The immediate action is to call emergency services (911 in the US) and begin high-quality chest compressions on the person's lower sternum at 100–120 per minute, pushing at least 2 inches deep. Do this before anything else: before rescue breaths, before an AED arrives, before you finish checking for a pulse. Compressions move oxygenated blood to the brain and heart. Every minute without CPR removes roughly 7–10% of survival probability. The single best predictor of survival is whether a bystander started compressions immediately. If a second rescuer is present, they call 911 while you compress; if you are alone with a phone, put it on speaker and compress while you talk.
Mechanism: Why Compressions Buy Time
Cardiac arrest means the heart has stopped pumping effectively. Electrical activity may persist for a few minutes, but without mechanical output, no blood reaches the brain or coronary arteries. Brain cells begin dying within 4–6 minutes of zero flow, and irreversible injury accumulates fast. The person is unconscious, not breathing normally, or only gasping. Gasping is not breathing; it is agonal respiratory effort from a brainstem reflex that can last 1–2 minutes. Treat agonal gasps as cardiac arrest and act.
CPR works because chest compressions physically squeeze the heart between the sternum and the spine. This generates a pressure gradient that pushes blood into the aorta and pulmonary arteries. It also changes intrathoracic pressure, which helps draw blood back into the chest during recoil. Hand positioning matters: the heel of one hand on the lower half of the sternum, the other hand on top, arms straight, shoulders over hands. Compression fraction, the proportion of time spent actively compressing, is the metric that correlates with survival. Professional guidelines target a compression fraction above 60%, and high-performing systems exceed 80%. Every pause to reposition, check a pulse, or fumble with an AED drops that number.
Depth and rate have mechanistic reasons. Compressions shallower than 2 inches do not generate enough stroke volume to perfuse the brain. Compressions faster than 120 per minute reduce the time the heart has to refill between squeezes, which lowers output. Full recoil, letting the chest return completely to its resting position, is what allows venous return. Leaning on the chest between compressions is one of the most common and most harmful errors. Plainly, push hard, push fast, let go completely, and do not stop.
Quantified Context: The Numbers That Define Survival
Out-of-hospital cardiac arrest kills more than 300,000 people in the US each year. Survival depends heavily on bystander action. When a bystander performs CPR, survival roughly doubles compared with no bystander CPR. When a bystander applies an AED before EMS arrives, survival can reach 50–70% in shockable rhythms, versus under 10% in many untreated cases. Time is the dominant variable. Survival falls about 7–10% for every minute that CPR and defibrillation are delayed. That means a 5-minute delay is roughly a 35–50% loss of the initial chance.
Consider the gap between arrest and defibrillation. EMS response times in urban US settings often run 6–10 minutes. The brain tolerates only 4–6 minutes of no flow without injury. That gap is exactly what bystander CPR fills. In systems where bystanders compress and apply AEDs, survival to hospital discharge can exceed 30%; in systems with no bystander action, it often sits below 10%. The difference is not exotic medicine. It is a person on the ground and a person on their knees.
Age and cause shift the numbers but not the first move. About 70–80% of out-of-hospital arrests are cardiac in origin, often from ventricular fibrillation or pulseless ventricular tachycardia, which are shockable. Roughly 20–30% are non-shockable rhythms like asystole or pulseless electrical activity, where CPR sustains circulation until a reversible cause (hypoxia, hemorrhage, hypothermia) is corrected. The first action does not change: compress and call.
Timeline Table: What Happens Minute by Minute
| Time since arrest | Physiology | What the rescuer should be doing |
|---|---|---|
| 0–1 min | Electrical activity still present; agonal gasps may appear | Recognize, call 911, start compressions |
| 1–4 min | Brain oxygen reserves depleting; irreversible injury begins near 4–6 min | Continue compressions; second rescuer retrieves AED |
| 4–6 min | Brain cell death advancing; shockable window narrowing | Apply AED as soon as it arrives; minimize pauses |
| 6–10 min | Typical EMS arrival in many urban systems; survival falling 7–10% per minute | Continue compressions through AED analysis and shocks |
| 10–15 min | Survival without bystander CPR often below 5% | Continue until EMS takes over; do not stop on your own |
| 15+ min | Neurological injury likely without early flow | High-quality CPR and advanced care; outcomes depend on cause |
The table is a model, not a guarantee. Cold-water drowning and hypothermia arrests can extend the tolerable window considerably, and pediatric arrests often reflect respiratory causes where early rescue breaths matter more.
Edge Cases: When the Rules Bend
Drowning. The arrest is usually hypoxic, meaning oxygen ran out before the heart stopped. Guidelines for drowning recommend rescue breaths in addition to compressions, and starting with 2 breaths if you are trained, because the blood may still carry usable oxygen once ventilation resumes. If untrained, compress only and call.
Hypothermia. Cold slows metabolism and can protect the brain for longer than the usual 4–6 minutes. Do not withhold CPR because the person feels cold or appears stiff. Continue until warming and advanced care are available.
Children and infants. Pediatric arrest is more often respiratory. For infants under 1 year, use two fingers or the heel of one hand on the lower sternum and compress about 1.5 inches. For children, use one or two hands to at least one-third of chest depth, roughly 2 inches. Call for help and start CPR; if alone with no phone nearby, give about 2 minutes of CPR before leaving to call.
Pregnancy. Compress at the standard rate and depth, and if the uterus is visibly enlarged, manual left uterine displacement (pushing the uterus to the patient's left) can improve venous return. Defibrillation is not withheld because of pregnancy.
Opioid overdose. If the person is unresponsive with slow or absent breathing but has a pulse, rescue breathing and naloxone take priority. If there is no pulse, standard CPR applies. In many US states, laypeople can obtain naloxone without a prescription.
Do not compress a person who is responsive, breathing normally, or has a clear pulse. That is the one case where the first action is to assess, not to act.
Misconceptions and the Counterintuitive Caveat
Misconception one: "I might hurt them." Good Samaritan laws in all 50 US states protect lay rescuers who act in good faith, and the risk of injury from CPR is small compared with certain death without it. Rib fractures occur in roughly 30% of CPR attempts and are an accepted cost. A cracked rib that saves a life is a good trade.
Misconception two: "I need to check for a pulse first." Pulse checks by laypeople are unreliable and waste 10–30 seconds. If the person is unresponsive and not breathing normally, treat as arrest. Professionals limit pulse checks to under 10 seconds.
Misconception three: "Rescue breaths are mandatory." Hands-only CPR, continuous compressions without breaths, is recommended for untrained bystanders and produces outcomes comparable to compression-plus-ventilation in adult cardiac arrest for the first few minutes. The compressions are the non-negotiable part.
The counterintuitive caveat, and one professionals emphasize: the most common failure in real arrests is not too-shallow compressions or skipped breaths. It is excessive interruption. Rescuers pause for AED pad placement, for pulse checks, for switching compressors, and each pause costs perfusion pressure that takes several compressions to rebuild. Professional guidance now emphasizes compressing during AED charging and analyzing rhythm with minimal hands-off time. If you are switching rescuers, do it in under 5 seconds. Do not stop to watch the AED; keep compressing until it tells you to stand clear.
Table: Comparison of First Actions by Situation
| Situation | First action | Second action | What to avoid |
|---|---|---|---|
| Adult, unresponsive, not breathing | Call 911 and start compressions | Retrieve and apply AED | Delaying compressions to search for a pulse |
| Adult with agonal gasps | Treat as arrest; compress | AED as soon as available | Assuming gasping means breathing |
| Drowning victim | Call 911, start compressions | Add rescue breaths if trained | Skipping breaths if trained and able |
| Child or infant, unresponsive | Start CPR (2 min if alone) | Call 911 | Waiting for someone else to call |
| Opioid overdose with pulse | Rescue breathing, naloxone | Call 911 | Compressing a person with a pulse |
| AED available | Apply pads, follow prompts | Resume compressions immediately after shock | Touching the person during analysis |
| Second rescuer present | One compresses, one calls and gets AED | Switch every 2 minutes | Both leaving to find help |
Practical Actions: The Exact Sequence
Call 911 first or compress first? If you are alone with a phone, call on speaker while starting compressions. If you are alone without a phone, give about 2 minutes of CPR, then leave to call. If a second person is present, they call while you compress. The order depends on resources, not on a rigid script.
Position the person on a firm, flat surface, face up. Kneel beside the chest. Place the heel of your hand on the lower half of the sternum, center of the chest, and stack the second hand. Lock elbows, shoulders over hands, and push straight down at least 2 inches at 100–120 per minute. Let the chest recoil fully between compressions. If trained, give 2 breaths every 30 compressions using a barrier device if available. If untrained, compress continuously.
Use an AED as soon as it arrives. Turn it on, follow the voice prompts, attach pads to bare skin, and stand clear during analysis and shock delivery. Resume compressions immediately after the shock or after the "no shock advised" prompt. Do not remove the pads or turn off the device. Continue until EMS arrives, the person starts breathing normally, or you are physically unable to continue.
Red flag: The person is unresponsive but breathing normally. Do not compress; place them in a recovery position and monitor breathing until help arrives. Red flag: The person has a pulse but is not breathing. Rescue breathing and naloxone (if opioid suspected) take priority over compressions. Red flag: You are exhausted and alone. Compressions degrade within 1–2 minutes; if no one relieves you, do your best and keep going. Red flag: The AED says "stand clear" during analysis or shock. Stop compressions and do not touch the person. Red flag: The person is in water or on a metal surface. Move them to a dry, firm surface before defibrillation if it takes seconds, not minutes.
Limitations: What This Article Cannot Tell You
This article describes general mechanisms and public guidance. It cannot assess a specific person, cannot confirm whether an arrest is shockable, and cannot substitute for hands-on CPR training. Guidelines are updated periodically, and local EMS protocols vary. The best preparation is a certified CPR and AED course, which gives you supervised practice on a manikin. If you have never taken one, the American Heart Association and Red Cross offer courses that take a few hours.
Outcomes depend on factors this article cannot measure: the cause of arrest, the person's baseline health, the time to EMS, and the quality of advanced care. Survival statistics describe populations, not individuals. A person who receives immediate compressions and an early shock can survive with normal neurological function; a person with a prolonged delay may not survive despite perfect technique. Nothing here promises an outcome.
This article is general educational information, not a diagnosis, treatment plan, or substitute for professional medical guidance. For a real emergency, call your local emergency number immediately and follow dispatcher instructions. Consult a qualified local healthcare professional or certified CPR instructor for training and any personal medical questions.