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]}