A bag-valve mask (BVM), often called an Ambu bag, is a portable manual resuscitator used in emergency medicine. It provides immediate, positive-pressure ventilation for individuals who are not breathing or whose breathing is inadequate. The BVM delivers breaths by forcing air or oxygen into the patient’s lungs until a more advanced airway can be secured or the patient can breathe effectively on their own. It is a crucial intervention for respiratory failure or cardiac arrest in both hospital and out-of-hospital settings.
How a Bag-Valve Mask Delivers Ventilation
The bag-valve mask functions by providing positive pressure ventilation, manually pushing air into the patient’s lungs to inflate them. The device is composed of three main parts: a flexible, self-inflating bag, a one-way valve system, and a face mask that forms a seal over the patient’s nose and mouth. Squeezing the self-inflating bag forces the gas inside through the one-way valve and into the patient’s airway.
The self-inflating bag automatically re-expands after each squeeze, drawing in fresh gas for the next breath. The integrated non-rebreathing valve ensures that the air being delivered travels only into the patient’s lungs and not back into the bag. This valve also prevents the patient’s exhaled air, which is high in carbon dioxide, from mixing with the gas supply in the bag.
This mechanical action simulates a natural breath by physically inflating the lungs under pressure. For an adult, an effective breath, known as tidal volume, typically requires delivering between 500 to 600 milliliters of air. This volume is usually achieved by only partially squeezing the bag, as full compression of an adult bag can deliver an excessive amount of air.
The Composition of the Delivered Breath
The concentration of oxygen delivered by a bag-valve mask depends on whether supplemental oxygen is attached. If the BVM is used without an external oxygen source, the self-inflating bag draws in room air, which contains approximately 21% oxygen. While this is an improvement over no ventilation, it is often insufficient for a critically ill patient.
In professional medical settings, the BVM is almost always connected to an oxygen tank via tubing and an oxygen reservoir bag. This reservoir fills with 100% oxygen, and when the rescuer squeezes the bag, the gas is drawn primarily from this oxygen-rich source. By supplying oxygen at a high flow rate (typically 10 to 15 liters per minute), the BVM can deliver an oxygen concentration approaching 100%. This high concentration is essential for patients experiencing respiratory or cardiac arrest, distinguishing the BVM from standard resuscitation methods.
Proper Technique and Essential Considerations
For the bag-valve mask to effectively deliver a breath, two conditions must be met: the patient’s airway must be open, and a tight seal must be maintained between the mask and the face. Rescuers often use maneuvers such as the head-tilt/chin-lift or the jaw-thrust to ensure the tongue does not block the back of the throat. Airway adjuncts, like an oropharyngeal or nasopharyngeal airway tube, are frequently inserted to help maintain this open path.
The proper seal is achieved using the C-E grip technique. The thumb and index finger form a “C” around the mask to press it onto the face, while the remaining fingers form an “E” to lift the patient’s jaw toward the mask. This tight seal prevents delivered air from escaping around the edges instead of entering the lungs. The two-person technique, where one person focuses solely on maintaining the seal with both hands, is often preferred for better effectiveness.
A major concern during BVM ventilation is the risk of gastric inflation, which occurs when air enters the stomach instead of the lungs. This complication is caused by delivering breaths too forcefully or with excessive volume, which can open the esophageal sphincter. To minimize this risk, each breath should be delivered gently over about one second, using only enough volume to cause a visible, slight rise of the chest.