A Bag-Valve-Mask (BVM) device is a handheld medical tool used to provide manual positive pressure ventilation to individuals who cannot breathe adequately on their own. Commonly known by the brand name Ambu bag, this apparatus consists of a self-inflating bag, a one-way valve system, and a face mask. The specific amount of oxygen delivered to the patient is highly variable and depends entirely on the device setup and the technique of the rescuer.
BVM Operation Using Ambient Air
When a BVM device is used without connecting it to an external oxygen source, the patient receives only the concentration of gas drawn directly from the surrounding environment. This gas, known as ambient or room air, contains a fixed oxygen concentration of approximately 21%. The BVM’s self-inflating bag automatically refills with this air after each squeeze through an intake valve.
Patients requiring manual ventilation are often experiencing respiratory failure or cardiac arrest, conditions that demand a far higher oxygen concentration. Relying solely on ambient air is inadequate in these emergency situations because the patient’s body is already severely deprived of oxygen. The baseline delivery of 21% oxygen is therefore a starting point, but it underscores the necessity of adding supplemental oxygen to the system for life support.
Maximizing Oxygen Concentration with Adjuncts
To move beyond the 21% concentration of ambient air, the BVM must be connected to an oxygen supply, and a specific component called the oxygen reservoir bag is required. The reservoir bag attaches to the intake port of the BVM and acts as a storage vessel for high-concentration oxygen. This setup is crucial because it ensures the BVM bag refills with oxygen from the reservoir rather than pulling in room air.
To achieve the highest possible oxygen concentration, the flow meter on the oxygen tank must be set to a high rate, typically between 10 and 15 Liters Per Minute (LPM). This high flow rate ensures that the reservoir bag remains fully inflated throughout the ventilation process, preventing the unintentional entrainment of lower-concentration ambient air. When properly attached and with a flow rate of 15 LPM, the BVM system can theoretically deliver an inspired oxygen concentration (FiO2) approaching 90% to nearly 100%.
Without the reservoir bag adjunct, the BVM would rapidly draw in surrounding air to refill, significantly diluting the delivered gas mixture. Achieving a concentration in the 90-95% range is the goal for maximizing oxygen delivery, which provides a reserve of oxygen to the patient’s lungs. This high concentration is especially important during critical events like cardiac arrest to combat severe tissue oxygen deprivation.
Factors Influencing Actual Delivery
While the proper equipment setup can deliver a theoretical maximum of nearly 100% oxygen, the actual concentration reaching the patient’s lungs is often lower due to practical factors and rescuer technique. One of the most significant variables is the quality of the mask seal over the patient’s nose and mouth. If the seal is poor, the positive pressure forces the high-concentration oxygen to leak out, and the patient may simultaneously draw in ambient air from around the edges of the mask, diluting the inspired oxygen concentration.
Techniques like the two-hand or “two-thumbs-up” grip are often necessary to create a tight, leak-free seal, especially in patients with difficult facial anatomy or without teeth. The volume of air delivered with each squeeze, known as the tidal volume, also affects efficacy. Delivering an appropriate volume, roughly 500 to 600 milliliters for an average adult, ensures adequate chest rise without over-inflating the lungs.
Excessively fast ventilation rates or squeezing the bag too forcefully can also compromise effectiveness and oxygen delivery. Hyperventilation, or breathing too quickly, can raise pressure within the chest, which may decrease blood flow returning to the heart. The recommended rate for an adult is approximately 10 to 12 breaths per minute, or one breath every five to six seconds. Maintaining this measured rate and volume, alongside a secure mask seal, is necessary for translating the high theoretical oxygen concentration into effective gas exchange.