Defibrillation, cardioversion and transcutaneous pacing — branching viva

Branching viva — defibrillation, cardioversion and pacing

The examiner releases the stem and then branches into five probes. A strong candidate answers the mode and dose first, defends the physiology, places the pads correctly, runs the drug sequence, and contrasts the two other electrical therapies without prompting. [1] [6]

Opening (examiner)

"An eight-year-old collapses in the waiting room. Cardiopulmonary resuscitation is in progress. The rhythm strip shows ventricular fibrillation. There is no pulse. Walk me through what you do." [1]

Branch 1 — Mode, dose and delivery (expected answer)

This is a shockable arrest rhythm, so deliver an unsynchronised shock — SYNC off — at 4 joules per kilogram, biphasic. The synchronisation function is not used because ventricular fibrillation has no organised R wave; leaving SYNC on would stop the machine from firing. Apply pads anterolateral or anteroposterior, charge while compressions continue, call stand clear, deliver, and resume compressions immediately without a pulse check. [1] [2]

Probe. "Why 4 joules per kilogram?" — The dose is weight-based because transthoracic impedance, the resistance the shock must overcome, scales with chest size relative to mass; 4 joules per kilogram is the biphasic standard taught by the European Resuscitation Council and APLS, and the American Heart Association accepts an initial 2 to 4 joules per kilogram. [1] [3]

Branch 2 — The shockable-arrest algorithm (expected answer)

After the shock, resume cardiopulmonary resuscitation for two minutes and reassess. If ventricular fibrillation persists, deliver a second shock at 4 joules per kilogram, then give adrenaline 10 micrograms per kilogram and repeat it every three to five minutes. If a third shock is needed, give amiodarone 5 milligrams per kilogram after the shock and flush. Continue cycles of shock, two minutes of compressions, and reassessment, treating reversible causes throughout. [1]

Probe. "Why no pulse check immediately after the shock?" — A shock can stun the myocardium, and the mechanical pump must keep the brain and coronaries perfused while the rhythm recovers; the pulse is reassessed after the next two-minute cycle. [1]

Branch 3 — Physiology (expected answer)

Ventricular fibrillation is a re-entry phenomenon: disordered wavelets prevent coordinated contraction, cardiac output is zero, and the shock depolarises a critical mass of myocardium at once to extinguish every wavelet and hand the sinus node a silent heart it can recapture. [3] [6]

Probe. "What is the vulnerable period, and why does it matter for cardioversion?" — The upstroke of the T wave is the interval in which a stray stimulus fragments an organised rhythm into ventricular fibrillation — the R-on-T phenomenon. A perfusing tachyarrhythmia has an R wave, so the defibrillator synchronises the discharge to it and dodges the T wave. [1] [6]

Branch 4 — Distinguishing cardioversion and pacing (expected answer)

If the child instead had a perfusing tachyarrhythmia such as supraventricular tachycardia, the shock would be synchronised at 1 joule per kilogram, escalating to 2 joules per kilogram, with SYNC on. If the child had symptomatic bradycardia with complete heart block unresponsive to adrenaline, the treatment would be transcutaneous pacing at a rate of 80 to 100 per minute, increasing the output until electrical and mechanical capture, with analgesia. [4] [5]

Probe. "The child is under one year and only an automated external defibrillator is available — what now?" — A manual defibrillator is preferred so 4 joules per kilogram can be delivered exactly, but an automated external defibrillator is acceptable; use attenuated paediatric pads, and place them anteroposterior if the chest is too small for anterolateral. [8]

Branch 5 — Complications (expected answer)

Name and prevent them: the mode error of leaving SYNC on for ventricular fibrillation (the machine never fires) or off for a perfusing tachyarrhythmia (an R-on-T shock provokes ventricular fibrillation); skin burns from poor pad coupling or arcing between touching pads; myocardial injury from high or repeated shocks; omitted drugs in the algorithm; and for pacing, failure to capture and treating the monitor rather than the child. [1] [5]

Examiner's wrap

The pulse and the rhythm decide the therapy. Outcome is driven by the underlying cause and the speed of resuscitation, not the shock alone; the shock buys the chance of a perfusing rhythm, and definitive cardiology care decides survival. [1] [6]