4.3 VF and Pulseless VT Algorithm

4.3 VF and Pulseless VT Algorithm

Ventricular fibrillation (VF) and pulseless ventricular tachycardia (pVT) are the shockable arrest rhythms, and their algorithm is the most heavily tested ACLS pathway. The single most powerful intervention is early defibrillation layered on top of high-quality CPR; every other step — drugs, airway, reversible-cause search — supports those two.

The loop, step by step

The algorithm is a repeating 2-minute cycle:

1. Identify VF/pVT on the monitor.
2. Deliver one shock as soon as the team is clear. Resume compressions the instant the shock fires — do not pause to check a pulse.
3. CPR for 2 minutes (about 5 cycles of 30:2, or continuous compressions with an advanced airway). During this time, establish IV/IO access.
4. Rhythm check (about 10 seconds). If still VF/pVT, shock again.
5. After the second shock, give epinephrine 1 mg IV/IO, then repeat every 3-5 minutes.
6. After the third shock, give the first antiarrhythmic — amiodarone 300 mg IV/IO (or lidocaine).
7. Continue the loop, adding the second antiarrhythmic dose and continuing epinephrine on schedule, while searching the H's and T's.
8. Exit on ROSC (organized rhythm + pulse, or a sudden ETCO2 rise) to post-arrest care, or convert to the PEA/asystole pathway if the rhythm becomes nonshockable.

Defibrillation energy

Biphasic waveforms terminate fibrillation at lower energy with less myocardial damage, which is why nearly all modern devices are biphasic. Pad position (anterolateral or anteroposterior), good skin contact, and clearing the patient all matter for an effective shock.

ACLS drug doses for VF/pVT

Note that amiodarone and lidocaine are alternatives, not given together; the team picks one antiarrhythmic for refractory VF/pVT.

How this is tested and common traps

Exam stems typically place you at a specific point in the loop ("after the second shock, what drug?" or "the rhythm is still VF after three shocks and epinephrine — what next?"). Anchor your answers to the sequence: shock → CPR → epinephrine after shock 2 → amiodarone after shock 3 → repeat.

Common traps:

• Pausing for a pulse check immediately after a shock. You resume CPR instead; reassess at the next rhythm check.
• Giving an antiarrhythmic before any shock or epinephrine. Amiodarone/lidocaine are for refractory VF/pVT, after early shocks and epinephrine.
• Stacking shocks without intervening CPR (an outdated practice — single-shock strategy is current).
• Combining amiodarone and lidocaine — choose one.
• Forgetting the basics in refractory VF: check pad position/contact, compressor fatigue (swap every 2 minutes), and the H's and T's.

The governing principle: drugs and airway are adjuncts. If a choice would delay a shock or interrupt compressions, it is almost certainly the distractor.

A full two-cycle walkthrough

Tracing the algorithm minute by minute cements the sequence and the drug timing:

Notice that epinephrine repeats on its own 3-5 minute clock (here roughly every other cycle) while amiodarone is a one-time 300 mg then 150 mg sequence. Real timing is approximate because rhythm checks happen on the 2-minute boundary.

Single-shock strategy and CPR fraction

Current AHA practice uses a single-shock strategy — one shock, then immediate CPR — rather than the older stacked three-shock sequence, because resuming compressions quickly preserves coronary perfusion pressure. Teams should also protect a high chest-compression fraction (CCF), ideally at least 60-80% of code time spent actively compressing. Charging the defibrillator during compressions and pre-assigning the post-shock compressor are concrete ways to keep pauses near 5 seconds and the CCF high.

Refractory VF that persists after several shocks should prompt a deliberate re-check of pad position (consider switching to anteroposterior), pad-skin contact, compressor fatigue, and the reversible H's and T's — not simply more of the same.