In ACLS, interpreting an ECG tracing means quickly classifying the rhythm into one of a few categories that determine your next action: shockable, non-shockable, fast, or slow. The goal isn’t a full 12-lead deep dive. It’s identifying the rhythm fast enough to choose the right algorithm. Every ACLS scenario begins with the same core question: what is the rhythm, and is the patient stable or unstable?
Whether you’re preparing for an ACLS exam or reinforcing what you learned in a provider course, here’s how to work through an ECG tracing systematically and recognize the rhythms that matter most.
The Systematic Approach to Rhythm Analysis
ACLS-level ECG interpretation follows a structured sequence so you don’t miss critical findings under pressure. Most teaching programs break this into roughly seven steps, each one narrowing the diagnosis.
- Rate. Is the heart beating at a normal rate (60 to 100 beats per minute), too fast (tachycardia), or too slow (bradycardia)? This single observation routes you into the correct ACLS algorithm immediately.
- Regularity. Are the QRS complexes evenly spaced? If the rhythm is irregular, is it chaotically irregular (think atrial fibrillation) or irregular in a repeating pattern (think certain heart blocks)?
- QRS width. A narrow QRS (under 0.12 seconds, or three small boxes on standard ECG paper) means the electrical signal is traveling normally through the ventricles. A wide QRS (0.12 seconds or greater) means the signal is either originating in the ventricles or taking an abnormal path, both of which change your treatment options.
- P waves. Are they present? Are they uniform and upright in Lead II? Missing P waves can point to atrial fibrillation, junctional rhythms, or ventricular rhythms. Abnormally shaped P waves may indicate the impulse is coming from somewhere other than the sinus node.
- PR relationship. Do the P waves and QRS complexes march together in a 1:1 ratio, or are they acting independently? Independent activity (AV dissociation) is a hallmark of complete heart block and a strong clue favoring ventricular tachycardia in wide complex rhythms.
- Onset and termination. If you catch the rhythm starting or stopping, an abrupt on-off pattern suggests a re-entrant circuit (like SVT), while a gradual speed-up and slow-down points to an automatic focus.
- Response to interventions. Vagal maneuvers or adenosine can help distinguish rhythms. Sinus tachycardia gradually slows during a vagal maneuver but speeds right back up. SVT often terminates abruptly. Ventricular tachycardia typically doesn’t respond at all.
In a cardiac arrest scenario, you won’t have time for all seven steps. You’re really answering two questions: is this a shockable rhythm, and is it organized? Outside of arrest, when the patient has a pulse, you work through the full sequence more carefully.
Shockable Rhythms: VF and Pulseless VT
The two shockable rhythms in ACLS are ventricular fibrillation (VF) and pulseless ventricular tachycardia (pulseless VT). Recognizing them on the monitor is the single most time-sensitive interpretation you’ll make, because defibrillation is the definitive treatment and every minute of delay reduces the chance of survival.
Ventricular fibrillation looks chaotic. The tracing shows rapid, grossly irregular waveforms with no identifiable P waves, QRS complexes, or T waves. The electrical rate is typically above 300 beats per minute, but nothing is organized enough to pump blood. On the monitor it appears as a wildly fluctuating, jagged baseline. Coarse VF has taller, more prominent waves and generally responds better to defibrillation than fine VF, which can look almost like a flat line.
Pulseless VT, by contrast, looks organized. You’ll see wide QRS complexes (0.12 seconds or greater) firing at a rate above 100 beats per minute, often much faster. Monomorphic VT has a uniform, repeating QRS shape from beat to beat. Polymorphic VT shows a QRS that changes shape continuously. Torsades de pointes is a specific type of polymorphic VT where the complexes appear to twist around the baseline. All of these are shockable when the patient has no pulse.
Non-Shockable Rhythms: Asystole and PEA
The two non-shockable arrest rhythms are asystole and pulseless electrical activity (PEA). Neither responds to defibrillation, so the treatment focus shifts to CPR, epinephrine, and identifying reversible causes.
Asystole is a flat line: no P waves, no QRS complexes, no T waves. It represents a complete absence of electrical activity in the heart. Before calling asystole, confirm it in more than one lead, check that your leads are attached, and increase the gain on the monitor. Fine VF can mimic a flat line, and misidentifying it means withholding a shock that could save a life.
PEA is trickier because the monitor shows an organized-looking rhythm, sometimes even something that resembles a normal sinus rhythm, but the patient has no palpable pulse. The electrical tracing alone doesn’t tell you it’s PEA. You have to pair what you see on the screen with what you feel (or don’t feel) at the carotid or femoral artery. PEA can present as any organized rhythm, from a slow, wide complex pattern to a relatively normal-looking tracing at 80 beats per minute. It can also deteriorate into asystole if the underlying cause isn’t corrected.
Tachycardia With a Pulse
When the patient has a pulse and the heart rate is fast, ACLS asks you to determine whether the tachycardia is causing hemodynamic instability (altered mental status, chest pain, hypotension, signs of shock). If it is, synchronized cardioversion takes priority regardless of the specific rhythm. If the patient is stable, you have more time to analyze the tracing and choose targeted treatment.
The key branch point is QRS width. A narrow QRS (under 0.12 seconds) tells you the rhythm is coming from above the ventricles: sinus tachycardia, atrial fibrillation, atrial flutter, or SVT. A wide QRS (0.12 seconds or greater) raises the possibility that the rhythm is ventricular tachycardia. The rate threshold that suggests a true tachyarrhythmia, rather than an appropriate response to pain or fever, is typically 150 beats per minute or above.
Narrow and regular often means SVT. Narrow and irregularly irregular almost always means atrial fibrillation. Narrow and irregular with a sawtooth baseline pattern suggests atrial flutter with variable conduction.
Wide Complex Tachycardia: VT vs. SVT With Aberrancy
Wide complex tachycardias present one of the most debated questions in ECG interpretation. The rhythm could be ventricular tachycardia or it could be a supraventricular rhythm conducting abnormally through the ventricles (called aberrancy, often from a bundle branch block). The distinction matters because the treatments differ, but in ACLS the practical rule is straightforward: if you aren’t sure, treat it as VT. Treating SVT as VT is safe. Treating VT as SVT can be fatal.
Several clues favor VT over SVT with aberrancy. AV dissociation, where P waves and QRS complexes march independently of each other, is strong evidence for VT. Fusion beats (a hybrid between a normal and a ventricular complex) and capture beats (a normal-looking QRS appearing briefly during the wide complex rhythm) also point toward VT. A QRS axis that falls into an unusual quadrant or QRS complexes wider than 0.16 seconds make VT more likely as well. Accurate assessment requires multiple leads, so a full 12-lead ECG is valuable when the patient is stable enough to obtain one.
Bradycardia Interpretation
A heart rate below 60 beats per minute triggers the ACLS bradycardia pathway if the patient is symptomatic. The ECG interpretation centers on where the conduction delay is occurring.
First-degree heart block shows a prolonged PR interval (greater than 0.20 seconds) but every P wave still conducts to a QRS. It’s usually benign and rarely needs treatment. Second-degree Type I (Wenckebach) shows a PR interval that progressively lengthens until a beat is dropped, then the cycle restarts. This is also often well-tolerated. Second-degree Type II is more concerning: the PR interval stays constant, but some P waves simply fail to conduct. The dropped beats are unpredictable and this type can progress to complete heart block without warning. Third-degree (complete) heart block shows P waves and QRS complexes firing at completely different rates with no relationship between them. The ventricles are beating on their own escape rhythm, often slowly and unreliably.
In ACLS, the symptomatic patient with bradycardia gets atropine first. If the rhythm is a high-grade block (Type II second-degree or third-degree), pacing becomes the priority because atropine is unlikely to work when the block is below the AV node.
ECG Clues to Reversible Causes
ACLS emphasizes searching for reversible causes of arrest, commonly taught as the H’s and T’s. Several of these leave fingerprints on the ECG tracing that can guide your treatment even before lab results come back.
Hyperkalemia produces a characteristic progression of changes as potassium levels rise. Early on, T waves become tall, narrow, and peaked. As potassium climbs further, the P wave flattens and may disappear entirely, the PR interval lengthens, and the QRS complex widens. In severe cases, the QRS and T wave merge into a smooth, sinusoidal wave pattern that can deteriorate into VF or asystole. Recognizing this sine-wave appearance on the monitor is a signal to treat for hyperkalemia immediately with calcium and other potassium-lowering strategies.
Hypothermia can produce Osborn waves (a distinctive hump at the end of the QRS complex) and a generally slow rate. Massive pulmonary embolism may show right heart strain patterns including a new right bundle branch block or the classic S1Q3T3 pattern, though these findings are neither sensitive nor specific in the arrest setting. Cardiac tamponade and tension pneumothorax won’t show specific ECG changes, but electrical alternans (a beat-to-beat variation in QRS height) can hint at a large pericardial effusion.
These findings won’t always be present, and their absence doesn’t rule out the condition. But when they do appear on the tracing, they give you a head start on treatment that can make the difference between return of spontaneous circulation and a prolonged, unsuccessful resuscitation.