A resuscitation bag mask (commonly called a bag-valve-mask or BVM) is made up of several connected devices that work together to push air into a person’s lungs during an emergency. The core assembly includes three main parts: a self-inflating bag, a one-way valve, and a face mask. Additional devices like oxygen reservoirs, pressure-relief valves, and filters can be attached depending on the situation.
The Three Core Components
Every BVM has the same basic structure. A self-inflating bag sits at the center of the device. This is the part a rescuer squeezes to deliver a breath. It refills on its own after each squeeze, drawing in air automatically. For adults, the bag holds between 1,500 and 2,000 mL of air. Pediatric versions hold 500 to 1,000 mL.
Attached to the front of the bag is a non-rebreathing valve. This is the critical piece that directs airflow in one direction only: fresh air goes into the patient’s lungs when the bag is squeezed, and exhaled air exits through a separate port rather than flowing back into the bag. Without this valve, a patient would rebreathe their own carbon dioxide. Two main valve designs exist. Flap-style valves (like the Ambu single shutter) use a hinged flap that swings open during a squeeze and closes during exhalation. Duckbill valves use a flexible slit that opens under pressure. Duckbill models without a separate expiratory valve can allow room air to leak in through the exhaust port if a patient is breathing on their own, which reduces oxygen delivery, particularly during respiratory distress.
The face mask connects to the valve and seals over the patient’s nose and mouth. It has a soft, inflatable cushion around its rim that conforms to the contours of the face. A tight seal is essential because any gap lets air escape instead of reaching the lungs. The American Heart Association’s 2025 guidelines note that a two-handed technique with jaw thrust creates a better seal than the traditional one-handed “E-C clamp” grip, where the thumb and index finger form a C over the mask while the remaining fingers lift the jaw in an E shape. Two-rescuer ventilation, with one person holding the mask and another squeezing the bag, is the most effective approach.
The Oxygen Reservoir
On the opposite end of the bag from the mask, you’ll typically find a reservoir bag (sometimes called a tail bag) connected to an oxygen supply line. When the BVM is used without supplemental oxygen, it delivers room air, which is only about 21% oxygen. Connecting 100% oxygen to the inlet and adding a reservoir bag allows oxygen to collect between breaths so that each squeeze delivers a much higher concentration. Most emergency protocols call for this setup as the standard configuration.
Pressure-Relief Valve
Many BVM devices, especially pediatric models, include a built-in pressure-relief valve (sometimes called a pop-off valve). This device automatically vents air if the pressure inside the system gets too high, which helps prevent lung injury from overinflation. In some adult resuscitation scenarios, rescuers may need to override or disable this valve to deliver adequate breaths, particularly when airway resistance is high. The valve is typically a small button or dial integrated into the bag or valve assembly.
Viral and Bacterial Filters
A filter can be placed between the valve and the face mask (or between the valve and an advanced airway tube) to trap exhaled pathogens. This became standard practice during infectious disease outbreaks to protect rescuers and bystanders from airborne particles in exhaled breath. The recommended setup uses two filters: one on the patient side to catch exhaled organisms before they enter the device or room, and a second on the exhalation port to further protect the surrounding environment.
These filters do add a small amount of extra space in the breathing circuit, which means a portion of each breath stays in the filter housing rather than reaching the lungs. This “dead space” effect matters more in smaller patients, so pediatric teams weigh the infection control benefit against the ventilation tradeoff.
PEEP Valve
A PEEP (positive end-expiratory pressure) valve is an optional attachment that threads onto the exhalation port of the non-rebreathing valve. It keeps a small amount of pressure in the lungs even after the patient exhales, which prevents the tiny air sacs in the lungs from collapsing between breaths. This is especially useful for patients whose lungs have trouble staying open on their own, such as those with fluid in the lungs or severe pneumonia. PEEP valves are adjustable, letting the rescuer dial in the right amount of back-pressure for the situation.
How the Parts Work Together During Use
During resuscitation, the rescuer seals the mask to the patient’s face, squeezes the bag over about one second, and watches for the chest to rise. For an adult with a pulse who isn’t breathing normally, the current guideline is one breath every six seconds, or about 10 breaths per minute. During CPR with chest compressions, breaths are delivered in the pauses of a 30-compressions-to-2-breaths cycle, keeping interruptions as short as possible.
Each device on the BVM serves a specific role in that sequence. The reservoir collects high-concentration oxygen between squeezes. The self-inflating bag delivers it under pressure. The non-rebreathing valve routes fresh gas in and exhaled gas out. The filter catches pathogens in the exhaled stream. The PEEP valve, when attached, maintains residual pressure. And the mask creates the seal that makes the whole system work. Remove or misconnect any one piece and the ventilation either fails or becomes significantly less effective.