Bag-mask ventilation (BMV) is a manual technique used in emergency medicine to deliver oxygen and provide breathing support to patients who cannot breathe adequately on their own. The process involves using a self-inflating bag attached to a face mask to push air into the lungs. While BMV is an immediate and life-saving intervention, it carries a significant complication known as gastric inflation: the unintentional forcing of air into the patient’s stomach. When air fills the stomach instead of the lungs, it severely undermines the effectiveness of ventilation and can compromise the patient’s respiratory status.
How Air Enters the Stomach During Ventilation
Air is mistakenly directed into the stomach due to the close anatomical relationship between the trachea (lungs) and the esophagus (stomach). Both pathways share the pharynx as a common entry point. The esophagus is normally closed off by the lower esophageal sphincter (LES), which acts as a protective valve. Gastric inflation occurs when the positive pressure generated during the bag squeeze exceeds the opening pressure of this sphincter.
In adults, the LES typically opens when airway pressure reaches approximately 19 to 20 centimeters of water (cm H₂O). If the pressure applied is greater than this threshold, air follows the path of least resistance into the esophagus. Several technique errors increase the likelihood of exceeding this pressure threshold. Delivering breaths too quickly or with excessive volume creates a high peak inspiratory pressure that easily overcomes the LES resistance. Improper head positioning, which can obstruct the trachea, also necessitates higher pressure to ventilate the lungs, increasing the risk of air being forced into the stomach.
Mechanical Restriction of Diaphragm Movement
Once a significant volume of air accumulates in the stomach, the primary mechanism of ventilation impairment is mechanical. The distended stomach inflates the abdominal cavity, exerting an upward force on the diaphragm. The diaphragm is the primary muscle responsible for breathing, separating the chest cavity from the abdomen. For a successful breath, the diaphragm must descend into the abdominal space to expand the thoracic cavity.
When the stomach is inflated, it prevents this downward movement, essentially splinting the diaphragm in an elevated position. This physical restriction limits the space available for the lungs to expand during positive pressure ventilation. The result is a significant decrease in the patient’s tidal volume. Reduced tidal volume means less fresh oxygen reaches the air sacs in the lungs where gas exchange occurs, leading to inadequate air exchange and poor oxygenation. The stomach’s upward pressure also reduces lung compliance, making the lungs stiffer and requiring higher pressures to achieve minimal chest rise.
The Danger of Regurgitation and Aspiration
Beyond the mechanical barrier, gastric inflation introduces a secondary, life-threatening complication: regurgitation and aspiration. The air forced into the stomach raises the internal pressure. This increased pressure can overcome the sealing function of the lower esophageal sphincter from the inside, leading to the expulsion of stomach contents back up the esophagus and into the pharynx.
Regurgitated material, which often includes stomach acid, partially digested food, and bile, can then be inhaled, or aspirated, into the lungs. This event is hazardous because the acidic contents cause a severe inflammatory reaction in the lung tissue known as chemical pneumonitis. This chemical injury damages the lining of the airways and alveoli, disrupting the lungs’ ability to transfer oxygen into the bloodstream. Aspiration of solid food particles can also physically obstruct smaller airways, leading to a sudden failure of oxygenation. The resulting aspiration pneumonia and the need for prolonged mechanical support significantly worsen the patient’s prognosis and are associated with extended stays in the intensive care unit. The risk of gastric inflation is notably high during BMV, occurring in up to 69% of patients in some out-of-hospital cardiac arrest settings.
Techniques to Prevent Gastric Inflation
Preventing air from entering the stomach relies on meticulous technique and pressure control during ventilation. Proper positioning of the patient’s head is the primary step; maneuvers like the head-tilt/chin-lift help align the airway to favor air entry into the trachea rather than the esophagus. Maintaining a secure, leak-free seal between the mask and the patient’s face is also important to ensure the delivered air is directed into the airway. Using a two-person technique, where one rescuer focuses solely on the mask seal with both hands, can greatly improve this seal.
The most impactful preventative action is controlling the volume and speed of the breath delivered. Rescuers should use the minimal effective pressure required to achieve a visible, slight rise of the patient’s chest. Breaths should be delivered slowly, taking approximately one second to squeeze the bag, which helps keep the peak inspiratory pressure low and below the LES opening threshold. For advanced providers, applying gentle, downward pressure on the cricoid cartilage (cricoid pressure) can physically compress the esophagus against the vertebrae, helping prevent air entry when airway pressures are below 20 cm H₂O.