His Bundle Pacing

His bundle pacing in humans was first described in 1970 by Narula et al. They demonstrated that it was possible to stimulate the His bundle to produce normal physiological ventricular activation via the His-Purkinje system. However, the first report of permanent His bundle pacing, by Deshmukh et al., did not occur until 2000. In that study, His pacing was performed in a series of patients with impaired left ventricular systolic function and AF prior to atrioventricular (AV) node ablation.

The lack of dedicated tools for implantation initially hampered enthusiasm; however, the development of specially designed sheaths and leads for delivering permanent His bundle pacing has led to a renewed interest. The potential role of His pacing in heart failure is large: it may prevent the development of pacing-induced cardiomyopathy; it may be used as an alternative to biventricular pacing in patients with heart failure and left bundle branch block (LBBB); and it may extend pacing therapy in heart failure to patients with narrow QRS and PR prolongation by providing AV synchrony without inducing ventricular dyssynchrony.

His Bundle Pacing: Conduction System and Outcomes (A) Schematic representation of the His-Purkinje conduction system. The membranous septum is indicated in yellow. Image courtesy of K. Shivkumar, MD, PhD, UCLA Cardiac Arrhythmia Center, Wallace A. McAlpine MD collection. Reproduced with permission. (B) Clinical outcomes of HBP. Kaplan-Meyer survival curves demonstrating a statistically significant reduction in the primary endpoint (composite endpoint of all-cause mortality, HFH, or upgrade to biventricular pacing) with His bundle pacing (HBP) compared with right ventricular pacing (RVP) in all patients and in patients with ventricular pacing (VP) >20%. Reprinted from Abdelrahman et al. (62). AVN = atrioventricular node; CS = coronary sinus; HB = His bundle; IVC = inferior vena cava; LBB = left bundle branch; LV = left ventricle; PA = pulmonary artery; RA = right atrium; RBB = right bundle branch; SVC = superior vena cava

Anatomy of the His Bundle and Implant Technique

The bundle of His extends from the compact AV node to the membranous interventricular septum, and measures approximately 20 mm in length. The bundle is a cord-like structure made up of multiple strands, which, even before the branching, are predestined to become the right or left bundle branches. His bundle pacing can be achieved by placing the lead at the atrial portion against the septum.

The most commonly used lead for His bundle pacing is the 69 cm Select Secure™ 3830 (Medtronic). This is a non-stylet-driven active fixation lead. Importantly, the screw forms part of the tip electrode allowing penetration of the capsule of the bundle of His and, therefore, direct stimulation of the His bundle fibres. The 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). Unlike traditional lead placement that primarily uses fluoroscopic guidance, His lead placement primarily uses electrical mapping. An electrogram from the lead tip is displayed using placement via either a lab electrophysiology system or a standard pacing system analyser.

A His signal is targeted, aiming for the local ventricular electrogram to be approximately twice the amplitude of the atrial signal . To confirm successful His capture, a 12-lead ECG is used to assess the QRS morphology with pacing. Criteria used to establish whether His capture has occurred are well described.Recently-published data suggests thresholds of <2.5 at 1 ms should be achieved. An increase in pacing threshold is observed in ~10 % of patients, leading to shorter battery duration. There is also a higher rate of lead revisions (6.7 %) due to loss of capture or increased threshold.

Anatomic Variations of the His Bundle

 (A) Type 1: The His bundle (AVB) runs under the membranous part of the interventricular septum (MS). (B) The type II His bundle runs within the muscular part of the interventricular muscle apart from the lower border of the membranous part of the interventricular septum. (C) The type III His bundle (arrow) is naked running beneath the endocardium with no surrounding myocardial fibers. AT = attachment of septal tricuspid leaflet; AVB = atrioventricular bundle; AVN = atrioventricular node; CS = coronary sinus

Despite recent advances and interest in HBP, several unanswered questions and concerns remain . Although permanent HBP may be an attractive option for physiological pacing in several groups of patients, its reliability and long-term performance are yet to be fully validated in large prospective studies. Particularly relevant are patients with infranodal, intra-Hisian AV block and BBB, where long-term safety of HBP has not been well studied. In such patients, should a backup RV lead be placed with HBP? What happens to the His bundle when it is traumatized by the screw on the tip of the lead in the long term? Can a second His Bundle pacing lead be placed successfully if the earlier lead fails in the long run? Considerable effort needs to go into improving the design and structure of the lead and the delivery tools to allow for easier implantation and stabilization of the lead. Beyond implant, what are the implications of extracting a chronic HBP lead? And beyond pacing hemodynamics, what is the impact of HBP on arrhythmia? Does HBP reduce the risk of ventricular tachyarrhythmias in the presence of myocardial scar? These and other questions remain.

What is certain is that this technique holds potential and requires further validation in larger studies with longer follow-up. It is also clear that collective and collaborative efforts from physician scientists, industry partners, scientific societies, and regulatory authorities will be required to successfully develop this technology and advance our understanding of the physiology of pacing.

Conclusions

HBP is an attractive mode of physiological pacing with significant promise for future applications in patients who are traditional candidates for RV pacing as well as CRT. Widespread adaptation of this technique is dependent on the improvement of tools and further validation of its efficacy in large randomized clinical trials.

Sources :

http://www.onlinejacc.org/content/accj/72/8/927.full.pdf

https://www.aerjournal.com/articles/his-bundle-pacing-heart-failure

http://www.onlinejacc.org/content/72/8/927

https://link.springer.com/article/10.1007/s10741-023-10296-4

Bundle Branches Block

What are the bundle branches?

Normally, your heart beats in a nice, regular fashion. The heartbeat starts in the upper chambers of the heart in an area called the SA node. The impulse then moves to the AV node. The AV node is an area of tissue that carries the impulse through the Bundle of His and then splits into two wire-like branches (the bundle branches). The branches carry the impulse to the Purkinje fibers, which are located in the muscular walls of the ventricles, and cause them to contract.

What is a bundle branch block?

A bundle branch block means the electrical impulses that control the heartbeat cannot move properly throughout the heart. A block in the branches causes the impulses to travel slower than normal.

Diagnosis of bundle branch block

A bundle branch block can be seen on an electrocardiogram (EKG).

How LBBB can affect other cardiovascular tests and conditions

Stress testing and heart attack

Left bundle branch block can make it more difficult to diagnose left ventricular hypertrophy (LVH) and ischemia (decreased blood supply to the heart) during exercise stress testing and during a heart attack.

If you have LBBB, make sure your doctor knows about the condition before you have a stress test. You may need additional testing or a different kind of stress test (done with medicine, not exercise).

You should also carry a copy of your EKG with you.

If you think you are having a heart attack, be sure to tell your doctor or nurse that you have LBBB and give them the copy of the EKG. This will help them see changes in your heart.

If you have signs of LVH, your doctor will use an echocardiogram to diagnose the condition.

Symptoms

In most people, bundle branch block doesn't cause any symptoms. Sometimes, people with the condition don't even know they have a bundle branch block.

For those people who do have signs and symptoms, they may include:

• Fainting (syncope)
• Feeling as if you're going to faint (presyncope)

Causes

Normally, electrical impulses within your heart's muscle signal it to beat (contract). These impulses travel along a pathway, including the right and the left bundles. If one or both of these branch bundles become damaged — due to a heart attack, for example — this change can block the electrical impulses and cause your heart to beat abnormally.

The underlying cause for bundle branch blocks may differ depending on whether the left or right bundle branch is affected. It's also possible that this condition can occur without any known underlying cause. Specific causes may include:

Left bundle branch block

• Heart disease
• Thickened, stiffened or weakened heart muscle (cardiomyopathy)
• A viral or bacterial infection of the heart muscle (myocarditis)
• High blood pressure (hypertension)

Right bundle branch block

• A heart abnormality that's present at birth (congenital) — such as atrial septal defect, a hole in the wall separating the upper chambers of the heart
• A heart attack (myocardial infarction)
• A viral or bacterial infection of the heart muscle (myocarditis)
• High blood pressure (hypertension)
• A blood clot in the lungs (pulmonary embolism)

Risk factors

Risk factors for bundle branch block include:

• Increasing age. Bundle branch block is more common in older adults than in people who are middle-aged.
• Underlying health problems. People who have high blood pressure or heart disease are more likely to have bundle branch block than people without those conditions.

Complications

The main complication of bundle branch block is a slow heart rate, which can sometimes cause fainting.
People who have a heart attack and develop a bundle branch block have a higher chance of complications, including sudden cardiac death, than do people who have heart attacks and don't develop a bundle branch block.
Because bundle branch block affects the electrical activity of your heart, it can sometimes complicate the accurate diagnosis of other heart conditions, especially heart attacks, and lead to delays in proper management of those problems

Diagnosis

Tests that may be used to diagnose a bundle branch block or the underlying problem causing it include:

• Electrocardiogram. An electrocardiogram records the electrical impulses in your heart through wires attached to the skin on your chest and other locations on your body. Abnormalities may indicate the presence of bundle branch block, as well as which side is being affected.
• Echocardiogram. An echocardiogram can be used to pinpoint an underlying condition that caused the bundle branch block. This test uses sound waves to produce images of the heart, allowing your doctor to see your heart in motion.
  
  An echocardiogram provides detailed images of the heart's structure and shows the thickness of your heart muscle and whether your heart valves are moving normally

Right bundle branch block (RBBB)

Right bundle branch block (RBBB) is very common, and the risk of developing the condition increases with age. Once your doctor sees RBBB on an EKG, the next step is to see if you have underlying heart disease.
If you do not have other heart disease, symptoms of heart disease, or other blocks in your electrical conduction system, no treatment is necessary.
If you have RBBB and another heart condition, such as heart attack or heart failure; dizziness or fainting; or other blocks in your electrical system, you will be treated for the heart disease. You may need a pacemaker if you have symptoms or another heart block.

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     In right bundle branch block, the right ventricle is activated via cell-to-cell transmission of the action potential.  This is illustrated in the following animation:

Notice the delay in the time of right ventricular depolarization.  This means that the right ventricle is activated after the left ventricle (which is depolarizing normally) and therefore contracts a "split second" after the left ventricle. Mechanically, this difference doesn't greatly influence the performance of the heart.  However, it does produce some characteristic changes in the ECG. 

Electrocardiographic changes in RBBB. As you might be able to guess from the animation, because the depolarization of the ventricle has been altered, the changes show up in the QRS complex and the T wave.  The following pictures illustrate right bundle branch block in different leads. 

Appearance of three different leads in RBBB:  From top to bottom:  Lead I, Lead V1 and Lead V6.  Due to the RBBB, the right ventricle is depolarizing after the left ventricle.  In all three leads, there is a prolongation of the QRS complex due to the blockade.  In leads I and V6 (which "see" the left ventricle best), the delayed depolarization of the right ventricle produces a late, negative going wave (S wave) that is very broad (there is no point to the wave) and prolonged.  In lead V1 (which has a good view of the right ventricle, the delay in depolarizing the right ventricle leads to the aptly-named "bunny ears"- the QRS complex is composed of an initial positive wave (the R wave), followed by a negative deflection (the S wave) produced when the left ventricle depolarizes, and then a second positive wave (called R' (spoken "R prime")) as the right ventricle finally depolarizes.  Because the normal conduction pathway was not followed for depolarization, the process of repolarization does not occur in the normal sequence leading to abnormalities in the T wave (seen best in V1).

Summary of electrocardiographic changes in RBBB:

• P wave of normal morphology (shape).
• rate and rhythm normal.
• QRS complex is prolonged (> 0.12 seconds).
• QRS complex is bizarrely shaped in leads with "good" views of the right ventricle, such as V1 and V2. 
  • The appearance of "bunny ears" is a hallmark of a bundle branch.
• The amplitude of the individual waves in the QRS complex is usually increased because the depolarization in the left and the right ventricles are no longer simultaneous (when they are simultaneous, they cancel each other out to some extent). 
• The T wave is abnormal in appearance because of alterations in the repolarization of the heart. 

Left bundle branch block (LBBB)

Left bundle branch block (LBBB) is less common than RBBB. As with RBBB, the risk of developing the condition increases with age. Once your doctor sees LBBB on an EKG, the next step is to see if you have underlying heart disease.

If you do not have other heart disease, symptoms of heart disease, or other blocks in your electrical conduction system, no treatment is necessary.

If you have another heart condition, such as hypertension, coronary artery disease or heart attack, myocarditis or heart failure; dizziness, fainting or chest pain; or another block in your electrical system, you will be treated for the heart disease. You may need a pacemaker if you have another type of heart block or have symptoms of heart block.

Left Bundle branch block (LBBB) is very similar to right bundle branch block, as the following animation illustrates. 

As with the RBBB, the major change is that the left ventricle is now depolarized due to the spread of the action potential via cell-to-cell conduction.  This results in the left ventricle depolarizes (and therefore contracting) a "split-second" after the right ventricle does.  As with RBBB, the mechanical effects of this on the heart are minimal. 

Electrocardiographic changes in LBBB: 

The following pictures illustrate left bundle branch block as seen in three different leads: 

The appearance of LBBB in three different leads:  As with right bundle branch block, the major change is in the shape and duration of the QRS complex (since it is ventricular depolarization that has been altered).  All of the leads show a prolonged QRS complex due to the time it takes the left ventricle to depolarize.  Leads I and V6 record long R waves (lead I shows a small initial R wave (you might be mistaking it for a P wave) and a larger R' leading to a somewhat odd pair of "bunny ears") because the depolarization is heading "at" the lead for a prolonged period of time as the ventricle depolarizes.   Lead V1 (on the right side of the sternum sees an initial very small positive wave as the faster right ventricle depolarizes, followed by a prolonged S wave produced by the left ventricular depolarization (note that the S wave also has two peaks - an inverted pair of "bunny ears").

Summary of electrocardiographic changes in LBBB:

• P wave of normal morphology (Note:  like a lot of things in this world, the ECG tracing shown above for LBBB has more than one thing wrong with it - no P waves are visible because this person is in atrial fibrillation).
• Normal rate and rhythm. 
• Normal PR interval
• QRS complex is prolonged and bizarrely shaped.
  • Bunny ears
  • other leads may show QRS complexes that aren't "pointy", instead they are rounded at the top/bottom.
• The amplitude of the QRS complex peaks are often increased, due to the failure of the left and right ventricle to depolarize simultaneously. 
• The T wave is abnormal in morphology because repolarization cannot occur normally if depolarization did not. 

Treatment

Most people with bundle branch block are symptom-free and don't need treatment.

However, if you have an underlying heart condition causing bundle branch block, treatment of the underlying condition is recommended. Treatment of underlying conditions may involve using medications to reduce high blood pressure or lessen the effects of heart failure, or the use of a coronary angioplasty to open up the artery leading to your heart.

Additionally, depending on your symptoms and whether you have other heart problems, your doctor might also recommend:

• A pacemaker. For some people with bundle branch block and a history of fainting, doctors may recommend implanting a pacemaker. A pacemaker is a compact device implanted under the skin of your upper chest (internal pacemaker) with two wires that connect to the right side of your heart. The pacemaker provides electrical impulses when needed to keep your heart beating regularly.
• Cardiac resynchronization. Also known as biventricular pacing, this procedure is similar to having a pacemaker implanted. However, in cardiac resynchronization, there's a third wire that's connected to the left side of the heart so the device can keep both sides in proper rhythm.