Bag-mask ventilation (BVM) is a common method used in emergency and medical settings to provide oxygen and air pressure to a person who is unable to breathe adequately. This procedure involves manually squeezing a self-inflating bag attached to a face mask to push air into the patient’s airway. While highly effective, a frequent and serious complication is gastric inflation, which occurs when air is inadvertently forced into the stomach instead of the lungs. This misplaced air severely compromises the effectiveness of the entire ventilation effort.
The Goal of Bag-Mask Ventilation
The primary purpose of bag-mask ventilation is to deliver a sufficient volume of air, known as tidal volume, into the patient’s lungs to ensure adequate oxygen exchange. The desired anatomical pathway travels through the pharynx and down the trachea into the lungs. For the procedure to be successful, the rescuer must see a visible, gentle rise and fall of the patient’s chest. This simple visual confirmation is the most immediate sign that the delivered air is reaching the lungs as intended, facilitating respiration.
Why Air Enters the Stomach
The body’s airway and the digestive tract are closely situated in the pharynx, making it possible for air to be misdirected during positive pressure ventilation. Air is diverted from the trachea into the esophagus and subsequently the stomach when the pressure applied by the bag exceeds the opening pressure of the lower esophageal sphincter (LES). This ring of muscle acts as a valve between the esophagus and the stomach, normally remaining tightly closed. In unconscious or critically ill patients, the tone of the LES is often reduced, meaning it can be forced open by relatively low pressure.
The most common cause of high pressure is the technique used by the rescuer, particularly when they ventilate too quickly or with too much force. Rapid breaths or the delivery of an excessive volume of air can cause the airway pressure to spike sharply. Studies suggest that the LES can be overcome when inspiratory pressures exceed approximately 20 cm of water (H₂O). Once this pressure threshold is crossed, the air follows the path of least resistance, which becomes the open esophagus leading to the stomach.
Physical Restriction of the Diaphragm
Once air enters and begins to accumulate in the stomach, the resulting gastric distension creates a direct physical impediment to effective breathing. The inflated stomach pushes upward against the underside of the diaphragm, the large, dome-shaped muscle that separates the chest cavity from the abdomen. As the primary muscle of respiration, the diaphragm must be able to descend fully to allow the chest cavity to expand and the lungs to inflate.
This upward pressure restricts the diaphragm’s ability to move downward, effectively limiting the space available for the lungs within the thoracic cavity. This reduction in available chest volume is often described as decreased lung compliance, meaning the lungs become stiffer and more difficult to inflate. Consequently, the amount of air that can be delivered to the lungs with each breath is reduced, and the effective ventilation needed for oxygenation fails.
The Danger of Regurgitation and Aspiration
Beyond the physical restriction of the diaphragm, gastric inflation presents a serious secondary complication: the risk of regurgitation and pulmonary aspiration. As the stomach fills with air, the pressure inside the organ increases significantly. This elevated pressure forces the gastric contents—which may include food particles, digestive juices, and stomach acid—back up through the now-compromised lower esophageal sphincter.
Regurgitation is the process of these contents flowing back into the pharynx and upper airway. If these contents are then inhaled into the lungs, a process called aspiration, it causes immediate and severe problems. Aspiration can lead to acute airway obstruction. Furthermore, the highly acidic stomach contents cause chemical injury to the delicate lung tissues, resulting in aspiration pneumonitis, a condition that can lead to severe respiratory distress.
Techniques for Risk Minimization
To prevent the cascade of problems caused by gastric inflation, rescuers must adhere to specific ventilation techniques. One of the most effective methods is to deliver the minimal effective tidal volume, which is just enough air to cause the chest to rise visibly and gently. For an adult, this volume is typically between 500 and 800 milliliters, which is often achieved by only partially squeezing the bag-mask device.
Ventilation rate is another factor that must be carefully managed to avoid excessive pressure buildup. The recommended rate for adults is approximately one breath every five to six seconds, translating to 10 to 12 breaths per minute. Maintaining this slow, deliberate rhythm helps keep the inspiratory pressure below the threshold that would force open the lower esophageal sphincter. Additionally, proper airway maneuvers, such as the head-tilt/chin-lift or the jaw-thrust, help ensure the air is directed toward the trachea and lungs by opening the upper airway and reducing the resistance the delivered air must overcome.