Aspiration of gastric contents is a rare but significant concern during the perioperative period. The incidence of pulmonary aspiration ranges between 0.7 and 4.7 per 10 000 administered anesthetics in nonpregnant adults, 5.3 per 10 000 anesthetics in pregnant patients, and 3.8 and 10.2 per 10 000 anesthetics in children. Pulmonary aspiration increases the risk of perioperative morbidity (ARDS, prolonged intubation, infection) and mortality (3.8%-4.6% in the general population, 0%-12% in the obstetric patients).
General anesthesia increases the risk of aspiration. Patients with certain comorbidities are at higher risk for pulmonary aspiration than the general population. Appropriate identification of these high-risk patients, as well as the implementation of risk-reduction interventions, are important for the safe delivery of anesthesia.
PATHOPHYSIOLOGY OF ASPIRATION
Natural barriers to aspiration include the lower esophageal sphincter, the upper esophageal sphincter, and the intrinsic protective airway reflexes.
Lower Esophageal Sphincter
The lower esophageal sphincter (LES) is a complex anatomic region which combines both circular and longitudinal fibers, and forms a barrier between the esophagus and the stomach. The left border of the esophagus aligns with the gastric fundus. The right crus of the diaphragm forms a sling around the abdominal esophagus, forming the “extrinsic LES.” The intrinsic LES is the band of circular muscle fibers that lie deeper into this extrinsic LES.
Gastroesophageal reflux is caused by a defect in the combined LES tone with transient relaxation of its tone that allows transit of gastric contents into the distal esophagus. Anesthetic agents and techniques can further exacerbate such a defect (Table 108-1). The net effect of a standard IV induction is a decrease in the LES tone. Conditions associated with chronic increased intraabdominal pressure, such as obesity and pregnancy, are associated with a high incidence of gastroesophageal reflux.
TABLE 108-1Factors Affecting LES Tone ||Download (.pdf) TABLE 108-1 Factors Affecting LES Tone
|Lower LES Tone ||Increased LES Tone ||No Effect on LES Tone |
|Anticholinergic agents ||Antiemetics ||Atracurium |
|Opioids ||Succinylcholine ||Vecuronium |
|Thiopental ||NMBD ||H2 antagonists |
|Propofol ||Cholinergic agents ||Sleep |
|Inhaled anesthetics ||Antacids || |
|Cricoid pressure || || || |
|NG tube || || || |
|Alkalinization || || || |
|Protein feeding || || |
Upper Esophageal Sphincter
Once gastric contents are present in the esophagus, the upper esophageal sphincter (UES) presents the next barrier to pulmonary aspiration. The cricopharyngeus muscle acts as a functional UES, assisting the actual UES to maintain a barrier between the hypopharynx and the proximal esophagus. Its tone is reduced during both general anesthesia and normal sleep. In fact, with the exception of ketamine, most anesthetic agents will cause relaxation of the UES.
Intrinsic Protective Airway Reflexes
If gastric contents make it past the UES, four reflexes help mitigate aspiration: apnea with laryngospasm, coughing, expiration, and spasmodic panting. Laryngospasm ...