Transcatheter aortic valve implantation (TAVI) has been shown to be as effective as surgical aortic valve replacement and perhaps better than surgery in certain patients with severe aortic stenosis (AS) [Citation1–Citation5]. Therefore TAVI has become an important medical intervention for patients with severe AS. As this technique evolves, damage to the conduction system including complete heart block requiring pacemaker implantation has been identified as one of the adverse outcomes of this procedure [Citation1–Citation5]. In this context, many important medical questions have been raised which require further investigation and research to address. For example, there is no reliable predictor of complete heart block occurrence in patients who undergo TAVI and it is not known which group of patients with partial damage to the conduction system, usually in the form of new left bundle branch block (LBBB), will require pacemaker implantation.
It is plausible that deployment of the bioprosthetic valve over the usually heavily calcified native valve exerts pressure on the left bundle branch (LBB) and the atrioventricular node (AVN) due to the proximity of those structures to where the valve is deployed. That direct pressure over the LBB and AVN, and impingement of those structures can result in significant damage to the conduction system. Some degree of damage to these structures may be inevitable; however, there is a great variability in occurrence of conduction disease depending on different TAVI techniques [Citation6,Citation7] and that supports an important role for technical details of the procedure in resulting conduction disease. For example, the rate of conduction impairment and pacemaker requirement has been generally higher in Medtronic CoreValve than in Edwards SAPIEN valves probably due to shape of the valve and the extension of the CoreValve into the left ventricular outflow tract. TAVI techniques and types of implantable bioprosthetic valves continue to evolve. For example, initially two main types of valves available for this procedure were Edwards Sapien and Medtronic CoreValve. However, there are many more types of valves available today and the original Sapien and CoreValves have undergone revision for having easier implantation and better outcome. For example, the Medtronic Corevalve EvolutR has a different shape than the previous generation of CoreValves, with less radial forces, which may decrease the occurrence of conduction disorders, whereas the newer Edwards Sapiens 3 valve has a higher radial strength and an outer skirt, enlarging the lower portion of the valve into the left ventricular outflow tract in proximity of the LBB. In addition, procedural steps of implantation and the delivery systems have been revised. Therefore, the technically related aspects of damage to the conduction system may continue to change and perhaps decrease.
Currently, the patient population who undergo TAVI are generally elderly patients with significant cardiovascular and other medical comorbidities who are at high risk for surgical aortic valve replacement. Nevertheless, as TAVI has been shown to be a relatively safe and effective treatment for AS, the indications are expected to expand. For example, a recently published randomized clinical trial that evaluated 795 patients at high risk for surgery but not inoperable who underwent TAVI versus surgical aortic valve replacement in 45 centers in the United States and the study concluded that TAVI was associated with significantly higher survival and less risk of vascular complications, bleeding, renal injury and atrial fibrillation [Citation8]. In addition, studies are being conducted to compare TAVI versus surgical aortic valve replacement in patients with moderate risk for surgery [Citation9]. Therefore, TAVI indications may expand and this expansion to patients with less comorbidity and perhaps younger patients may affect the incidence of conduction damage during this procedure.
It is important to be able to predict which patients will develop complete heart block and will require pacemaker implantation immediately after TAVI. Nevertheless, because of the above-mentioned expected changes in TAVI techniques and TAVI patient population, the current and past experience may provide only limited information to guide future practice. Persistent damage to the conduction system after TAVI can be due to damage to the AVN and His bundle causing complete heart block or damage to the proximal LBB presenting as new LBBB or presenting as complete heart block in the setting of pre-existing right bundle branch block (RBBB). It has been shown that such injury is mainly acute and presents fairly quickly after the procedure. Therefore, it can usually be recognized and treated appropriately with pacemaker implantation within the same hospitalization [Citation6,Citation10]. Therefore, management of persistent complete heart block post-TAVI implantation usually is not a clinical dilemma since it requires permanent pacemaker implantation. A central question that remains critical in this field, however, is the challenge of those patients who demonstrate partial damage to their conduction system.
The damage to the proximal LBB without complete heart block is not uncommon during TAVI procedures [Citation10]. Immediately after TAVI, this may present as new LBBB captured on the 12-lead electrocardiogram (ECG). Current guidelines from major cardiology and cardiac electrophysiology societies do not recommend implantation of pacemaker in asymptomatic patients with new LBBB, and cardiac resynchronization therapy may be indicated only in symptomatic patients with reduced left ventricular ejection fraction and LBBB [Citation11,Citation12]. In some of the post-TAVI patients, assessment of symptoms and relating them to one versus another of their cardiovascular diseases is a difficult task. Most patients with severe AS suffer from left ventricular hypertrophy and diastolic dysfunction with normal left ventricular ejection fraction. In addition, if the left ventricular ejection fraction is reduced, it may improve in some of these patients as a result of TAVI. Therefore, most patients with new LBBB post TAVI do not clearly meet conventional criteria for pacemaker implantation or cardiac resynchronization therapy.
The damage to the LBB during TAVI is usually at its very proximal portion as it is expected from the proximity of the valve deployment and the course of the LBB. Therefore, in post-TAVI patients with new LBBB, AV conduction mainly relies on conduction over AV node/His bundle and then to the right bundle branch (RBB). Therefore, whether the new LBBB progress to complete heart block mainly depends on two factors: (1) the potential recovery of the LBB or expansion of fibrosis and calcification in that area and (2) the integrity of the impulse conduction via AV node to the His bundle and RBB after TAVI. The likelihood of recovery of LBBB after TAVI is a relatively new question in clinical cardiology practice and there have not been many examples of similar conditions from which prior lessons can be applied to this particular question. A recently published review on this subject gathers many relevant studies and examines available data [Citation13]; however, there are many important questions that remain to be answered and therefore it is important to study this subject in the post-TAVI patients while keeping in mind the evolution of TAVI techniques and changes in the TAVI patient population. This may, therefore, limit the applicability of findings of such studies for future TAVI indications.
The progression of conduction disease from bundle branch block (BBB) to complete heart block has been studied and it has been shown that the progression from either RBBB or LBBB to complete heart block is rare [Citation14,Citation15], and those studies are basis for the reasons that major cardiology and cardiac electrophysiology societies do not generally recommend pacemaker implantation for BBB. Nevertheless, those studies were small in size and conducted long ago on patients who were younger and with considerably less cardiovascular comorbidities than today’s TAVI patients. Therefore, it may not be accurate to assume based on those studies that fragile elderly post-TAVI patients with several cardiovascular problems and new LBBB will not progress to complete heart block. The consequence of developing complete heart block for post-TAVI patients may be syncope and fall with major orthopedic or head trauma. The development of complete heart block may also lead to sudden cardiac death in this population. Therefore, it is reasonable to assume that the presence of RBBB in patients who develop LBBB post TAVI is associated with a higher risk of complete heart block in the future and a pacemaker implantation is indicated even if the new LBBB is transient. Predicting complete heart block in those with new LBBB but without apparent RBBB is a more difficult and critical question. A recently published study evaluated 75 consecutive patients who underwent TAVI [Citation10]. Electrophysiology testing was performed before and after TAVI in all patients. AV block occurred in five of the six patients with preprocedural RBBB, supporting the idea that the LBB is the main culprit of TAVI-induced conduction damage and the presence of RBBB is associated with considerably higher risk of developing complete heart block. In addition, 30 patients with normal baseline conduction developed new LBBB with 7 of them developing AV block later after the procedure. The investigators showed that more than a 13 ms increase in HV interval or an HV interval greater than 65 ms post TAVI are associated with considerably increased risk of developing complete heart block in patient with LBBB post TAVI [Citation10]. The HV interval in those patients was perhaps a surrogate of how robust conduction is over the RBB.
In conclusion, persistent conduction abnormalities post TAVI usually presents as either complete heart block or new LBBB. Complete heart block requires permanent pacemaker implantation. A critical medical issue in post-TAVI patients with new LBBB is to determine how reliable conduction is over the RBB. Investigations with vigorous and detailed assessment of RBB conduction, studying less-invasive surrogates of RBB conduction, such as ECG markers, and testing the findings in different TAVI techniques and in a prospective manner are important steps to demystify these important clinical questions.
Declaration of interest
The authors have no relevant affiliations or financial involvement with any organization or entity with a financial interest in or financial conflict with the subject matter or materials discussed in the manuscript. This includes employment, consultancies, honoraria, stock ownership or options, expert testimony, grants or patents received or pending, or royalties.
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