Basic Life Support (BLS) is a standardized sequence of emergency procedures used to manage cardiac arrest, airway obstruction, and respiratory failure until advanced medical help arrives. The effectiveness of these interventions relies heavily on immediate, coordinated action, which is why global organizations establish uniform protocols. The International Liaison Committee on Resuscitation (ILCOR) provides a consensus on the science of resuscitation, which national organizations adopt for training guidelines. The ILCOR 2020 consensus re-emphasized the importance of high-quality cardiopulmonary resuscitation (CPR) and rapid defibrillation. This recommended sequence provides a framework designed to maximize the chance of survival for a person experiencing a sudden cardiac event.
Scene Safety and Initial Assessment
The first action upon encountering a potential victim is to quickly survey the environment to ensure the safety of both the rescuer and the person requiring assistance. Rescuers must check for hazards like traffic, fire, or electrical sources before approaching. Once the scene is determined to be safe, the rescuer should immediately attempt to assess the person’s level of consciousness.
This initial assessment involves gently tapping the victim and shouting loudly to determine responsiveness. If the person does not respond, the rescuer simultaneously looks for signs of normal breathing. This check for breathing, or for abnormal gasping that may indicate cardiac arrest, should take no longer than ten seconds to complete. If the person is unresponsive and not breathing, or only gasping, the rescuer must assume cardiac arrest and proceed immediately with the next steps of the sequence.
Activating Emergency Medical Services
The immediate priority following the recognition of cardiac arrest is to activate the emergency response system. For an adult who collapses suddenly, the protocol is to “call first,” meaning the rescuer should immediately call the local emergency number, such as 911.
If the rescuer is alone, they should put the phone on speaker mode to communicate with the dispatcher while performing the remaining steps. The dispatcher can provide guidance and confirm that the emergency medical services (EMS) are en route. The rescuer should also specifically request that an Automated External Defibrillator (AED) be brought to the scene as soon as possible, as this device is a time-sensitive component of the overall resuscitation effort.
High Quality Chest Compressions
The most impactful part of the BLS sequence is the delivery of high-quality chest compressions, which circulate oxygenated blood to the brain and heart. The ILCOR 2020 guidelines specify a compression rate between 100 and 120 compressions per minute to optimize blood flow. The rescuer must push hard and fast on the center of the person’s chest, specifically the lower half of the breastbone.
For an adult, the compression depth should be at least 2 inches, or approximately 5 to 6 centimeters. The rescuer must allow the chest to fully recoil after each compression, which permits the heart to refill with blood. Minimizing interruptions to compressions is important, as every pause reduces the pressure needed to perfuse the vital organs. Rescuers should aim to limit interruptions to less than ten seconds, particularly when transitioning between compressions and ventilations.
Airway Opening and Rescue Breathing
Following the initial set of compressions, the sequence requires opening the airway to deliver rescue breaths. The recommended method for achieving a patent airway is the Head Tilt–Chin Lift maneuver, which gently lifts the chin while tilting the forehead back. This action moves the tongue away from the back of the throat, which is the most common cause of airway obstruction in an unresponsive person.
For conventional CPR, the adult sequence maintains a compression-to-ventilation ratio of 30 compressions followed by 2 rescue breaths. Each breath should be delivered over approximately one second and must be sufficient to cause a visible chest rise. Lay rescuers who are unwilling or unable to provide mouth-to-mouth ventilation are encouraged to perform compression-only CPR, as continuous compressions are still better than no action at all.
Integrating Defibrillation
The final step in the sequence is the integration of defibrillation, which must be performed as soon as an AED is available at the scene. The AED is a sophisticated device that analyzes the person’s heart rhythm and delivers an electrical shock to reset a chaotic rhythm. The rescuer must power on the AED and follow the clear, spoken prompts delivered by the device.
Pads are applied directly to the person’s bare chest, typically one below the right collarbone and the other on the person’s side below the left armpit. Once the pads are connected, the rescuer must ensure that no one is touching the person while the AED analyzes the heart rhythm. If a shock is advised, the rescuer must clear everyone away before delivering the electrical impulse, and then immediately resume chest compressions without delay.