Consultant Opinion #1

Akash R. Patel, MD

Ronn E. Tanel, MD

This patient illustrates the challenges with regard to the evaluation and management of acquired AV conduction disease after congenital heart surgery. Although the patient’s heart block occurred after closure of a perimembranous ventricular septal defect, the discussion points below can be more generally applied to acquired postoperative AV conduction disease. In addition, rarely, tachyarrhythmias may arise in patients with postoperative AV block that occurs as a result of corrective congenital heart surgery (i.e., new scar), underlying anatomy, development of noncardiac diseases that may affect the AV conduction system, and bradyarrhythmias.

Currently, the overall incidence of postoperative heart block is between 1% and 3% for all congenital heart surgery. The heart defects most commonly associated with the occurrence of acquired AV conduction disease include surgical repair of certain types of ventricular septal defects, aortic or AV valve surgery, septal myomectomy, and congenital defects with abnormal or displaced AV conduction tissues (i.e., congenitally corrected transposition of the great arteries, AV canal defects, and atrial isomerism). This patient demonstrated no additional anatomic risk factors for early or late development of AV conduction disease based on anatomic considerations besides surgical repair of the perimembranous ventricular septal defect. In addition, the majority of postoperative AV block is transient with 43%–95% recovering in the first 7–10 days. This patient initially had resolution of transient complete heart block within 4 days in the absence of residual conduction disease or symptoms (i.e., unexplained syncope), thus not necessitating the need for placement of pacemaker.

Over time, this patient demonstrated progressive conduction disease with the development of second-degree AV block and bifascicular block that would warrant pacing. This finding is not surprising as a small portion of patients (9%–16%), who develop transient postoperative heart block that resolves will go on to develop late-onset heart block. AV node recovery can be seen late beyond 14 days which has been reported in up to 10%–32% of patients. However, these patients whose early postoperative complete heart block lasts beyond 7 days are at highest risk of late-onset AV block. The presence of early recovery still warrants careful follow-up evaluation including clinical and electrocardiographic monitoring.

The identification of significant and progressive AV conduction disease based on clinical electrocardiography over the course of this patient’s postoperative history provided data to support the initiation of pacemaker therapy. In addition, the use of invasive testing was used to confirm clinical electrocardiographic concerns of pathologic AV conduction disease. The presence of untreated permanent postoperative complete heart block places patients at significant risk of symptoms and mortality (28%–100%).

Finally, the presence of ventricular arrhythmias in the setting of complete heart block is a rare but known occurrence. These life-threatening proarrhythmic events can be a result of several factors including consequences of surgery—ventricular scar or anatomic barriers due to surgical repair techniques—leading to reentrant VT, premature ventricular ectopy-induced ventricular arrhythmias, prolonged QT-dependent torsade de pointes, and pause-dependent ventricular arrhythmias. The lack of pacing makes management of proarrhythmia in this patient challenging and more importantly may be the primary modality for treatment.

Acquired AV conduction disease after congenital heart surgery resolves in the majority. However, this case illustrates the nontrivial aspect that even temporary AV nodal injury can have long-term implications including ongoing conduction and arrhythmia issues. This raises several important questions regarding optimal monitoring and evaluation, timing and indications for intervention, and appropriate treatment of unexpected arrhythmias.

• 1.
  

  Investigations in the patient with atrioventricular conduction disturbance

Clinical electrocardiography is the mainstay of investigating postoperative AV conduction disease. The resting electrocardiogram provides the initial evaluation of conduction disease. The preoperative ECG is important to determine underlying conduction disease that can be present including congenital complete heart block that should not be attributed to surgery and carries different implications. In this case, there was a normal preoperative electrocardiogram. The additional time points for monitoring include immediately postoperative to determine early-onset AV conduction disease, continuous telemetry monitoring after surgery, and during follow-up evaluations as AV conduction disease can be paroxysmal, progressive, or variable in recovery or development. Additionally, follow-up ECGs should be used during evaluation of symptoms such as unexplained syncope and during routine follow-up to identify significant asymptomatic conduction disease as was evident here. In particular, chronic bifascicular block carries a small risk of progressing to advanced second-degree AV or complete heart block.

Ambulatory electrocardiographic monitoring provides additional information that resting electrocardiograms cannot, as AV conduction disease can be paroxysmal and asymptomatic. The choices of ambulatory monitoring technique vary based on modality of recording–event, continuous or autotriggered—and duration from <1 min for event monitors to >1 year for implantable loop recorders (ILRs). The choice of monitoring is dependent on symptoms and/or surveillance evaluation. The monitor used here was a Holter monitor, which can be used for surveillance of pathologic AV block and should be considered as part of routine follow-up with postoperative AV block or chronic bifascicular block. The presence of first- and second-degree AV blocks should be evaluated to determine whether this is physiologic and associated with sleep or increased vagal tone or if it occurs during awake hours and/or faster heart rates which would raise concern for underlying AV conduction disease. In addition, in younger children who are unable to perform exercise testing, ambulatory electrocardiographic monitoring may be used to demonstrate rate-related AV conduction disease that may result in impaired AV synchrony and chronotropic incompetence.

• 2.
  

  Indications for loop-recorded implantation and electrophysiological study

The particular role of an ILR in these patients is unclear. This patient already demonstrated significant AV conduction disease with noninvasive monitoring making the use of the ILR unnecessary. However, in patients with unexplained syncope with prior history of AV block and/or residual conduction disease (i.e., bifascicular block) the use of an ILR may be considered if other forms of ambulatory monitoring are unrevealing. In addition, the later development of palpitations and presyncope may have been identified earlier as due to polymorphic VT by using an ILR.

Exercise stress testing can be important for the evaluation of rate-related AV conduction disease. This should be considered in patients with symptoms or who are unable to achieve fast physiologic rates on Holter monitoring. The patient here was unable to comply with the exercise stress test.

The role of invasive evaluation of AV conduction disturbances with an electrophysiology study is limited in this case, as the clinical electrocardiographic findings have already demonstrated significant AV conduction abnormalities. However, electrophysiology study could be considered to assess level of conduction disease (intranodal vs. infra-Hisian) in situations that are unclear, such as 2:1 AV block or to exclude pseudo AV block due to premature, concealed junctional depolarizations suspected to cause a second- or third-degree AV block pattern. This patient demonstrated significant infra-Hisian disease (HV > 100 ms) which, if untreated, has a poor prognosis, as patients with this finding progress to higher degrees of AV block and become symptomatic with syncope.

• 3.
  

  Time and indications for pacemaker implant in congenital heart disease

The decision and timing for pacemaker implantation in congenital heart disease for postoperative AV conduction disease has been well established. For postoperative advanced second- or third-degree AV block that is not expected to resolve or that persists at least 7 days after cardiac surgery, a permanent pacemaker is a Class I indication. This patient had recovery of AV node function at 4 days, thus deferring the need for pacemaker implantation unless there was transient postoperative third-degree AV block that reverted back to sinus rhythm with residual bifascicular block (Class IIB indication). However, the evidence for pacemaker therapy in this context is less clear. In the absence of transient AV block, postoperative bifascicular block with or without first-degree block does not warrant pacemaker therapy.

Once there is demonstration of higher degree AV block (Mobitz type II second-degree or higher) late after cardiac surgery, as was seen at 3 years of age, pacemaker therapy should be considered. The presence of asymptomatic Mobitz type II second-degree AV block with a narrow QRS complex would warrant pacemaker therapy (Class IIa) when not occurring during sleep or not thought to be vagally mediated. The presence of bifascicular block with a wide QRS, asymptomatic Mobitz type II second-degree AV block with a wide QRS, including isolated right bundle branch block, as seen at 5 years of age, would be a Class I indication for pacemaker therapy. The EP study provides further evidence for pacemaker implantation with asymptomatic second-degree AV block at intra- or infra-His levels IIA (HV > 100 ms) or if pacing-induced infra-Hisian block is seen (Class IIa).

Finally, The presence of advanced second-degree or third-degree AV blocks at any anatomic level that is associated with bradycardia and symptoms (including heart failure) or ventricular arrhythmias presumed to be due to AV block should warrant pacemaker therapy (Class I). At an age of 16 years, this patient developed episodes of symptomatic runs of polymorphic VT.

The indications for pacemaker therapy in the setting of postoperative AV conduction disturbances vary based on timing relative to surgery, symptoms, presence or absence of cardiac dysfunction or ventricular arrhythmias, and extent of conduction disease at the time a decision is undertaken. Additional factors to consider after the decision for pacemaker therapy is determined are the age and size of patient which have implications on type of device—single versus dual chamber and transvenous versus epicardial. The lack of symptoms had an impact on the parents’ reluctance to proceed with pacemaker therapy but the long-term implications without therapy are clear, as demonstrated by the development of life-threatening polymorphic ventricular arrhythmias.

• 4.
  

  Use of antiarrhythmic drugs in patients with atrioventricular conduction disturbance

Ventricular arrhythmias in patients with postoperative AV conduction disease (advanced second-degree or third-degree AV block) after congenital heart surgery warrant immediate attention and initiation of pacemaker therapy as outlined above. The initiation of pacing can treat potential proarrhythmic factors such as bradycardia, pauses, ectopy suppression if paced at faster rates, and shortening and stability of the QT interval with uniform ventricular pacing. The use of antiarrhythmics in the setting of AV conduction disease is challenging and problematic and is thus not generally considered a first-line therapy. These agents may exacerbate AV conduction, slow the ventricular escape, induce or prolong pauses, and cause or exacerbate symptoms as a result of bradycardia, thus limiting their efficacy and potentially worsening the problem. Acutely, if temporary pacing cannot be initiated, isoproterenol can be used to increase the heart rate, minimize bradycardia, and pause-dependent events, and shorten the QT interval to reduce the risk of torsade de pointes. If after initiating pacemaker therapy, there continues to be tachyarrhythmias, identification of other substrates should be considered in postoperative congenital heart surgery patients, as medications and/or ablation may be required.