Bradyarrhythmias, heart block and pacing — formative SAQs

SAQ 1 (10 marks)

A newborn infant is found at the routine postnatal check to have a heart rate of fifty-five beats per minute. She is feeding reasonably well but is mildly mottled. The midwife attributes the slow rate to "a fit mother." A twelve-lead ECG shows P waves and QRS complexes that are independent of each other, with a narrow-QRS escape rhythm at fifty-five per minute. The mother has a history of systemic lupus erythematosus and is known to be anti-Ro positive. [2] [4]

1. Give the diagnosis, the mechanism, and explain why the damage is permanent. (3) [2] [4]
2. Outline the initial inpatient management, including the threshold for acute intervention and the pharmacological options. (3) [1] [6]
3. Describe the long-term management and prognosis, including the indication for a permanent pacemaker and the implications for future pregnancies. (4) [1] [3]

Model answer — SAQ 1

(1) Diagnosis and mechanism (3). The diagnosis is congenital complete (third-degree) atrioventricular block, most likely immune-mediated, given the maternal systemic lupus erythematosus and anti-Ro positivity. The mechanism is the transplacental passage of maternal immunoglobulin G anti-Ro/SSA (and anti-La) antibodies from approximately sixteen weeks of gestation, which bind the developing fetal atrioventricular node and His-Purkinje tissue, trigger inflammation and apoptosis, and culminate in fibrosis. The block is complete because scarred conduction tissue cannot conduct — the atria and ventricles beat independently, with a slow junctional escape rhythm holding the circulation. The damage is permanent because fibrotic tissue does not recover, so the child faces lifelong dependence on a pacemaker. The midwife's attribution of the bradycardia to maternal fitness is the error — a slow rate in a newborn is never normal and is complete heart block until an ECG proves otherwise. [2] [4]

(2) Initial inpatient management (3). The infant needs admission with continuous cardiac monitoring, intravenous access, a full assessment of perfusion and ventricular function, and an echocardiogram to exclude associated structural congenital heart disease and endocardial fibroelastosis. The mildly mottled appearance suggests she is becoming symptomatic from the slow rate. The threshold for acute intervention is any sign of haemodynamic compromise — poor perfusion, hypotension, heart failure, or a ventricular rate that is too slow for age. If she is unstable, atropine is unreliable for atrioventricular block because the blocked node does not respond to vagal blockade, so the bridge is an isoprenaline or epinephrine infusion to accelerate the intrinsic escape, and definitive acute support is transcutaneous pacing followed by transvenous pacing while a permanent system is planned. Maternal and infant anti-Ro/SSA and anti-La titres confirm the mechanism. [1] [6]

(3) Long-term management and prognosis (4). Because immune-mediated complete block is permanent, the long-term management is a permanent pacemaker. The PACES consensus and the ACCF/AHA/HRS guidelines place this infant in a Class I pacing category on the basis of her congenital third-degree block with symptomatic low cardiac output (mottling) and her infant age — the natural history of untreated congenital complete block includes Stokes-Adams syncope and sudden death, as established by Michaëlsson's prospective study of adults with isolated congenital block, which underpins prophylactic pacing even in the apparently asymptomatic child. Because she is small, an epicardial pacing system is the standard choice, with lifelong surveillance of generator battery, lead thresholds and impedance, and growth-related complications; Fortescue's work established that lead failure is driven by patient, procedural, and hardware factors in children. The prognosis with appropriate pacing is excellent — she is expected to grow, attend school, and be active. For future pregnancies, the mother must be counselled that anti-Ro positivity carries an approximately fifteen to twenty per cent recurrence risk of congenital heart block, and that serial fetal echocardiography from sixteen weeks of gestation is the surveillance that detects first- or second-degree block, the stages at which transplacental treatment may halt progression. [1] [3]

SAQ 2 (10 marks)

A three-year-old boy is admitted to the paediatric intensive care unit three days after surgical repair of an atrioventricular septal defect. The cardiac monitor shows a slow rate with sudden dropped beats, a constant PR interval where present, and a wide-QRS escape rhythm at forty-five per minute. He is drowsy and his blood pressure is borderline. [2] [6]

1. Interpret the rhythm, classify the atrioventricular block, and explain the immediate priority. (3) [1] [2]
2. Describe the acute management of the unstable bradycardic child, including the role and limitations of atropine. (3) [1] [6]
3. Explain the decision rule for permanent pacing in this child, the time window, and the rationale for it. (4) [1] [6]

Model answer — SAQ 2

(1) Rhythm interpretation and priority (3). This child has post-operative atrioventricular block — the constant PR interval with suddenly dropped beats and a wide-QRS escape rhythm is Mobitz II second-degree atrioventricular block progressing toward high-grade or complete block, a recognised complication of atrioventricular septal defect repair where the sutures and patch sit immediately adjacent to the atrioventricular node and His bundle. The wide-QRS escape indicates an infranodal pacemaker, which is slower and less reliable than a junctional escape, and his drowsiness and borderline blood pressure mean he is haemodynamically compromised. The immediate priority is to support his circulation with pacing and treat the bradycardia, because an unstable escape rhythm can fail altogether and produce a Stokes-Adams arrest. [1] [2]

(2) Acute management (3). The resuscitation is the standard paediatric approach with one critical modification: the unstable blocked child is paced early. Support the airway and breathing, confirm intravenous access, attach continuous cardiac monitoring, and obtain a twelve-lead ECG with a rhythm strip. Atropine at 0.02 milligrams per kilogram (minimum 0.1 milligram, maximum 0.5 milligram per dose) is effective for sinus bradycardia driven by high vagal tone, but it is unreliable for atrioventricular block because the blocked node does not respond to vagal blockade — so atropine 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, but 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. He is monitored continuously in intensive care. [1] [6]

(3) Permanent pacing decision (4). The decision rule for post-operative atrioventricular block 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 this point has almost certainly been permanently injured — transected or irreversibly scarred by the surgical repair — 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. Until that window closes, the team observes for recovery of conduction on serial ECGs with the temporary system in place; once the window closes without recovery, a permanent pacemaker is implanted, with the system chosen for his size and anatomy — in a three-year-old, typically an epicardial system. [1] [6]