Defibrillation, cardioversion and transcutaneous pacing — formative SAQs

SAQ 1 — Defibrillation of ventricular fibrillation (15 marks, 15 minutes)

An eight-year-old weighing 25 kilograms collapses in the waiting room. Cardiopulmonary resuscitation is in progress. The monitor shows ventricular fibrillation. There is no pulse. [1] [2]

Question. Describe your immediate management of this shockable cardiac arrest rhythm. State the shock mode, the energy dose, the sequence of shocks and drugs, and the pad placement you would use. Explain the physiology that justifies the dose and the immediate resumption of compressions.

Model answer

Mode and energy (3 marks). Ventricular fibrillation is a shockable arrest rhythm and requires an unsynchronised shock — SYNC must be off. Deliver the first biphasic shock at 4 joules per kilogram, which for this child is 100 joules (4 times 25). The synchronisation function is not used because ventricular fibrillation has no organised R wave for the machine to lock onto; leaving SYNC on would prevent the shock from firing altogether. [1] [2]

Sequence of shocks and drugs (4 marks). Resume cardiopulmonary resuscitation immediately after the shock without a pulse check, for two minutes, then reassess the rhythm. If ventricular fibrillation persists, deliver a second shock at 4 joules per kilogram. Give adrenaline 10 micrograms per kilogram (0.1 millilitre per kilogram of 1 in 10,000) after the second shock and repeat it every three to five minutes. If a third shock is required, give amiodarone 5 milligrams per kilogram after the shock, followed by a flush. Reassess after each two-minute cycle of compressions. [1]

Pad placement (3 marks). Apply self-adhesive pads anterolateral — one to the right of the sternum below the clavicle and the other on the left mid-axillary line at the level of the nipple — or anteroposterior, with one pad on the anterior chest and the other on the back between the scapulae. Either is effective; the pads must be firm and gel-coupled, must not touch each other, and must be clear of an implanted device. Charge while compressions continue, call stand clear, confirm no contact, and deliver. [1]

Physiology (3 marks). Ventricular fibrillation is a re-entry phenomenon in which disordered wavelets prevent coordinated contraction and cardiac output is zero. The shock depolarises a critical mass of myocardium at once, extinguishing every wavelet and handing the sinus node a silent heart it can recapture. The dose must overcome transthoracic impedance, which is higher relative to mass in children and falls with successive shocks. Compressions resume at once because a shock may transiently stun the myocardium, and the mechanical pump must keep the brain and coronaries perfused while the rhythm recovers. [3]

Transatlantic nuance (2 marks). The American Heart Association accepts an initial dose of 2 to 4 joules per kilogram with escalation to up to 10 joules per kilogram; the European Resuscitation Council and APLS teach 4 joules per kilogram from the first shock. Either is defensible; the taught Australasian standard is 4 joules per kilogram. [1] [2]

SAQ 2 — Cardioversion versus transcutaneous pacing (12 marks, 12 minutes)

Two children arrive in the resuscitation bay. The first is a six-month-old infant with a narrow-complex tachycardia at 240 per minute, pale and mottled, in whom vagal manoeuvres and two doses of adenosine have failed. The second is a neonate with a heart rate of 55 and complete heart block on a maternal anti-Ro background, poorly responsive to adrenaline. [4] [5]

Question. Contrast the cardiac electrical therapy you would use for each child. For each, state the indication, the mode, the energy or pacing setting, the pad placement, and one key complication to prevent. Explain why the two treatments differ.

Model answer

Infant — synchronised cardioversion (5 marks). The infant has a perfusing tachyarrhythmia (supraventricular tachycardia) with haemodynamic compromise, and drug therapy has failed. The treatment is synchronised cardioversion at 1 joule per kilogram, escalating to 2 joules per kilogram. SYNC must be on so the machine times the discharge to the R wave and avoids delivering the shock on the vulnerable T wave, which would risk an R-on-T shock provoking ventricular fibrillation. Place pads anteroposterior in the small infant so they do not touch, sedate if time permits but do not delay, and watch the sync markers flash on each R wave before discharging. The complication to prevent is a mode error — shocking in unsynchronised mode and precipitating ventricular fibrillation. [1] [4]

Neonate — transcutaneous pacing (5 marks). The neonate has symptomatic bradycardia from complete heart block that has not responded to adrenaline, after reversible causes (hypoxia, hypothermia, drugs) are excluded and addressed. The treatment is transcutaneous pacing: place pads anteroposterior, set the rate near 80 to 100 per minute, and increase the output in milliamperes from zero until each pacing spike is followed by a captured QRS (electrical capture) and a palpable pulse (mechanical capture). A conscious or lightly sedated child needs analgesia because each pacing stimulus is painful. Pacing is a bridge to a transvenous wire and to definitive management of the conduction disease, which may be a permanent pacemaker. The complication to prevent is failure to recognise failed capture — a spike without a QRS, or electrical capture without a pulse. [5]

Why they differ (2 marks). The mode follows the rhythm and the pulse. A perfusing tachyarrhythmia has an R wave to synchronise to, so a timed shock terminates it safely. A bradycardia from conduction failure is not a re-entry rhythm to be shocked; it needs an external pacemaker to take over the role of the absent intrinsic pacemaker, stimulus by stimulus. The unifying principle is that the pulse and the rhythm, not the device, decide the therapy. [1] [5]