Physiological pacing

Question 1

How do we locate the HIS bundle?

Answer 1

We are looking for a HIS bundle signal.
Small A signal and big V signal (3:1 ratio) - to prevent oversensing issues.
Pacemapping can be performed in patients with CHB and no HIS signal.

Question 2

How to prove HIS conduction pacing?

Answer 2

1. Change in QRS morphology with changes in pacing output
2. His-end QRS = Stim-end QRS
3. Morphology of mid to end QRS same as during intrinsic conduction

Question 3

What is the difference between selective and non-selective HIS capture?

Answer 3

Selective: the stimulus captures only the HIS bundle: manifests as a delay (isoelectric line) between the stim and the onset of the QRS (Stim-V interval = HV interval) (which typically is very similar to the HV time). The QRS itself will look identical to native QRS
Non-selective: capture of HIS as well as local myocardial capture: manifests on ECG as a pacing stimulus followed by an immediate upslope to a QRS complex (referred to as pseudodelta wave - represents local myocardium capture). The Stim-V interval is shorter than the HV interval

Question 4

How do we know we are aiming for the LBBA pacing?

Question 5

What are we looking for in LBBA pacing at implant?

Question 6

LBBAP criteria

Answer 6

Stim- peak deflection in the QRS in V5/V6 (gives an idea about the LV activation time)

Question 7

What are the recommendations for CPP (conduction physiological pacing, i.e HIS, LBBAP and CRT) in patients with indication for pacemaker therapy and expected substantial ventricular pacing? (2023 HRS guidelines)

Answer 7

In patients with LVEF 36-50% who are anticipated to have substantial ventricular pacing, CPP is reasonable to reduce risk of PICM (pacing-induced cardiomyopathy).
In patients with normal EF and expected substantial ventricular pacing, CPP may be acceptable to prevent risk of PICM.
It is reasonable to implant a “backup” lead when the primary pacing lead is a HIS and the patient is expected to have substantial ventricular pacing to mitigate the risks of high threshold, loss of capture, lead dislodgement or oversensing.

Question 8

What are the recommendations for CPP in patients with indications for pacemaker therapy and expected less than substantial ventricular pacing? (2023 HRS guidelines)

Answer 8

Patients who require less than substantial ventricular pacing will have a smaller clinical impact of the pacing strategy selected compared to those expected to have substantial VP.
Therefore, RV lead placement with minimization of RVP, as well as CSP, are acceptable strategies for patients with normal or mildly depressed LVEF.
CRT with BiVP has not been found to benefit patients who are not anticipated to require substantial pacing and who have normal LVEF.

Question 9

Describe the 3 types of HIS bundle.

Answer 9

Type I: 46.7%, it courses along the lower border of the membranous septum and it´s covered only by a thin layer of myocardial fibers
Type II: 32.4%, the HB runs within the interventricular muscle (“burried HB” and is separated from the lower border of the membranous part of the interventricular septum - most difficult to implant
Type III: 21%, “naked HIS bundle”, HB traverses immediately beneath the endocardium and courses onto the membranous part of the interventricular septum

Question 10

What are the possible adverse events with HIS bundle pacing?

Answer 10

Atrial oversensing
Atrial capture
HIS signal oversensing
Ventricular undersensing
Transient or permanent RBBB
In AV block and LBBB there is risk for CHB and asystole - back up pacing should be available
Failure to implant 10-20%
High thresholds > 2.5V@1 ms (10-15% patients)
Lead revisions (3-13.3%)

Question 11

What are the ECG characteristics of pacing close to LBBA?

Answer 11

QRS is characterized by a positive terminal component in V1, pseudo-delta in leads V5/V6, and V6RWPT of 95-80 ms.

Question 12

Describe the paced LBB ECG morphology

Answer 12

Capture of the LBB is characterized by deeper S wave in lead I and V5/V6, more prominent R wave in V1-V3, and V6RWPT usually <80ms.

Question 13

What are the ECG characteristics of pacing the RV septum?

Answer 13

“W” morphology QRS in V1, notched rr´morphology in lateral leads and V6RWPT > 120 ms.

Question 14

What is the ECG appearance in deep septal capture?

Answer 14

QRS is narrower than RV septal capture, the notch in V1 moves towards the end of the QRS, notches or rr´pattern is absent in lateral leads, and V6RWPT is usually between 120 -95ms.

Question 15

What are the NS-HBP capture possibilities when the lead is located directly in the HIS bundle?

Answer 15

When the lead is situated directly in the HIS bundle, capture progression may occur as follows (in order of decreasing output):
1. Non-selective HBP
2. Selective HBP with bundle branch block recruitment
3. Selective HBP without bundle branch block recruitment

In some patients with NS-HBP, the lead tip is located in the RV itself, passing just between the septal and inferior tricuspid leaflets.

Question 16

What are the NS-HBP capture possibilities when the lead is located near (but not on) the HIS bundle?

Answer 16

When the lead is located near (but not on) the HIS bundle, capture progression may occur as follows (with decreasing output):
1. Non-selective HBP
2. Septal (RV only) capture

Question 17

What is S-HBP and how does is manifest on ECG?

Answer 17

Ventricular activation&repolarization occurs solely over the His-Purkinje system, evidenced by ECG concordance of QRS&T wave similar to baseline
Stimulus to V interval is nearly identical to the H-V interval
May result in normalization of BBB: if a high pacing output is applied it may overcome the site of block and normalize the QRS. However as pacing stimulus is increased, ventricular tissue surrounding the His bundle may also be captured resulting in NS-HBP.
HBP capture is an all-or-none phenomenon, as demonstrated by absence of QRS widening at a lower pacing output

ECG manifestations: HBP mimics the HV interval and QRS morphology of the intrinsic pattern
Visible isoelectric line between the pacing pulse and the beginning of the QRS
When a low output stimulus is applied, only the His bundle will be captured
As more current is applied, more tissue will potentially be recruited

Question 18

What is NS-HBP and how does it manifest on the ECG?

Answer 18

Occurs when there is local activation of local myocardium and His bundle conductive tissue.
It appears on the ECG as a pacing stimulus followed by an immediate upslope to a QRS complex.
The initial upslope represents the local myocardium capture, referred to as pseudodelta wave.
The stimulus to V interval is shorter than the H-V interval

Question 19

Describe the possible LBBAP capture during a threshold test

Answer 19

There are three possible capture outcomes with LBBAP during threshold testing (at near threshold outputs):
1. Transition from non-selective LB capture to selective LB capture
2. Transition from non-selective LB capture to LV septal myocardial only capture

Question 20

What is NS-LBBP?

Answer 20

Simultaneous LBB and local septal myocardial capture
No distinct isoelectric segment interval (No Stim-QRS latency)
QRS onset is immediately after the pacing artefact with pseudo-delta wave
No clear change is demonstrable on the local EGM

Question 21

What is S-LBBP?

Answer 21

Capture of the LBB alone, without local ventricular septal myocardium
Characterized by a distinct isoelectric interval before the local EGM at low pacing output (Stim-QRS latency)
Duration of the segment corresponds to the interval between LB potential to the onset of the surface QRS
Can be demonstrated by change in paced QRS morphology from qR to rSR in V1 with fixed Stim - V6RWPT while doing unipolar threshold measurement

Question 22

What is LV septal pacing?

Answer 22

Capture of the left side of the IVS without capture of the conduction system
Transition from NS-LBBP to LVSP noted as changes in V6RWPT (becomes longer >75 ms) and QRS morphology (diminished terminal R wave in V1)

Question 23

What is left fascicular pacing (LFP)?

Answer 23

Capture of one of the LB fascicles with capture of local ventricular septal myocardium
Characterized by short potential to QRS interval (< 25 ms) often with:
Abnormal paced QRS axis (different to intrinsic QRS axis)
Presence of criteria for conduction system pacing

Question 24

What is anodal stimulation?

Answer 24

Anodal stimulation occurs when there is simultaneous LBB capture by the tip of the electrode and capture by the ring electrode in the right side of the IVS
Anodal stimulation usually occurs at higher pacing outputs than cathodal stimulation

Question 25

List advantages of LBBP

Answer 25

Technically less challenging than HBP
Stable, low thresholds
Wide target zone
High success rates in AV block
Pacing beyond the site of block
Large R waves, no oversensing
Left septal myocardial capture

Question 26

LBBAP follow up steps

Answer 26

1. Presenting rhythm and rate
2. Underlying rhythm (to observe intrinsic QRS morphology)
3. VVI pacing at current outputs (if the patient has got underlying BBB and intact AV conduction consider running rhythm strips at different AV delays and/or pacing polarity/ outputs to optimize QRS morphology
4. Sensing is comparable to RV apex pacing
5. Check lead impedance in bipolar and unipolar configuration - Imp is low if unipolar < 400 ohms (loss of capture will likely be noted when pacing unipolar)
6. Run threshold tests with 12 lead ECG (compare the current QRS duration/morphology to QRS at implant) - check for LBBAP capture (LBBAP QRS should always be narrower than a typical RV septal or apical paced QRS):
   - Assess voltage-dependent changes in QRS morphology at different pacing outputs during bipolar threshold testing (VVI mode preferable to avoid fusion).
   Start at higher output (5V) and decrement until loss of capture and determine if there is loss of anodal capture while decrementing voltage and document the voltage at which this occurs
7. Perform a unipolar threshold test - paced unipolar and bipolar QRS morphologies are often comparable with the unipolar paced QRS morphology resembling the low output bipolar paced QRS

Question 27

HBP follow up steps

Answer 27

1. Evaluate presenting rhythm QRS morphology and rhythm on EGMs (12 lead ECG)
2. Sensing: the amplitude of HIS bundle EGMs is often low
   Possible adverse events: P wave oversensing, HIS potential oversensing, Ventricular oversensing - test which unipolar/ bipolar sensing configuration yields a better sensing signal
3. Threshold. Multiple thresholds may be observed:
   - septal
   - His-bundle (Selective or non-selective/ with or without BBB recruitment)

Question 28

What are the 3 main theories for LBBB correction with His bundle pacing?

Answer 28

1. Longitudinal dissociation of His bundle:
   pre-destination of fibers
   localized intra/inter-Hissian disease
2. Left bundle conduction delays:
   reset/accelerate with pacing impulse
   source/sink mismatch overcome with current load
3. Left bundle block- proximal/high:
   leapfrog with virtual electrode