ACLS, or Advanced Cardiovascular Life Support, is a set of standardized emergency protocols used by healthcare providers to treat cardiac arrest, dangerous heart rhythms, and other life-threatening cardiovascular emergencies. Published and updated by the American Heart Association, these protocols give medical teams a step-by-step framework for restoring a heartbeat and keeping the brain alive during the most critical minutes of a cardiac event. The most recent update came in 2025.
ACLS builds on basic CPR skills by adding medications, electrical therapies like defibrillation, airway management, and structured team communication. It’s used in emergency departments, intensive care units, operating rooms, and by paramedics in the field.
The Cardiac Arrest Algorithm
The core of ACLS is the cardiac arrest algorithm, a branching decision tree that guides treatment based on the type of heart rhythm detected. Every cardiac arrest starts the same way: begin CPR immediately, provide oxygen with a bag-mask device, and attach a cardiac monitor or defibrillator. From there, the team checks the rhythm and follows one of two pathways.
Shockable Rhythms: VF and Pulseless VT
Ventricular fibrillation (VF) and pulseless ventricular tachycardia (pVT) are chaotic or overly rapid heart rhythms that prevent the heart from pumping blood. These are called “shockable” because a defibrillator can reset the heart’s electrical activity. Early defibrillation, combined with CPR, is the single most important factor in surviving these rhythms.
The protocol follows repeating two-minute cycles. After the first shock, the team performs two minutes of CPR, establishes IV access, and begins giving epinephrine (1 mg every 3 to 5 minutes). If the rhythm is still shockable after the next check, another shock is delivered, followed by two more minutes of CPR plus an antiarrhythmic medication. If the rhythm persists through additional cycles, the team continues alternating shocks, CPR, and medications while investigating reversible causes.
For biphasic defibrillators (the modern standard), initial shock energy is typically 120 to 200 joules, following the manufacturer’s recommendation. If the setting is unknown, providers use the maximum available energy. Older monophasic machines use 360 joules.
Non-Shockable Rhythms: PEA and Asystole
Pulseless electrical activity (PEA) means the heart’s electrical system is firing but the heart isn’t actually pumping. Asystole is a flatline, with no electrical activity at all. Neither responds to defibrillation. Instead, the protocol focuses on high-quality CPR, epinephrine given as soon as possible, and aggressively searching for a treatable underlying cause. Every two minutes, the team rechecks the rhythm. If it shifts to a shockable rhythm, they switch to the VF/pVT pathway.
High-Quality CPR Standards
ACLS emphasizes that CPR quality directly determines survival. The specific targets are precise: compress the chest at least 2 inches deep in adults, at a rate of 100 to 120 compressions per minute, and allow the chest to fully recoil between compressions. Interruptions in compressions should be as brief as possible. The person doing compressions should switch out every two minutes to avoid fatigue-related decline in quality.
Without an advanced airway in place, the ratio is 30 compressions to 2 breaths. Once an advanced airway is secured, compressions become continuous while breaths are delivered once every 6 seconds (10 per minute). Overventilating is a common mistake that raises pressure inside the chest and reduces blood flow, so the protocol explicitly warns against it.
Waveform capnography, which measures carbon dioxide in exhaled air, serves as a real-time indicator of CPR effectiveness. A reading above 20 mmHg at the 20-minute mark suggests a higher chance of getting the heart back. A reading below 10 mmHg at that point means the odds are very low. A sudden spike in the reading during CPR often signals the heart has restarted, sometimes before a pulse is even detectable.
The H’s and T’s: Reversible Causes
One of the most practical frameworks in ACLS is the list of reversible causes, organized as the “H’s and T’s.” These are conditions that can trigger or sustain cardiac arrest, and identifying them quickly can change the outcome entirely. There are twelve in total.
The H’s cover:
- Hypovolemia: severe blood or fluid loss
- Hypoxia: dangerously low oxygen levels
- Hydrogen ion (acidosis): too much acid in the blood, impairing circulation
- Hypokalemia/Hyperkalemia: abnormally low or high potassium, which disrupts the heart’s electrical signals
- Hypothermia: core body temperature dropping below 86°F (30°C)
The T’s cover:
- Toxins: drug overdoses or poisonings, including opioids, cocaine, and certain heart medications
- Tamponade: fluid compressing the heart inside its surrounding sac
- Tension pneumothorax: trapped air in the chest cavity pressing on the heart and lungs
- Thrombosis (coronary): a blood clot blocking blood flow to the heart muscle
- Thrombosis (pulmonary): a blood clot blocking the main artery to the lungs
Experienced providers may use bedside ultrasound during cardiac arrest to identify some of these causes in real time, as long as it doesn’t interrupt chest compressions.
Bradycardia and Tachycardia Protocols
ACLS doesn’t only cover cardiac arrest. It also provides algorithms for dangerous heart rhythms in patients who still have a pulse.
For bradycardia (a heart rate typically below 50 beats per minute), the key question is whether the slow rate is causing problems: low blood pressure, confusion, signs of shock, chest pain, or heart failure. If the patient is stable, the team monitors and evaluates. If the slow heart rate is causing cardiovascular compromise, the first-line treatment is atropine (1 mg, repeated every 3 to 5 minutes up to 3 mg). If atropine doesn’t work, the protocol moves to transcutaneous pacing or a medication drip to speed the heart rate.
For tachycardia (a heart rate typically 150 or above), the algorithm splits based on stability and the width of the electrical signal on the monitor. An unstable patient showing low blood pressure, altered consciousness, shock, or chest pain gets synchronized cardioversion, which is a precisely timed electrical shock. For stable patients with a narrow electrical complex, treatments include vagal maneuvers (like bearing down) and adenosine, a short-acting medication given as a rapid IV push at 6 mg initially and 12 mg if needed. Stable patients with a wide complex may receive antiarrhythmic medications or need expert consultation.
Post-Resuscitation Care
Getting the heart beating again, known as return of spontaneous circulation (ROSC), is only half the battle. The 2025 guidelines outline specific targets for the hours and days that follow. Oxygen saturation should be maintained between 90% and 98%, avoiding both low oxygen and excessive oxygen delivery. Blood pressure is managed to keep the mean arterial pressure at or above 65 mmHg, preventing the kind of low blood flow that causes brain damage.
Temperature control is a major component of post-arrest care. For patients who remain unresponsive after ROSC, controlled temperature management should be maintained for at least 36 hours. Clinical trials have tested target temperatures ranging from about 88°F to 100°F (31°C to 37.7°C), and current evidence shows no clear survival advantage of cooling to 32-34°C versus maintaining a near-normal temperature of 36°C. The priority is preventing fever, which worsens brain injury.
Team Roles and Communication
ACLS places significant emphasis on how teams work together during a code. A typical resuscitation involves a team leader who directs the effort and team members assigned to specific roles: airway management, chest compressions, defibrillation, IV access and medications, and documentation.
The communication standard is called closed-loop communication. The team leader gives an order directed to a specific person by name. That person repeats the order back. The team leader confirms it was heard correctly. This three-step loop prevents the kind of miscommunication or assumed compliance that leads to errors when seconds matter. It sounds rigid, but in the controlled chaos of a cardiac arrest, it keeps critical steps from being missed or duplicated.