BLS (Basic Life Support) prioritizes chest compressions above all else. The entire framework is built around getting blood moving as quickly as possible during cardiac arrest, with every other action, including opening the airway and delivering rescue breaths, coming second. This priority is reflected in the CAB sequence: Compressions, Airway, Breathing.
Why Compressions Come First
Before 2010, BLS followed an ABC sequence: Airway, Breathing, Compressions. The problem was that opening the airway and delivering breaths took time, and chest compressions were consistently delayed. The American Heart Association flipped the order to CAB based on a straightforward finding: the majority of cardiac arrests happen in adults, and the single most critical factor in survival is getting chest compressions started quickly.
There was also a practical barrier with the old approach. Bystanders often skipped CPR entirely because they found rescue breaths difficult or were unwilling to perform mouth-to-mouth on a stranger. By moving compressions to the front of the sequence, more people were willing to act, and the most important intervention started sooner. Ventilation is only minimally delayed by this reordering, typically by about 18 seconds for the first cycle of 30 compressions.
The CAB Sequence Step by Step
When you encounter someone who is unresponsive and not breathing normally, the BLS priority sequence works like this:
- Compressions: Begin chest compressions immediately. Push hard and fast in the center of the chest, delivering 30 compressions before anything else.
- Airway: After 30 compressions, tilt the head back and lift the chin to open the airway.
- Breathing: Deliver 2 rescue breaths, then return to compressions. This 30:2 cycle repeats until help arrives or an AED is ready to use.
Before starting compressions, you should check for a pulse, but this check is capped at 10 seconds. If you don’t feel a definite pulse within that window, you start compressions. Spending too long searching for a pulse delays the thing that matters most.
What High-Quality CPR Looks Like
BLS doesn’t just prioritize starting compressions. It prioritizes doing them well. The American Heart Association defines high-quality CPR through a handful of specific performance targets:
- Rate: 100 to 120 compressions per minute. Faster than one per second, but not so fast that the chest can’t fully recoil between pushes.
- Depth: At least 2 inches (5 centimeters) in adults. For infants and children, at least one-third the depth of the chest.
- Chest compression fraction: More than 80% of total resuscitation time should be spent actively compressing the chest. Every pause for pulse checks, breaths, or switching rescuers eats into this number.
- Full recoil: Let the chest come all the way back up between compressions. Leaning on the chest between pushes reduces blood flow back to the heart.
- No excessive ventilation: Over-ventilating increases pressure in the chest, which reduces blood return to the heart and undermines the compressions you’re working so hard to deliver.
That 80% compression fraction target is worth paying attention to. It means that in a 10-minute resuscitation, the chest should be actively compressed for at least 8 of those minutes. Pauses are inevitable for breaths, rhythm checks, and AED shocks, but keeping them short is a core BLS priority.
Where AEDs Fit In
An Automated External Defibrillator is the other major priority in BLS, and the guideline is simple: use one as soon as it is available. If someone nearby can grab an AED while you’re doing compressions, that’s the ideal scenario. You don’t stop CPR to go find one yourself unless there’s no other option.
The reason defibrillation ranks so high is that the most survivable type of cardiac arrest involves an abnormal heart rhythm that an AED can correct. Every minute without defibrillation reduces the chance of survival by roughly 7 to 10 percent. Compressions buy time by keeping some blood flowing to the brain and heart, but the AED is what can actually restore a normal rhythm.
How Priorities Shift for Children and Infants
The CAB sequence still applies to pediatric patients, but the underlying reasoning shifts. Most cardiac arrests in adults are caused by a heart problem. In children, the cause is usually respiratory: an airway obstruction, an infection, drowning, or some other event that cuts off oxygen first and then leads to cardiac arrest secondarily. This means that while compressions still come first in the sequence, rescue breaths carry more weight in pediatric resuscitation than in adult cases.
The compression-to-breath ratio also changes when two rescuers are available for a child or infant. A single rescuer uses the standard 30:2 ratio regardless of the patient’s age. But two rescuers working on a pediatric patient switch to a 15:2 ratio, delivering breaths more frequently to address the oxygen deficit that likely caused the arrest in the first place.
For adults, compression-only CPR (no rescue breaths at all) is considered acceptable for bystanders who are untrained or uncomfortable with mouth-to-mouth. This option exists precisely because BLS prioritizes compressions so heavily. In the first several minutes of a sudden cardiac arrest in an adult, the blood still contains enough oxygen that compressions alone can keep vital organs alive. For children, where oxygen depletion is more likely the root problem, rescue breaths are more important to include if you’re able.
Why Minimizing Interruptions Matters
A theme runs through every BLS guideline: don’t stop compressing unless you absolutely have to. Pulse checks should take less than 10 seconds. Switching between rescuers should happen quickly during a planned pause. AED analysis and shock delivery should interrupt compressions for as short a window as possible.
CPR is also physically exhausting. Compression quality starts to decline after about two minutes, even if the rescuer doesn’t feel tired yet. When two people are available, BLS protocols recommend swapping the compressor role every two minutes, timed to coincide with rhythm checks so you’re not adding extra pauses. The priority isn’t just starting compressions. It’s sustaining effective compressions for as long as the resuscitation takes.