Mouth-to-mouth resuscitation (MMR), often called rescue breathing, is an emergency procedure designed to manually supply oxygen to a person who is not breathing adequately or has stopped breathing entirely. The technique involves a rescuer exhaling air directly into the victim’s lungs. Due to the widespread adoption of hands-only cardiopulmonary resuscitation (CPR) for many adult emergencies, there is public confusion about whether MMR is still necessary. However, rescue breaths remain a scientifically proven intervention required in specific circumstances where the underlying problem is a lack of oxygen rather than a primary heart event. This article explores the scientific basis for MMR’s effectiveness and clarifies precisely when it is required.
How Rescue Breathing Delivers Necessary Oxygen
The scientific basis for mouth-to-mouth resuscitation relies on the composition of the rescuer’s exhaled air. While the air we breathe in contains approximately 21% oxygen, the air we exhale still retains a significant amount, typically ranging from 16% to 17%. This residual oxygen concentration is more than enough to sustain the vital functions of a person whose own breathing has failed.
The simple act of exhaling into a non-breathing person’s mouth forces this oxygen-rich air into the alveoli of their lungs, where gas exchange can occur. Although chest compressions are needed to circulate this newly oxygenated blood, the rescue breath guarantees the delivery of oxygen required by the brain and other organs. Without this manual ventilation, the existing oxygen reserves in the body are rapidly depleted, especially in respiratory-related emergencies.
When Mouth-to-Mouth is Required in Modern CPR
The decision to use rescue breaths depends entirely on the suspected cause of the collapse. Hands-only CPR is recommended for most adult victims of sudden collapse, which is typically caused by an electrical problem in the heart, known as primary cardiac arrest. In these events, the victim’s blood still holds enough oxygen for the first few minutes, and the immediate need is to circulate that blood with continuous compressions. For lay rescuers, the simplicity of hands-only CPR encourages immediate action and minimizes interruptions to chest compressions.
Rescue breaths are mandatory whenever the cause of the emergency is a primary respiratory problem, leading to oxygen deprivation. These scenarios create a state of respiratory arrest, where the blood’s oxygen levels drop almost immediately and must be manually replenished. Specific situations where rescue breathing is a required component of CPR include:
- Drowning, where the lungs are filled with water.
- Drug or opioid overdose, which causes breathing to slow or stop.
- Smoke inhalation.
- Carbon monoxide poisoning.
In these instances, just pressing on the chest will only circulate oxygen-depleted blood.
Pediatric patients (infants and children) also require rescue breaths. For children, cardiac arrest is most frequently a secondary result of respiratory failure, often due to conditions like severe asthma, choking, or respiratory infections. Since the problem begins with a lack of oxygen, the standard protocol for all pediatric CPR involves the combination of chest compressions and rescue breaths at a ratio of 30 compressions to two breaths for a single rescuer. This dual approach addresses both the lack of oxygen and the subsequent failure of the heart.
Proper Execution of Rescue Breaths
The technique must be executed precisely to ensure the air reaches the lungs. The first step involves opening the airway, which is accomplished using the head-tilt/chin-lift maneuver. This action moves the tongue away from the back of the throat, preventing it from obstructing the passage of air.
With the airway open, the rescuer must pinch the victim’s nose shut using the thumb and index finger of the hand placed on the forehead. This prevents the delivered air from escaping through the nasal passages. The rescuer then takes a normal breath and creates a complete, tight seal over the victim’s mouth with their own.
The rescuer delivers the breath by gently blowing steadily for about one second, watching the chest to confirm it visibly rises. If the chest does not rise, the rescuer should immediately reposition the head and attempt the breath again, as the airway may still be blocked. After a successful first breath, a second one-second breath is delivered, followed immediately by chest compressions to complete the cycle of two breaths for every 30 compressions. Barrier devices such as a pocket mask are recommended to provide a physical shield between the rescuer and the victim, offering an additional measure of safety.