Basic life support (BLS) includes four core skills: recognizing cardiac arrest, calling for emergency help, performing CPR (chest compressions and rescue breaths), and using an automated external defibrillator (AED). It also covers techniques for clearing a blocked airway in adults, children, and infants. Together, these skills form the first critical links in what the American Heart Association calls the Chain of Survival, the sequence of actions that gives someone in cardiac arrest the best chance of making it.
The C-A-B Sequence
Since 2010, BLS guidelines have followed a C-A-B order: circulation, airway, breathing. That replaced the older A-B-C approach. The reason is simple: getting blood moving matters more in the first moments than delivering breaths. Studies using training manikins show that starting with chest compressions leads to faster first compressions, faster first breaths, and faster completion of the entire first CPR cycle compared to starting with ventilation. A 2024 international review reaffirmed this approach for adults.
In practice, C-A-B means you start pushing on the chest immediately after confirming someone is unresponsive and not breathing normally, rather than tilting the head back and blowing air in first.
High-Quality Chest Compressions
Chest compressions are the single most important part of CPR. They manually pump blood from the heart to the brain and other organs. But not all compressions are equal. High-quality CPR has specific standards:
- Rate: 100 to 120 compressions per minute, roughly the tempo of the song “Stayin’ Alive.”
- Depth: At least 2 inches (5 cm) in adults. Pushing too shallow doesn’t generate enough blood flow.
- Full chest recoil: Let the chest come all the way back up between compressions. Leaning on the chest prevents the heart from refilling with blood.
- Minimal interruptions: Every pause in compressions drops blood pressure to near zero. Keep breaks as short as possible.
- Compression-to-ventilation ratio: For adults, 30 compressions followed by 2 breaths per cycle when two rescuers are present (or when a single rescuer is trained and willing to give breaths).
If you’re untrained or uncomfortable giving breaths, hands-only CPR (continuous compressions without mouth-to-mouth) is still far better than doing nothing.
Rescue Breathing
Rescue breaths deliver oxygen to someone whose lungs have stopped working. Each breath should last about one second and produce a visible rise in the chest. Overinflating the lungs forces air into the stomach, which can cause vomiting and complicate the rescue.
There’s an important distinction between cardiac arrest and respiratory arrest. In cardiac arrest, the heart has stopped and you perform full CPR. In respiratory arrest, the heart is still beating but the person isn’t breathing. When a pulse is present but breathing is absent, the correct approach is rescue breathing alone: one breath every 4 to 5 seconds, which works out to about 10 to 12 breaths per minute. You check for signs of circulation every few minutes to make sure the heart is still going.
Using an AED
An AED is a portable device that analyzes heart rhythm and delivers an electric shock if the heart is in a rhythm that can be corrected. Many cardiac arrests involve a chaotic electrical pattern called ventricular fibrillation, where the heart quivers instead of pumping. An AED can reset that rhythm. The device is designed so anyone can use it, even without training.
The steps are straightforward. Turn the AED on and it will give you voice prompts. Remove clothing from the person’s chest and wipe the skin dry if needed. Place one adhesive pad on the upper right chest and the other on the lower left side, a few inches below the armpit. If the pads might touch (on a small person), place one on the center of the chest and one on the back between the shoulder blades. Plug in the connector cable if it isn’t already attached.
Before the AED analyzes the rhythm, make sure nobody is touching the person and say “Clear!” loudly. If the device advises a shock, confirm again that nobody is touching the person, say “Clear!” again, and press the shock button. Immediately after the shock, or if no shock is advised, resume CPR starting with compressions.
Choking and Airway Obstruction
BLS also covers what to do when someone is choking. The key indicator is that the person cannot talk, cry, or laugh forcefully. For adults and children older than 1 year, alternate between five firm back blows (struck between the shoulder blades with the heel of your hand) and five abdominal thrusts. For abdominal thrusts, stand behind the person, wrap your arms around their waist, place your fist just above the belly button, and thrust inward and upward. Keep cycling between back blows and thrusts until the object comes out.
Infants under 1 year get a different approach. Hold the baby facedown along your forearm, resting your arm on your thigh, with the head lower than the body. Give five gentle but firm back thumps with the heel of your hand. If that doesn’t clear the blockage, turn the infant faceup and give five chest compressions. If the person becomes unconscious at any point, begin CPR. Chest compressions can help dislodge the object.
If you can see the object in the mouth, reach in and sweep it out. Never do a blind finger sweep when you can’t see what you’re reaching for, as you risk pushing the object deeper.
Differences for Children and Infants
The core BLS sequence is the same for all ages, but the physical technique changes. For infants, the 2025 AHA guidelines eliminated the old two-finger compression method because it doesn’t achieve adequate depth. The recommended techniques are now the two thumb-encircling hands method (wrapping both hands around the infant’s chest and pressing with your thumbs) or the one-hand technique using the heel of one hand. Studies show both produce deeper, more effective compressions than two fingers.
For children ages 1 through 8, two-handed compressions generate greater depth than one hand, though one-handed compressions tend to stay closer to the recommended rate. The target compression depth for pediatric patients is about one-third of the chest’s front-to-back diameter. Research has found that reaching a compression depth of at least 2 inches in children is associated with better rates of return of spontaneous circulation and 24-hour survival.
Where BLS Fits in the Chain of Survival
The AHA’s Chain of Survival has six links: recognizing cardiac arrest and calling for help, early CPR with emphasis on compressions, rapid defibrillation, advanced care by paramedics and hospital teams, post-arrest medical care, and long-term recovery including rehabilitation and psychological support. BLS covers the first three links. Everything that follows, from IV medications to intensive care, builds on the foundation BLS provides. Without early compressions and defibrillation, advanced treatments have far less to work with.
Training and Legal Protections
A full BLS provider course through the AHA takes roughly 4.5 hours including practice and testing. A blended option lets you complete the knowledge portion online in 1 to 2 hours, then attend a hands-on skills session lasting 1 to 2 hours depending on your experience level. Certification is valid for two years, and the renewal course runs about 4 hours.
If you’re worried about legal consequences of helping someone, every U.S. state and the District of Columbia has a Good Samaritan law. These laws protect people who voluntarily provide emergency care from negligence claims, as long as they act in good faith and don’t expect payment. The protection covers ordinary mistakes made under pressure. It does not cover reckless or intentionally harmful behavior, but that’s a far cry from doing your best to help someone who has stopped breathing.