During resuscitation of a patient in refractory VF, the team considers double sequential defibrillation (DSED). What is the theoretical basis for this technique?

• A Two shocks generate sufficient heat to ablate the re-entrant VF focus
• B The first shock depolarises a critical mass of myocardium; the second, delivered milliseconds later, terminates residual fibrillation wavelets in newly repolarised tissue
• C Sequential shocks depolarise the whole myocardium including regions at the refractory period boundary that a single shock cannot reach ✓
• D Two defibrillators provide lower impedance path, increasing current density through the myocardium

Explanation

Standard defibrillation fails in refractory VF partly because the delivered shock, while depolarising much of the myocardium, cannot effectively depolarise tissue in its absolute refractory period or in regions of the myocardium lying in the periphery of the electrical field. DSED uses two separate defibrillator-pad vectors almost simultaneously, extending the electrical field to depolarise myocardial regions inaccessible to a single-vector shock, thus terminating the fibrillation wavefronts. The DOSE VF trial (2022) demonstrated superiority of DSED over standard defibrillation for refractory VF. Option B describes the critical mass hypothesis but incorrectly emphasises residual wavelets in repolarised tissue; the simultaneous field extension concept (option C) is the primary mechanism.

Reference: Morgan & Mikhail's Clinical Anesthesiology, 6th ed.

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Written and medically reviewed by the StethoPrep medical team.