2.3 Dysrhythmias, Cardiac Arrest, and ACLS

Cardiac Arrest: Shockable vs Non-Shockable

The ACLS arrest algorithms split on one question: is the rhythm shockable? High-quality CPR (push at least 2 inches deep, 100-120 compressions/min, full recoil, minimal interruptions) underlies every pathway.

Shockable rhythms - ventricular fibrillation (VF) and pulseless ventricular tachycardia (pVT):

1. Defibrillate immediately - 200 J biphasic (or device-specific; 360 J monophasic). Resume CPR right away for 2 minutes; do not pause to check a pulse after the shock.
2. After the second shock, give epinephrine 1 mg IV/IO every 3-5 minutes.
3. After the third shock, give amiodarone 300 mg IV/IO (second dose 150 mg), or lidocaine as an alternative.

Non-shockable rhythms - asystole and pulseless electrical activity (PEA):

• Do not defibrillate. Provide continuous high-quality CPR and give epinephrine 1 mg every 3-5 minutes as early as possible.
• Aggressively search for and treat reversible causes.

Reversible Causes: The Hs and Ts

For PEA and asystole especially, identifying a reversible cause is the difference between return of spontaneous circulation and a failed code.

For example, PEA in a dialysis patient suggests hyperkalemia - give calcium, insulin/dextrose, and bicarbonate. PEA with distended neck veins and tracheal deviation suggests a tension pneumothorax needing needle decompression. PEA after chest trauma with muffled heart sounds suggests tamponade needing pericardiocentesis. These cause-specific reversals are favorite CEN distractor sets.

Tachycardia and Bradycardia With a Pulse

Tachycardia (rate > 150) with a pulse: the first question is stability. Signs of instability are hypotension, acutely altered mental status, signs of shock, ischemic chest discomfort, or acute heart failure.

• Unstable + tachycardia: perform synchronized cardioversion.
• Stable narrow-complex regular (SVT): try vagal maneuvers first (Valsalva, carotid sinus massage), then adenosine 6 mg rapid IV push with a fast saline flush; if no conversion, give 12 mg. Adenosine briefly stops the heart and patients feel a frightening pause.
• Atrial fibrillation/flutter with rapid ventricular response: rate control with a calcium channel blocker (diltiazem) or beta-blocker if stable; cardiovert if unstable.

Symptomatic bradycardia: give atropine 1 mg IV (per the 2020 AHA update, increased from 0.5 mg), repeated every 3-5 minutes to a maximum of 3 mg. If atropine fails, start transcutaneous pacing or a dopamine (5-20 mcg/kg/min) or epinephrine (2-10 mcg/min) infusion. Atropine is less effective in high-degree (Mobitz II or third-degree) block, where pacing is the priority.

Reading the Strips: Rhythm Recognition

The CEN shows rhythm strips and expects rapid identification because the rhythm dictates the algorithm. Anchor your recognition on a few discriminators:

• Ventricular fibrillation: chaotic, irregular, no identifiable QRS - defibrillate.
• Ventricular tachycardia: wide, regular, fast QRS complexes; pulseless VT is defibrillated, while VT with a pulse follows the tachycardia algorithm.
• Asystole: a flat line - confirm in two leads and check lead/connection before calling it; never shock asystole.
• PEA: an organized rhythm on the monitor with no palpable pulse - the monitor lies, the patient is in arrest.
• Atrial fibrillation: irregularly irregular, no discernible P waves.
• Atrial flutter: sawtooth flutter waves, often a regular ventricular response.
• First-, second- (Mobitz I/II), and third-degree AV block: distinguished by the PR interval and the relationship of P waves to QRS complexes.

A tested trap is confusing fine VF with asystole; if in doubt about a flat-looking rhythm, confirm leads and consider it shockable VF rather than withholding defibrillation.

Post-Cardiac-Arrest Care

After return of spontaneous circulation (ROSC), the work is not over. The CEN tests the post-arrest bundle:

• Optimize oxygenation and ventilation: titrate to an SpO2 of 92-98% and a normal end-tidal CO2; avoid hyperoxia and hyperventilation.
• Support hemodynamics: treat hypotension (SBP < 90) with fluids and vasopressors to maintain perfusion.
• Obtain a 12-lead ECG: if STEMI is present, emergent coronary angiography is indicated.
• Targeted temperature management (TTM): for comatose patients, controlled temperature support improves neurologic outcomes.
• Identify and treat the underlying cause (the Hs and Ts) that precipitated the arrest.

These post-ROSC priorities frequently appear as "what is the next step" questions after a successful resuscitation.

Defibrillation versus cardioversion

A classic CEN distinction is defibrillation versus synchronized cardioversion. Defibrillation delivers an unsynchronized shock at full energy and is used for pulseless rhythms (VF and pulseless VT) - there is no organized QRS to synchronize to. Synchronized cardioversion times the shock to the R wave to avoid the vulnerable T-wave period (the R-on-T phenomenon that can induce VF) and is used for unstable tachycardias with a pulse (unstable SVT, atrial fibrillation/flutter, or VT with a pulse).

Pressing "sync" on a pulseless patient wastes time because the machine waits for a QRS that will not come; conversely, defibrillating a perfusing rhythm can provoke VF. Knowing which mode applies to which rhythm is a frequently tested decision point in this domain. A related caveat: polymorphic VT (torsades de pointes) in a pulseless patient is treated like VF with unsynchronized high-energy shocks, because the irregular, varying QRS morphology prevents reliable synchronization - and torsades specifically is treated with IV magnesium sulfate in addition to defibrillation when associated with a prolonged QT interval.