is observed before the loss of capture (LOC).17 Some examples follow for the beneficial effect of HBP on the electrocardiographic duration of ventricular depolarization compared to biventricular pacing and conventional apical RV pacing. The loss of pacemaker capture during threshold control, and the recording of selective and nonselective pacing on the electrocardiogram, are also shown. In addition, the position of the three electrodes in the fluoroscopic view during HBP implantation is apparent. (Examples 1-4 ) Except for QRS duration, other electrocardiographic parameters are examined, such as H-QRSend = duration from His potential to QRS offset, LBpo-QRS-LVAT = duration from left bundle potential to peak of R wave in lateral leads (where the time of the peak of the R wave in lead V5 or V6 is thought to represent lateral LV activation time, LVAT), Stim-QRSend = duration from pacing stimulus to QRS offset, Stim-QRS-LVAT = duration from pacing stimulus to the peak of R wave in lateral leads, and Stim-V = interval from pacing stimulus to the onset of QRS.15
Moreover, in cases of conduction system disease with BBB, it is examined whether or not there is a correction of the pre-existing disorder in the branches of the His Bundle. Even in cases where there is no complete correction of the block, the width of the QRS with HBP is narrower than the initial.11 Thus, electrocardiographic criteria have been proposed based on the various types of pacing in combination with the intrinsic electrocardiogram, and that is undoubtedly more demanding than simple conventional pacing.20
In HBP, there are also troubleshooting issues to take care of because they cause programming considerations, not only during implantation but also at outpatient follow-up.21 Two issues that should be taken care of during implantation, and have to do with the position of the His lead, are those of atrial oversensing and ventricular undersensing. Αttention should be paid to cases of atrial capture with the His lead due to loss of His Bundle capture, as there may be morphologies with a short PR interval (due to proximity to the AV node) but also with a narrow P-wave morphology (due to simultaneous bi-atrial activation).17 In addition, His leads may produce pacing artifacts, especially at high pacing thresholds, and morphologies that may be confusing when reading the electrocardiogram, such as retrograde P-wave and anodal capture.
Atrial ovesensing should be checked at each follow-up visit, as should any threshold increases that may occur during the follow-up period. HBP thresholds are usually higher when compared with other pacing sites, not only because His Bundle is encased in a fibrous sheath which is electrically non-conducting but also because of the concomitant His area disease, including local fibrosis and degeneration. Underlying conditions, such as right atrial dilatation, septal hypertrophy, or infiltrative situations, as in sarcoidosis or amyloidosis, should be considered for threshold issues, as well. Finally, the ports that His lead is placed in, especially at CRT devices (atrial, right ventricular, or left ventricular port), should be taken into account so that the timing intervals are accurately measured, and the correct algorithms are appliable.22, 23 For these reasons, a specialized Pacemaker Technician is required, who will actively participate with knowledge and suggestions during the implantation process. In conclusion, HBP associates with more clinical and technical issues, and an every six months closer surveillance, which for many elderly patients may not be so practical, unlike conventional pacemakers, where the in-office follow-up oughts to be annual.1