What Rhythms Do I Need to Know for ACLS?

ACLS decisions start with rhythm interpretation. Whether it’s identifying shockable rhythms during cardiac arrest or spotting unstable bradycardia, the ECG tells you what to do next. Your ability to read it quickly can make the difference between hesitation and life-saving action. In ACLS, rhythm recognition plays a central role in guiding interventions during cardiac emergencies. From knowing when to defibrillate to understanding when medications or pacing are needed, reading the rhythm correctly ensures the right actions are taken.

Whether you’re preparing for your ACLS certification or refreshing your knowledge, this guide covers the essential rhythms you need to recognize to provide effective, evidence-based care under pressure.

Core ACLS Rhythms to Know

ACLS divides rhythms into shockable and non-shockable categories, as well as organized rhythms that guide treatment in bradycardia or tachycardia algorithms. Here’s a breakdown of the core rhythms you need to know to be prepared for an emergency

1. Shockable Rhythms

Shockable rhythms are life-threatening arrhythmias that respond to defibrillation. The two shockable rhythms in ACLS are:

Ventricular Fibrillation (VF)

Ventricular Fibrillation is a chaotic, disorganized rhythm that results in no effective cardiac output. The ventricles quiver instead of contracting properly, which means the heart can’t pump blood. VF is always treated with immediate defibrillation, followed by high-quality CPR and medications such as epinephrine and amiodarone to support rhythm stabilization.

Pulseless Ventricular Tachycardia (VT)

Pulseless VT presents as wide, rapid QRS complexes on an ECG, but there’s no palpable pulse. It’s treated the same way as VF—with defibrillation as the first step, followed by CPR, epinephrine, and antiarrhythmic drugs like amiodarone to restore an effective rhythm.

2. Non-Shockable Rhythms

Non-shockable rhythms don’t respond to defibrillation, so the focus shifts to high-quality CPR, medication, and identifying reversible causes. There are two types of non-shockable rhythms in ACLS:

Asystole

Asystole is a true cardiac flatline. No electrical activity, no pulse, no output. Since there’s nothing to shock, defibrillation isn’t used. The priority here is immediate CPR, administering epinephrine, and quickly identifying and treating any reversible causes, like hypoxia or severe acidosis.

Pulseless Electrical Activity (PEA)

PEA shows up as an organized rhythm on the monitor, but there’s no pulse. It can trick you into thinking the heart is working when it’s not. Like asystole, treatment includes CPR and epinephrine, but success often depends on finding and fixing the underlying cause, commonly remembered using the Hs and Ts (hypovolemia, hypoxia, tension pneumothorax, etc.).

Also Read: Key Differences between PEA and Asystole

3. Bradycardia-Related Rhythms

Bradycardia refers to a slow heart rate typically under 60 bpm. In ACLS, these rhythms can range from harmless to life-threatening, depending on how well the patient is perfusing. Some require no treatment at all, while others call for medications or even pacing. Here’s a breakdown of the key bradycardia rhythms you need to recognize:

Sinus Bradycardia

This is a slow but regular heart rhythm, usually under 60 bpm. It’s common in athletes and during sleep, and often doesn’t need treatment. But if it causes symptoms like hypotension, dizziness, or chest pain, it may require atropine, or in some cases, pacing or dopamine/epinephrine infusion.

First-Degree AV Block

In this rhythm, all the electrical impulses get through the AV node—they’re just delayed. It shows up as a consistently prolonged PR interval on the ECG. Most of the time, it’s harmless and doesn’t require treatment unless it’s contributing to symptoms.

Second-Degree AV Block (Type I – Wenckebach)

Also known as Mobitz Type I, this rhythm shows progressively lengthening PR intervals until a beat is dropped. It’s often benign, especially if the patient is stable. If symptoms are present, treatment might include atropine or pacing.

Second-Degree AV Block (Type II)

Unlike Type I, this block causes sudden dropped beats without any PR interval warning. It’s more unpredictable and serious, with a higher risk of progressing to complete heart block. Pacing is typically required, even if symptoms are mild.

Third-Degree (Complete) Heart Block

In this complete AV dissociation, the atria and ventricles beat independently. The ventricles often take over with a slow escape rhythm, which isn’t enough to maintain adequate circulation. This rhythm always needs immediate pacing and often calls for a permanent pacemaker.

4. Tachycardia-Related Rhythms

Tachycardia refers to any rhythm with a heart rate over 100 bpm. In ACLS, the main concern is whether the patient is stable or unstable. Some rhythms originate above the ventricles (supraventricular), while others come from the ventricles themselves. The treatment depends on both the rhythm type and how it’s affecting the patient.

Supraventricular Tachycardia (SVT)

SVT is a narrow-complex, rapid rhythm that starts above the ventricles. Patients may feel palpitations, chest discomfort, or dizziness. If the patient is stable, try vagal maneuvers first—things like bearing down or a cold stimulus. If that doesn’t work, adenosine can be used to break the rhythm. If the patient is unstable, synchronized cardioversion is the treatment of choice.

Atrial Fibrillation (Afib)

Afib is an irregular rhythm with no clear P waves. It causes an erratic ventricular response and can lead to poor perfusion. Treatment depends on whether the patient is stable or unstable. For stable patients, rate control and medications like amiodarone are common. For unstable patients, immediate synchronized cardioversion is often needed.

Atrial Flutter

This rhythm shows characteristic “sawtooth” flutter waves on the ECG and typically has a regular ventricular response. Like Afib, treatment is based on the patient’s stability and duration of symptoms. Options include rate control, rhythm conversion with antiarrhythmics, or cardioversion if the patient is unstable.

Monomorphic Ventricular Tachycardia (with a pulse)

This form of VT has a consistent, wide-complex rhythm. If the patient is stable, antiarrhythmic medications like amiodarone may be used. If unstable, meaning low blood pressure, chest pain, or altered mental status, immediate synchronized cardioversion is needed to restore a perfusing rhythm.

Polymorphic VT / Torsades de Pointes

This is a twisting, wide-complex rhythm often linked to prolonged QT intervals or electrolyte imbalances. If there’s no pulse, treat it like VF with defibrillation. If a pulse is present, magnesium sulfate is the first-line treatment. Always check and correct underlying causes like low potassium or magnesium levels.

Bonus: Rhythms That Can Confuse You on the Exam

Some rhythms look scarier than they are—or mimic more serious ones. Here are a few commonly misunderstood rhythms that show up on exams:

Idioventricular Rhythm

A slow (20–40 bpm), wide-complex rhythm is often seen after ROSC. It looks ominous but may not require treatment unless the patient is unstable.

Accelerated Junctional Rhythm

This AV node rhythm (60–100 bpm) can resemble SVT. The absence or inversion of P waves is a clue. Don’t confuse it with atrial rhythms—it requires a different approach.

Artifact

Movement or interference can mimic VF or VT on the monitor. Always check the patient first. Never treat the screen without confirming what’s going on.

Also Read: Ventricular Fibrillation vs. Pulseless Ventricular Tachycardia

Tips for Rhythm Identification

ECG rhythm strips can look intimidating at first, but breaking them down step by step makes interpretation more manageable. Use these five core questions to analyze any rhythm:

• Rate – Is the heart beating too fast, too slow, or at a normal pace? This helps narrow down your options right away.
• Regularity – Are the R-R intervals consistent? Irregular rhythms (like Afib) stand out here.
• P Waves – Are they visible? Do they look normal? Are they related to the QRS complexes?
• PR Interval – Is the interval between the P wave and QRS normal (0.12–0.20 seconds) or prolonged? This helps you spot AV blocks.
• QRS Width – Is the QRS narrow (<0.12 sec) or wide? Narrow suggests supraventricular origin; wide suggests ventricular involvement.

Summary Table of ACLS Rhythms You Need to Know

You can use this table as a quick-reference guide to the core ACLS rhythms, their shock status, and their primary treatments:

Final Thoughts

ACLS rhythms are critical to emergency response, but it’s the ability to recognize them under stress and act quickly that truly makes a difference in saving lives. These essential skills are covered in ACLS classes, where comprehensive instruction helps you gain the confidence to handle emergencies effectively.

Heart Start CPR offers AHA-authorized ACLS courses, led by expert instructors who ensure that you not only learn the necessary rhythms but also develop the hands-on skills required to manage any emergency scenario with clarity and precision. With practical, real-world training, you’ll be well-prepared to make life-saving decisions in the moment.

Ready to enhance your ACLS skills? Enroll in an ACLS course at Heart Start CPR today and take the next step in your life-saving training!