Cardiac arrest is diagnosed by three immediate physical signs: the person is unresponsive, has no pulse, and is not breathing normally. Unlike many medical conditions that require lab work or imaging, the initial diagnosis happens in seconds through direct observation. Everything that follows, from CPR to advanced monitoring, builds on that rapid bedside assessment.
The Three Signs That Confirm Cardiac Arrest
Cardiac arrest looks dramatic and unmistakable once you know what to check for. The person suddenly loses consciousness, stops breathing (or breathes abnormally), and has no detectable pulse. All three happen because the heart’s electrical system has malfunctioned, causing it to stop pumping blood to the brain, lungs, and other organs. This is different from a heart attack, which is a blockage in blood flow to the heart muscle. A heart attack is a plumbing problem; cardiac arrest is an electrical one.
For trained healthcare providers, current guidelines recommend checking for a pulse at the neck (carotid artery) for no more than 10 seconds. If no definite pulse is felt within that window, cardiac arrest is assumed and CPR begins immediately. Bystanders without medical training skip the pulse check entirely and act on what they can see: the person is unconscious and not breathing normally.
Why Agonal Breathing Delays Recognition
One of the trickiest parts of recognizing cardiac arrest is agonal breathing. These are irregular, gasping breaths that can sound like snoring or labored gulping. They’re a brainstem reflex triggered by severe oxygen deprivation, not actual effective breathing. Because agonal gasps can look and sound like the person is still alive, they frequently cause bystanders to hesitate, delaying CPR and reducing the chance of survival.
The key distinction: normal breathing is rhythmic and steady. Agonal gasps are sporadic, noisy, and often separated by long pauses. If someone is unconscious and their breathing looks anything other than normal, treat it as cardiac arrest. The 2025 American Heart Association guidelines reinforce a simple framework for bystanders called “No-No-Go”: no response, no normal breathing, go (start CPR).
How an ECG Identifies the Type of Arrest
Once emergency responders arrive, a heart monitor or defibrillator reads the heart’s electrical activity and classifies it into one of four rhythms. This step determines what treatment the heart needs.
Two rhythms are “shockable,” meaning an electrical shock from a defibrillator can potentially reset them:
- Ventricular fibrillation (VF): The heart’s lower chambers quiver chaotically instead of pumping. This is the most survivable rhythm if caught quickly.
- Pulseless ventricular tachycardia: The heart beats extremely fast but produces no effective blood flow.
Two rhythms are “non-shockable,” meaning a defibrillator won’t help and other interventions are needed:
- Asystole: No electrical activity at all, often called “flatline.”
- Pulseless electrical activity (PEA): The monitor shows what looks like organized heart activity, but the heart isn’t actually pumping blood. This is particularly deceptive because the electrical tracing can appear almost normal.
Identifying which rhythm is present is one of the most important diagnostic steps because it splits the treatment path in two completely different directions.
What an AED Can Tell You
Automated external defibrillators, the devices found in airports, gyms, and offices, perform their own rhythm analysis. When you place the pads on someone’s chest, the AED reads the heart’s electrical signal and decides whether a shock is appropriate. Modern AEDs are highly accurate: studies show they detect shockable rhythms with about 89% sensitivity and 99.8% specificity. For standard ventricular fibrillation specifically, sensitivity reaches 97%.
Where AEDs struggle is with very fine ventricular fibrillation, where the electrical signal is so small it’s hard to distinguish from a flatline. Detection drops to around 50% for those cases. This is one reason why paramedics and hospital teams rely on additional tools beyond the AED’s automated analysis.
Finding the Underlying Cause
Diagnosing cardiac arrest doesn’t stop at confirming the heart has stopped. Medical teams simultaneously work to identify why it stopped, because many causes are reversible if caught in time. They use a standard checklist of possibilities organized into two categories, informally called the “H’s and T’s.”
The H’s cover metabolic and environmental problems: severe blood loss (hypovolemia), oxygen deprivation, acid buildup in the blood, dangerous potassium levels (too high or too low), and extreme cold (hypothermia). The T’s cover structural and toxic problems: poisoning or drug overdose, fluid compressing the heart (cardiac tamponade), a collapsed lung under pressure (tension pneumothorax), a blood clot blocking a coronary artery or the lungs, and major trauma.
Each of these has a specific treatment. A cardiac arrest caused by massive blood loss, for instance, needs volume replacement, not repeated defibrillation. One caused by an opioid overdose may respond to an opioid-reversing medication. The 2025 AHA guidelines now specifically include opioid reversal agents in the basic life support algorithm for suspected overdose cases, reflecting how common opioid-related arrests have become.
Advanced Monitoring During Resuscitation
In the emergency department or ambulance, two additional diagnostic tools give the medical team real-time information about whether resuscitation is working.
Exhaled CO2 Monitoring
When a breathing tube is placed, a sensor measures the amount of carbon dioxide in each exhaled breath. During cardiac arrest, CO2 levels drop because blood isn’t circulating through the lungs efficiently. If CO2 readings stay at or below 10 mmHg after 20 minutes of CPR, studies show the arrest is uniformly fatal. Conversely, a sudden rise in CO2 is often the very first sign that the heart has started beating again, sometimes appearing before a pulse is even detectable by hand.
Bedside Ultrasound
Point-of-care ultrasound lets clinicians see the heart in real time during resuscitation. This is especially valuable for non-shockable rhythms, where the monitor can’t tell you much about what’s mechanically happening. Ultrasound can reveal fluid squeezing the heart, a massive blood clot in the lungs, a collapsed lung, or severe blood loss. It can also distinguish true asystole (a completely still heart) from very fine ventricular fibrillation (a faintly quivering heart that might respond to a shock). Those two conditions look nearly identical on a monitor but require opposite treatments.
Cardiac Arrest vs. Heart Attack: The Diagnostic Difference
These two conditions are commonly confused, but the diagnostic picture is completely different. A heart attack usually presents with chest pain, shortness of breath, and nausea, and the person remains conscious. It’s diagnosed with blood tests and ECG tracings that show damage patterns. Cardiac arrest presents with sudden collapse, no pulse, and no breathing. There’s no time for blood tests; the diagnosis is made at the bedside in seconds.
The connection between them is that a heart attack can trigger cardiac arrest. A blocked artery damages heart muscle, which can destabilize the electrical system and send the heart into a fatal rhythm. So a heart attack is one of the reversible causes teams look for once they’ve confirmed the arrest and started resuscitation.