The Bag-Valve Mask (BVM) is a manual device used to provide positive pressure ventilation to a patient who is not breathing adequately or at all. This self-inflating device pushes air or oxygen into the lungs during emergency situations. The amount of oxygen delivered, known as the fractional inspired oxygen concentration (FiO2), is highly variable. The FiO2 depends entirely on how the device is set up and the technique used by the operator. Understanding the mechanics of the BVM is necessary to ensure the patient receives the required oxygen concentration.
BVM Operation Using Room Air
The BVM automatically re-expands after being squeezed, even without an external gas source. When the bag re-expands, it draws in ambient air through a valve in the rear of the device. Since the atmosphere contains approximately 21% oxygen, the delivered oxygen concentration (FiO2) in this baseline setup is only slightly greater than 21%.
The mechanical design allows the delivered FiO2 to reach a maximum of around 30% when used solely with room air. This low concentration is generally inadequate for patients experiencing severe respiratory distress or cardiac arrest. To significantly increase the oxygen concentration, supplemental oxygen and additional equipment are required.
The Role of Supplemental Oxygen and Reservoirs
Achieving high oxygen concentrations requires connecting the BVM to an external oxygen source and utilizing a reservoir bag. This large, collapsible pouch attaches to the rear of the BVM and collects pure oxygen during the device’s resting phase. To maximize delivery, the oxygen flow rate must be set to a high level, typically 10 to 15 liters per minute.
The reservoir bag’s primary function is to eliminate the entrainment of room air into the self-inflating bag. As pure oxygen flows in, it fills the reservoir, which should remain plump. When the operator releases the compression, the self-inflating bag draws gas from the full reservoir, ensuring the gas delivered to the patient is nearly 100% oxygen.
The combination of a high-flow oxygen source and a properly inflated reservoir bag allows the BVM to theoretically deliver an FiO2 of 100%. In practice, the maximum delivered concentration is usually in the range of 90% to 95%. This slight reduction occurs because most BVM devices have safety valves that allow for a small amount of room air entrainment, preventing a vacuum if the oxygen source were to fail.
Technique and Human Factors in Oxygen Delivery
Even when the BVM is correctly connected to a high-flow oxygen source and reservoir bag, the actual oxygen concentration reaching the patient can be reduced by practical limitations and human factors. The single most important factor is the quality of the seal between the mask and the patient’s face. A tight seal is necessary to ensure that the positive pressure breath is delivered entirely to the lungs, preventing leaks around the edges of the mask.
Any leak in the mask seal allows ambient air to be drawn in and mix with the gas being delivered. This entrainment of low-concentration air can dramatically dilute the intended high FiO2, even if the equipment setup is perfect. Providers often use a two-handed mask-holding technique, known as the E-C clamp, to maintain a firm seal and minimize dilution.
The rate at which the bag is squeezed also influences the delivered oxygen concentration. If the operator ventilates the patient too quickly, the self-inflating bag may not allow enough time for the reservoir bag to fully refill with pure oxygen. A partially deflated reservoir means the BVM will compensate by drawing in room air through the air intake valve, lowering the delivered FiO2. Delivering breaths at the recommended rate of 10 to 12 per minute is necessary to ensure optimal oxygenation.