Pulmonary aspiration is defined as the entry of gastric contents, oropharyngeal secretions, or other foreign materials into the trachea and lungs. Although it is a relatively rare complication, occurring in approximately 1 in every 2,000 to 3,000 cases, its potential consequences are severe. This event can lead to pulmonary injury, respiratory failure, and in the most serious instances, death. The risk is highest during the induction and emergence phases of anesthesia when a patient’s protective reflexes are lost or have not yet fully returned.
Why Aspiration Occurs During Anesthesia
The administration of general anesthetic agents creates conditions that compromise the body’s natural defenses against foreign material entering the lungs. Under normal circumstances, reflexes like the cough and gag reflex, along with the tight closure of the larynx, form an effective barrier. Anesthetic medications diminish or completely suppress these protective airway reflexes, leaving the respiratory tract vulnerable to any material that moves backward from the stomach or throat.
The second primary mechanism involves the lower esophageal sphincter (LES), which acts as a muscular valve separating the esophagus from the stomach. Many anesthetic drugs, including volatile agents, opioids, and certain sedatives, cause this sphincter to relax, reducing its resting pressure. This relaxation eliminates the pressure barrier that typically prevents stomach contents from moving back up into the esophagus, a process known as regurgitation. Once the contents reach the back of the throat, the absence of the cough and gag reflex allows them to enter the trachea and subsequently the lungs. Conditions that delay gastric emptying, such as trauma, obesity, pregnancy, or diseases like diabetes, significantly increase the volume of material available for potential regurgitation.
Immediate Damage and Symptoms in the Lungs
When gastric contents are aspirated, the immediate damage is primarily determined by the volume and acidity of the material inhaled. The resulting injury can manifest as two distinct pulmonary syndromes, often beginning with an acute chemical reaction. The most acute form is aspiration pneumonitis, a severe chemical burn caused by the stomach’s highly acidic fluid. Stomach acid typically has a pH below 2.5, and when inhaled, it rapidly destroys the delicate lining of the alveoli and small airways.
This chemical injury triggers an inflammatory response in the lung tissue, leading to pulmonary edema and the breakdown of the lung’s surfactant, which is necessary for maintaining open air sacs. Immediate clinical signs include a sudden onset of bronchospasm (a tightening of the airways) and a drop in the patient’s oxygen saturation (SpO2). Patients may develop a cough, wheezing, and may expel pink, frothy sputum as fluid fills the air sacs.
Aspiration can also lead to aspiration pneumonia, a bacterial infection that develops later. This occurs when the aspirated material contains a significant amount of bacteria, typically from the oral cavity or upper gastrointestinal tract. While pneumonitis is a sterile chemical injury, pneumonia is an infectious process that may take 24 to 48 hours to develop. Both outcomes can lead to acute respiratory distress syndrome (ARDS), a life-threatening condition requiring intensive support.
Protocols for Minimizing Aspiration Risk
The fundamental protocol for preventing aspiration is pre-operative fasting, often referred to as nil per os (NPO). This measure aims to reduce the volume of gastric contents, minimizing the risk of regurgitation and the severity of injury. Standard guidelines recommend that patients fast from clear liquids for a minimum of two hours before an elective procedure. Intake of light meals, infant formula, or non-human milk requires a minimum fasting period of six hours. Meals containing fried foods, fatty foods, or meat slow gastric emptying significantly and require a longer fasting period, generally eight hours or more.
For patients identified as high risk for aspiration, such as those with bowel obstruction or trauma, a specialized technique called Rapid Sequence Induction (RSI) is often used. RSI secures the airway with a cuffed breathing tube in the shortest possible time. This is achieved by rapidly administering a sedative and a fast-acting muscle relaxant, allowing for immediate intubation without the time-consuming step of manual ventilation.
A maneuver known as cricoid pressure was previously a standard component of RSI. This involves applying pressure to the cricoid cartilage to compress the esophagus against the vertebral column, intending to physically prevent the reflux of gastric contents into the pharynx. However, the use of cricoid pressure has become controversial, as studies show it is not consistently effective and may occasionally distort the airway, making intubation more difficult. Therefore, its application is now often based on the individual clinician’s judgment.
Acute Medical Management and Recovery
If aspiration is suspected or confirmed during a procedure, the initial response focuses on securing the airway and preventing further inhalation. The team administers 100% oxygen and performs suctioning of the patient’s mouth and throat to remove visible material. If the patient is not already intubated, an endotracheal tube is quickly placed to protect the lower airway and allow for mechanical ventilation. Once the tube is in place, suctioning of the trachea and bronchi clears material from the lungs.
Management following the acute event aims to maintain adequate oxygenation and ventilation while the lung tissue recovers from the inflammatory injury. Bronchodilator medications are administered if the patient exhibits bronchospasm or wheezing. For aspiration pneumonitis caused by stomach acid, antibiotics are generally withheld because the injury is chemical, not bacterial. If the patient’s condition does not improve within 48 hours or if signs of infection develop, antibiotics may be started to treat developing aspiration pneumonia. Most patients with mild pneumonitis show improvement within one to two days, but severe events may require an extended stay in the intensive care unit for mechanical ventilation.