A bag valve mask, abbreviated to BVM and sometimes known by the proprietary name Ambu bag, is a hand-held device used to provide positive pressure ventilation to a patient who is not breathing or who is breathing inadequately. The device is a normal part of a resuscitation kit for trained professionals, such as ambulance crew. The BVM is frequently used in hospitals, and is an essential part of a crash cart. The device is used extensively in the operating room to ventilate an anaesthetised patient in the minutes before a mechanical ventilator is attached. The device is self-filling with air, although additional oxygen (O2) can be added.
Use of the BVM to ventilate a patient is frequently called "bagging" the patient.[1] Bagging is regularly necessary in medical emergencies when the patient's breathing is insufficient (respiratory failure) or has ceased completely (respiratory arrest). The BVM resuscitator is used in order to manually provide mechanical ventilation in preference to mouth-to-mouth resuscitation (either direct or through an adjunct such as a pocket mask).
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The bag valve mask concept was developed in 1953 by the German engineer, Dr. Holger Hesse, and his partner, Danish anaesthetist Henning Ruben, following their initial work on a suction pump.[2] They named their resuscitator Ambu bag, and then formed their own company, also called Ambu, to manufacture and market it, starting in 1956. This position as first to market has led to the name Ambu becoming a generic trademark, with bag valve masks from any manufacturer being referred to as 'ambu bags'.
The BVM consists of a flexible air chamber (the "bag", about the size of an American football ), attached to a face mask via a shutter valve. When the face mask is properly applied and the "bag" is squeezed, the device forces air through into the patient's lungs; when the bag is released, it self-inflates from its other end, drawing in either ambient air or a low pressure oxygen flow supplied by a regulated cylinder, while also allowing the patient's lungs deflate to the ambient environment (not the bag) past the one way valve.
Bag and valve combinations can also be attached to an alternate airway adjunct, instead of to the mask. For example, it can be attached to an endotracheal tube or laryngeal mask airway. Often a small HME filter (Heat & Moisture exchanger, or humidifying / bacterial filter) is used.
A bag valve mask can be used without being attached to an oxygen tank to provide air to the patient, often called "room air" in the U.S. Supplemental oxygen increases the partial pressure of oxygen inhaled, helping to increase perfusion in the patient.
Most devices also have a reservoir which can fill from an oxygen source during the patient expiratory phase (a process which happens passively for patients in respiratory arrest), in order to increase the amount of oxygen that can be delivered to the patient to nearly 100%.[3]
Bag valve masks come in different sizes to fit infants, children, and adults. The mask size may be independent of the bag size; for example, a single pediatric-sized bag might be used with different masks for multiple face sizes, or a pediatric mask might be used with an adult bag for patients with small faces.
Most types of the device are disposable and therefore single use, while others are designed to be cleaned and reused.
A filter is sometimes placed between the mask and the bag (before or after the valve) to prevent contamination of the bag.
Some devices have PEEP valve connectors, for better positive airway pressure maintenance.
A covered port may be incorporated into the valve assembly, to allow inhalatory medicines to be injected into the airflow.
A pressure relief valve (often known as a "pop-up valve") is often included in pediatric versions, and sometimes in adult versions, to prevent overinflation of the lungs. A bypass clip is typically incorporated into this valve assembly, in case medical protocol calls for inflation at a rate beyond the maximum pressure cuttoff allowed by the pop-up valve. A connection for a pressure or flow meter may be included in the valve or mask assembly. Some bags have a built-in strap to assist the pressure provider.
Some bags are designed to collapse for storage. A bag not designed to store collapsed may lose elasticity when stored compressed for long periods, reducing its effectiveness. The collapsible design has longitudinal scoring so that the bag collapses on the scoring "pivot point," opposite to the direction of normal bag compression.
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.[1] 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).[4]
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 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.[5] 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.
Although the BVM is widely used as the primary ventilation device in most healthcare settings there are however some issues of concern when it is used by inexperienced providers or by providers utilizing the device improperly. The issue that arises the most is hyperventilation, (providing breaths too forcefully and/or too fast. Rescuers should deliver the breaths in order to achieve chest rise. Large volume breaths increase intrathoracic pressure or ITP; decrease venous return to heart. Long breaths interrupt compressions. Hyperventilation decreases coronary and cerebral perfusion pressures. Over-ventilation increases air in stomach; regurgitation/aspiration.
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