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