Bradyarrhythmias, heart block and pacing — branching viva

Examiner opening (Examiner)

You are the paediatric registrar on the neonatal unit. A two-day-old infant has a routine postnatal heart rate of fifty-five beats per minute. She is mildly mottled and feeding slowly, and the midwife has noted that the mother is "a runner" and attributed the slow rate to inherited fitness. The mother has systemic lupus erythematosus and is known to be anti-Ro positive. A twelve-lead ECG shows atrial and ventricular complexes that are independent of each other, with a narrow-QRS escape rhythm at fifty-five per minute. Talk me through your assessment and management. [2]

Exemplar opening (Candidate)

This infant has congenital complete (third-degree) atrioventricular block, almost certainly immune-mediated given the maternal systemic lupus erythematosus and anti-Ro positivity, and the midwife's attribution to inherited fitness is the classic error — a slow rate in a newborn is never normal and is complete heart block until an ECG proves otherwise. The ECG shows atrioventricular dissociation with a narrow-QRS junctional escape rhythm, which is the expected escape from below the blocked AV node. Her mottling and slow feeding mean she is becoming symptomatic from the low rate. My immediate plan is to admit her with continuous cardiac monitoring and intravenous access, obtain an echocardiogram to exclude associated structural congenital heart disease and endocardial fibroelastosis, send maternal and infant anti-Ro and anti-La titres, and assess whether she needs acute rate support or pacing while a permanent pacemaker is planned. [2] [4]

Branch 1 — the mechanism and why it is permanent (Examiner)

Explain the mechanism of immune-mediated congenital complete heart block, and tell me why the damage is permanent. [2]

Exemplar (Candidate)

The mechanism is the transplacental passage of maternal immunoglobulin G anti-Ro/SSA — also called anti-SSA, targeting the 52-kilodalton and 60-kilodalton Ro antigens — and anti-La antibodies, which begin to cross the placenta in significant quantities from approximately sixteen weeks of gestation. The antibodies bind the developing fetal atrioventricular node and His-Purkinje tissue, where they trigger an inflammatory cascade, apoptosis, and ultimately fibrosis of the conduction system. The conduction tissue is most vulnerable between eighteen and twenty-four weeks, which is why fetal heart block typically declares on serial echocardiography in the second trimester in an anti-Ro positive pregnancy. The damage is permanent because fibrotic conduction tissue does not recover — once scarred, the AV node cannot conduct, and the child faces lifelong dependence on a pacemaker. This irreversibility is the whole justification for definitive pacing rather than expectant management. [2] [4]

Branch 2 — classifying atrioventricular block on the ECG (Examiner)

How do you classify atrioventricular block on the ECG, and which patterns matter most for the pacing decision? [2]

Exemplar (Candidate)

I classify atrioventricular block by degree and by morphology. First-degree block is a PR interval beyond the age-adjusted upper limit in which every atrial impulse still conducts — it is usually benign and does not need pacing. Second-degree block is intermittent failure of conduction and has two subtypes. Mobitz I, or Wenckebach, shows progressive PR prolongation until a beat is dropped, is usually at the AV node with a narrow QRS, and is generally benign in children — it does not need pacing unless symptomatic. Mobitz II has a constant PR interval with suddenly dropped beats, is often infranodal with a wide QRS, and carries a real risk of progression to complete block — it is a pacing indication even if the child is asymptomatic. Third-degree, or complete, block is total dissociation of atrial and ventricular activity with a slow escape rhythm, and it is the pattern that most often mandates a device. The two patterns that matter most for the pacing decision are Mobitz II and complete block — and the key bedside skill is distinguishing Mobitz I from Mobitz II, because they look superficially similar and have opposite implications. [2] [5]

Branch 3 — the unstable bradycardic child (Examiner)

Suppose this infant deteriorates on the unit — she becomes more mottled and her blood pressure drops. How do you manage an unstable bradycardic child? [1]

Exemplar (Candidate)

The resuscitation is the standard paediatric approach with one critical modification — the unstable bradycardic child with atrioventricular block is paced early, because the blocked node does not respond to atropine. I would support the airway and breathing, confirm intravenous access, ensure continuous cardiac monitoring, and obtain a twelve-lead ECG. I would identify and treat reversible contributors — hypoxia, hypoglycaemia, electrolyte disturbance, drug toxicity. Atropine at 0.02 milligrams per kilogram, minimum 0.1 and maximum 0.5 milligrams per dose, is effective for sinus bradycardia driven by high vagal tone but is unreliable for atrioventricular block, so it must not delay pacing. Epinephrine at 0.01 milligrams per kilogram intravenously and an isoprenaline infusion support the rate and blood pressure as a bridge. The definitive acute intervention is pacing — transcutaneous pacing through chest pads is the fastest to deploy, then transvenous pacing through a temporary wire into the right ventricle for more reliable and comfortable sustained support, which is the standard bridge while the permanent decision is awaited. [1] [6]

Branch 4 — the permanent pacing decision and system choice (Examiner)

What is the indication for a permanent pacemaker in this infant, and how do you choose the system? [1]

Exemplar (Candidate)

Because immune-mediated complete block is permanent, this infant meets a Class I indication for a permanent pacemaker on the basis of congenital third-degree atrioventricular block with symptomatic low cardiac output (her mottling) and her infant age. The Michaëlsson prospective study established that the natural history of untreated congenital complete block includes Stokes-Adams syncope and sudden death, which underpins prophylactic pacing even in the apparently asymptomatic child. In terms of the system, because she is small, an epicardial pacing system with leads placed on the surface of the heart is the standard choice — it avoids the thromboembolic risk of a transvenous lead across a potential intracardiac shunt and accommodates her growth. Transvenous systems are reserved for larger children and adolescents with a structurally normal or repaired heart without residual shunt, and leadless pacemakers are an emerging option in selected older children and adolescents that address the long-standing problems of lead fracture and venous occlusion. Where the sinus node is intact, an atrial-based or dual-chamber approach is preferred to preserve atrioventricular synchrony and reduce the long-term risk of pacing-induced cardiomyopathy. [1] [3]

Branch 5 — post-operative AV block and the seven-to-ten-day rule (Examiner)

Now suppose a different child — a three-year-old after repair of an atrioventricular septal defect — has Mobitz II block on the monitor three days post-operatively. What is your decision rule? [1]

Exemplar (Candidate)

Post-operative atrioventricular block complicates repairs near the atrioventricular node and His bundle, of which atrioventricular septal defect repair is a classic example, and it presents in the intensive care unit in the days after surgery with dropped beats or dissociation on the monitor. The decision rule is to maintain temporary pacing and to implant a permanent pacemaker if normal conduction has not returned by seven to ten days after surgery. The reasoning is that conduction tissue that has not recovered by then has almost certainly been permanently injured — transected or irreversibly scarred — and the residual risk of late sudden death from unrecognised complete block outweighs the burden of a device. This seven-to-ten-day window is one of the most examined facts in paediatric pacing and is common to the PACES, ACCF/AHA/HRS, and ESC documents, all of which classify persistent post-operative second- or third-degree atrioventricular block beyond this window as a Class I indication for permanent pacing. In the meantime, the child is monitored continuously with the temporary system in place and observed for recovery on serial ECGs. [1] [5]

Branch 6 — long-QT syndrome with 2:1 block (Examiner)

Tell me about an infant who presents with 2:1 atrioventricular block. What are you thinking? [2]

Exemplar (Candidate)

An infant with 2:1 atrioventricular block has long-QT syndrome until proven otherwise. The mechanism is not conduction disease but extreme repolarisation prolongation — the QT interval is so prolonged that every alternate atrial impulse arrives while the ventricles are still refractory and is not conducted, producing a functional 2:1 block and a slow rate. The diagnosis hinges on a careful manual measurement of the QT interval, which is frequently overlooked because the eye is drawn to the dropped beats. The management is beta-blockade and genotype-specific care for the underlying channelopathy rather than pacing in isolation — though a pacemaker may be needed if the beta-blocker-induced bradycardia aggravates the QT interval. This is a critical distinction because pacing alone, without addressing the repolarisation disorder, does not protect the child from torsades and sudden death. [2] [5]

Examiner wrap-up (Examiner)

Thank you. Summarise the three points you most want the examiner to remember. [1]

Exemplar (Candidate)

First, a symptomatic bradycardic child has complete heart block until proven on a twelve-lead ECG — never attribute a slow rate to fitness without an ECG, and resuscitate the unstable blocked child with early transcutaneous and transvenous pacing, because atropine does not fix atrioventricular block. Second, congenital complete heart block is most often immune-mediated — maternal anti-Ro antibodies scar the fetal AV node irreversibly — so the child needs a permanent pacemaker, and Michaëlsson's prospective data on the natural history of untreated block underpin prophylactic pacing even in the asymptomatic child. Third, post-operative atrioventricular block persisting beyond seven to ten days is permanent and is a Class I pacing indication — one of the most examined facts in paediatric pacing, and the rule that governs the most common acquired paediatric AV block. [1] [3]