2.1 Adult BLS Sequence: Safety, Recognition, Activation, CPR
Key Takeaways
- The adult BLS sequence is C-A-B: chest compressions first, then airway, then breathing.
- A lone rescuer for an adult activates EMS and gets an AED before starting CPR (phone-first), because most adult arrests are cardiac and need a shock.
- Combine the breathing and pulse check into a single assessment of no more than 10 seconds; agonal gasps are not normal breathing.
- If there is no normal breathing and no definite pulse within 10 seconds, begin compressions immediately at 100-120/min, depth at least 2 inches (5 cm).
- BLS is the foundation of ACLS: every advanced action depends on perfusion generated by high-quality CPR.
2.1 Adult BLS Sequence: Safety, Recognition, Activation, CPR
Adult Basic Life Support (BLS) is the ordered series of actions a rescuer performs the moment an adult collapses. Per the current American Heart Association (AHA) Guidelines (2020, with 2023 and 2025 focused updates), the resuscitation order is C-A-B: Compressions, then Airway, then Breathing. The 2010 guidelines moved compressions ahead of airway/breathing because the most common cause of sudden adult arrest is a primary cardiac rhythm problem (ventricular fibrillation), and delaying compressions to open the airway costs perfusion the brain and heart cannot afford.
The six-step adult BLS sequence
| Step | Action | Key rule |
|---|---|---|
| 1. Scene safety | Survey for hazards before approaching | Do not become a second patient |
| 2. Recognition | Tap and shout; check for response | Unresponsive = potential arrest |
| 3. Activation | Call 9-1-1 / code team, get an AED | Lone rescuer for an adult: phone FIRST |
| 4. Assess | Simultaneous breathing + pulse check | No more than 10 seconds total |
| 5. Compressions | Start CPR if no pulse / no normal breathing | 30:2, 100-120/min, at least 2 in (5 cm) |
| 6. Defibrillate | Attach and use the AED as soon as it arrives | Minimize the pause around the shock |
Scene safety and recognition
The sequence opens with scene safety: look for electrical hazards, moving traffic, fire, water, toxic gas, violence, or unstable surfaces. A harmed rescuer cannot help. Next is recognition. Tap the patient's shoulder firmly and shout, "Are you okay?" If the patient does not respond, you treat this as a potential cardiac arrest and move immediately to activation. Recognition is the most time-sensitive link in the adult Chain of Survival because every minute without CPR and defibrillation drops survival roughly 7-10%.
Activation: why the adult is 'phone-first'
For an adult, a lone rescuer who finds an unresponsive person should activate the emergency response system and retrieve an AED before beginning CPR ("phone-first"). The logic: adult arrest is usually a sudden shockable rhythm, and the single most effective treatment is early defibrillation, so summoning a defibrillator and advanced help first improves survival. This is the opposite of the lone-rescuer pediatric rule ("CPR-first" / 2 minutes of CPR before leaving an unwitnessed child), because pediatric arrest is more often respiratory. If a phone is in reach, use the speaker function so you can call and start CPR at once.
When multiple rescuers are present, these tasks happen in parallel: one calls and gets the AED while another starts compressions, so nothing is delayed.
Assessment: breathing and pulse in one look
For a healthcare provider, check breathing and pulse simultaneously, in no more than 10 seconds. Scan the chest for normal breathing while palpating the carotid pulse. Two traps dominate exam items here:
- Agonal gasps are NOT normal breathing. Occasional gasping, snorting, or labored irregular breaths in an unresponsive adult signal arrest. Treat the patient as pulseless.
- Do not let the pulse check stall compressions. If you cannot definitively feel a pulse within 10 seconds, or you are unsure, start compressions. Lay rescuers skip the pulse check entirely and begin CPR for any unresponsive adult who is not breathing normally.
Starting CPR and linking to ACLS
If there is no normal breathing and no definite pulse, begin chest compressions at 100-120/min, depth at least 2 inches (5 cm), allowing full recoil, in cycles of 30 compressions to 2 breaths. Apply the AED the instant it arrives. Even in a fully monitored hospital arrest with a code team at the bedside, the first lifesaving treatment is still high-quality CPR while the defibrillator charges; epinephrine, advanced airways, and rhythm-specific drugs all build on the perfusion compressions create. This is why BLS is the literal foundation of Advanced Cardiovascular Life Support (ACLS).
How this shows up on exams
BLS/ACLS items test priority and sequence. Read the patient state first: age, pulse, breathing, rhythm, stability, and number of rescuers. A distractor often sounds advanced (intubate, give a drug, get a 12-lead) but delays compressions or defibrillation. When an adult is pulseless and not breathing normally, the answer is almost always "start/continue high-quality CPR and prepare to defibrillate."
The Adult Chain of Survival
The BLS sequence is the front half of the out-of-hospital Chain of Survival, the linked sequence of actions that maximizes survival: (1) recognition of arrest and activation of the emergency response system, (2) early high-quality CPR with an emphasis on chest compressions, (3) rapid defibrillation, (4) advanced resuscitation by EMS/ACLS providers, (5) post-cardiac-arrest care, and (6) recovery (the link added in recent updates, covering rehabilitation and survivorship).
The in-hospital chain replaces the first link with surveillance and prevention by rapid-response/early-warning systems, because many hospital arrests are preceded by hours of deteriorating vital signs that a trained team can catch first. Recognizing where each action sits in the chain is exactly the discrimination exam items reward: a lone rescuer's first job is recognition and activation, the next is compressions, and defibrillation follows the moment a device is available.
Mislabeling these links — for example, attaching the AED before recognizing arrest, or starting drugs before perfusion exists — is the most common conceptual error the test probes.
A lone healthcare provider witnesses an adult suddenly collapse and finds the person unresponsive with no normal breathing. What should the rescuer do FIRST?
In the current adult BLS sequence, what does the order C-A-B represent and why was compressions placed first?
During the assessment step, a healthcare provider sees an unresponsive adult making occasional gasping sounds. How should this be interpreted?