Bag Valve Mask - Method of Operation

Method of Operation

The BVM directs the gas inside it via a one-way valve when compressed by a rescuer; the gas is then delivered through a mask and into the patient's trachea, bronchus and into the lungs. In order to be effective, a bag valve mask must deliver between 500 and 800 milliliters of air to an adult patient's lungs, but if oxygen is provided through the tubing and if the patient's chest rises with each inhalation (indicating that adequate amounts of air are reaching the lungs), 400 ml may still be adequate. Squeezing the bag once every 5 seconds for an adult or once every 3 seconds for an infant or child provides an adequate respiratory rate (12 respirations per minute in an adult and 20 per minute in a child or infant).

Professional rescuers are taught to ensure that the mask portion of the BVM is properly sealed around the patient's face (that is, to ensure proper "mask seal"); otherwise, air pressure is relieved to the environment instead of to the lungs. This is difficult when a single rescuer maintains the seal with one hand while operating the bag with other. Therefore, common protocol uses two rescuers: one rescuer to hold the mask to the patient's face with both hands and focus entirely on mask seal, while the other squeezes the bag and focuses on tidal volume and timing. However, in a two-person ambulance crew, the other crew member is likely to be doing compressions in the case of CPR, or may be performing other interventions such as defibrillation or cannulation. In this case, or if no other options are available, the BVM can also be operated by a single rescuer who holds the mask to the patient's face with one hand, in the anaesthetists grip, and squeezes the bag with the other

When using a BVM, as with other methods of positive pressure ventilation, there is a risk of over-inflating the lungs. This can lead to pressure damage to the lungs themselves, and can also cause air to enter the stomach, causing gastric distension which can make it more difficult to inflate the lungs. Another consequence may be to cause the patient to vomit, which can cause additional airway problems beyond the original breathing difficulty. This can be usually be avoided by care on behalf of the rescuer. Alternatively, some models of BVM (usually pediatric) are fitted with a valve which prevents over-inflation, by venting the pressure when a pre-set pressure is reached. Nevertheless, the "Sellick maneuver" (application of cricoid pressure) is often applied to reduce the risk of aspiration of gastric contents whenever possible until the trachea can be intubated or until there is no longer any need for positive pressure ventilation.

An endotracheal tube can be inserted by a trained practitioner and can substitute for the mask portion of the BVM. This provides a more secure fit and is easier to manage during emergency transport, since the ET tube is sealed with an inflatable cuff in the trachea, so that any regurgitation is less likely to enter the lungs, and so that positive air pressure can only be relieved into the lungs. The ET also maintains an open airway at all times, even during CPR compressions; a BVM can typically only be operated during set pauses in compressions.

(Vomitus can severely damage the lung tissue, and in the absence of an ET tube, could choke the patient by obstructing the airway. Inhalation of stomach contents can be fatal; the after effects can cause Mendelson's syndrome or aspiration pneumonia.)

Some rescuers may also choose to use a different form of resuscitation adjunt, such as an oropharyngeal airway or Laryngeal mask airway, which would be inserted and then used with the BVM.

In a hospital, long-term mechanical ventilation is provided by using more complex automated devices such as an intensive care ventilator, rather than by a BVM, which requires at least one person to operate it constantly.

A flow-restricted, oxygen-powered ventilation device (FROPVD) is similar to a BVM in that oxygen is pushed through a mask into the patient's lungs, but unlike a BVM, in the FROPVD the pressure needed to push air into the patient's lungs is generated by oxygen via a pressure regulator from a cylinder rather than by squeezing a bag.