Agonal breathing is caused by the lower brainstem firing off reflexive signals as the brain loses oxygen. It happens most often during cardiac arrest, when the heart stops pumping blood and the brain’s higher functions begin shutting down. The brainstem, which controls basic survival reflexes, continues sending signals to the breathing muscles even as the rest of the brain goes dark. The result is slow, irregular gasps that look and sound nothing like normal breathing.
About 40% to 60% of people experiencing out-of-hospital cardiac arrest display agonal breathing, making it one of the most common visible signs that someone’s heart has stopped.
How the Brain Produces Agonal Gasps
Normal breathing is coordinated by multiple brain regions working together. When the heart stops and blood flow to the brain drops, the upper brain centers responsible for smooth, rhythmic breathing fail first. But the lower brainstem, which sits at the base of the skull and is more resistant to oxygen deprivation, keeps firing primitive respiratory signals for a short time afterward. These signals produce the characteristic gasps of agonal breathing: sporadic, labored, and completely ineffective at moving enough air into the lungs to sustain life.
Think of it as the brain’s last-resort reflex. The gasps aren’t conscious, voluntary, or purposeful. They’re the nervous system’s most basic circuitry operating on its own as everything above it shuts down.
Conditions That Trigger Agonal Breathing
Cardiac arrest is the most common trigger, but any event that cuts off oxygen to the brainstem can produce agonal gasps. The underlying causes fall into several categories:
- Heart-related events: ventricular fibrillation (a chaotic heart rhythm), cardiac tamponade (fluid compressing the heart), and blunt or penetrating trauma to the chest
- Neurological events: stroke, traumatic brain injury, and seizures that progress to respiratory failure
- Oxygen deprivation: drowning, strangulation, suffocation, and carbon monoxide poisoning
- Drug overdose: opioids and other substances that suppress the brain’s respiratory drive
- Other causes: electrocution and severe hemorrhage
In opioid overdoses specifically, respiratory depression is the primary mechanism of death. Heroin and other opioids suppress the brainstem’s breathing centers directly, and as oxygen levels plummet, the transition from slowed breathing to agonal gasping to complete respiratory arrest can happen rapidly. Research on heroin overdose deaths shows significant variation in respiratory distress patterns during the final minutes, with factors like vomit inhalation and alcohol use accelerating the process.
What Agonal Breathing Looks and Sounds Like
The American Heart Association describes agonal breathing as “slow, irregular gasping respirations that are ineffective for ventilation.” In practice, this means the person may make snoring, gurgling, or moaning sounds while their mouth opens and closes or their jaw moves. The gasps come at unpredictable intervals, sometimes seconds apart, sometimes with long pauses between them. The chest may barely move, or only one side may rise slightly.
This is critically different from labored breathing, where someone is struggling but still moving air in and out of their lungs in a recognizable rhythm. Agonal breathing has no rhythm. It looks more like a fish out of water than a person fighting to breathe. The person will be unresponsive and won’t react to your voice or touch.
Distinguishing agonal gasps from real breathing matters enormously, because misidentifying them is one of the most common reasons bystanders fail to start CPR. If someone collapses, is unresponsive, and is only making occasional gasps, they are in cardiac arrest and need chest compressions immediately.
Why Agonal Breathing Is Actually a Hopeful Sign
Counterintuitively, the presence of agonal breathing during cardiac arrest is associated with better survival odds. Because the brainstem can only sustain these gasps for a limited time after the heart stops, their presence signals that the arrest happened recently and the brain hasn’t been without oxygen for long.
A large multicenter study found that cardiac arrest patients who were still gasping when they arrived at the hospital had nearly double the rate of regaining a pulse compared to those who were not: 39.4% versus 19.4%. Survival at one month was also higher in the agonal breathing group (2.8% versus 1.7%), and the odds of a favorable neurological outcome, meaning the person could function independently afterward, were roughly twice as high.
These numbers are still sobering, but the pattern is clear. Agonal breathing is a marker of the early phase of cardiac arrest, when intervention is most likely to work. Every minute that passes without CPR reduces the chance of survival, and agonal gasping fades as that window closes.
How Long Agonal Breathing Lasts
In cardiac arrest, agonal gasping typically diminishes within minutes if no intervention occurs. The American Heart Association notes that it “diminishes the longer a person is in cardiac arrest,” which means the gasps become less frequent and weaker over time before stopping entirely. Without CPR or defibrillation, complete cessation of breathing follows.
In the context of the natural dying process, where someone is at the end of life due to terminal illness rather than a sudden cardiac event, a similar pattern of irregular breathing can occur. Breathing may slow, stop, and restart with long pauses between breaths. This pattern sometimes lasts hours or even a couple of days, though in many cases it persists only for minutes before breathing stops for good.
What to Do When You See It
The 2025 American Heart Association guidelines are unambiguous: if someone is unresponsive and not breathing normally, start CPR. Agonal gasps count as abnormal breathing. You do not need to check for a pulse if you are not a trained healthcare provider. Recognition should be based on whether the person is conscious and whether their breathing looks normal.
The guidelines also emphasize that the benefit of giving CPR to someone in cardiac arrest far outweighs any risk of giving chest compressions to someone who turns out not to need them. If you’re unsure whether the gasping you’re seeing is agonal breathing or real breathing, err on the side of starting compressions and calling emergency services. Hesitation during those first minutes costs lives.