Mendelson Syndrome

In Curtis Lester Mendelson’s original 1946 description of his namesake syndrome, chemical pneumonitis was described in young and healthy obstetrical patients following aspiration of gastric acid under anesthesia. In the case series by Mendelson, 66 obstetrical patients under anesthesia with ether aspirated gastric contents. Within two hours of the witnessed aspiration, patients developed respiratory distress and cyanosis. Unilateral or bilateral lower lobe infiltrates were present on chest radiography. Although Mendelson’s sample had a positive outcome, subsequent studies have revealed that patients may develop acute respiratory distress syndrome (ARDS) following aspiration pneumonia. Mendelson’s landmark study suggested that chemical pneumonitis was preventable by restricting oral intake during labor, which eventually led to the NPO guidelines we have in place today for parturients. The field of obstetrics and gynecology has come a long way since Mendelson’s time, as the use of general anesthesia is now infrequent for laboring women, and neuraxial analgesia is the standard of care for modern practice. At present, the American College of Obstetricians and Gynecologists encourage the ingestion of clear liquids and the avoidance of solid food during labor.

Mendelson’s study reviewed the aspiration of gastric contents among 44000 pregnancies at the New York Lying-In Hospital from 1932 to 1945. His paper has two parts: a clinical report and an animal model. “Mendelson syndrome” was initially described as aspiration of gastric contents causing a chemical pneumonitis characterized by fever, cyanosis, hypoxia, pulmonary edema, and potential death. Among the patients studied, there were 66 cases of aspiration (0.15%) and two deaths (0.0045%). Both patients tragically died following suffocation from solid food aspiration of full meals that were ingested six and eight hours before delivery, respectively. The remaining 64 patients experienced aspiration of liquid material, and they often went unrecognized with complete recovery.

Mendelson replicated the gastric acid in the respiratory distress syndrome he witnessed in human patients through his animal model. He placed both neutralized and untreated hydrochloric acid and vomitus from pregnant women into the respiratory tracts of rabbits. Mendelson found that during labor, there is prolonged retention of solids and liquids in parturients’ stomachs, and aspiration commonly occurs after abolishing laryngeal reflexes. During Mendelson’s time, the induction of general anesthesia was not limited to parturients undergoing a cesarean section but was also the method for spontaneous or operative vaginal deliveries. 21% of aspiration cases were among women who delivered via cesarean section, while 79% of women were undergoing general anesthesia for vaginal deliveries. The general anesthetic at this time consisted of a nonspecific mixture of gas, oxygen, and ether. The airway was left unsecured during delivery as parturients were subjected to mask induction and maintenance with an opaque black rubber mask. Following aspiration, the initial clinical course was severe including massive atelectasis with cyanosis, dyspnea, mediastinal shift, and radiographic signs of lung injury. Despite this, the 64 nonfatal cases were almost all liquid aspirations with radiographic resolution within seven days and clinical recovery within 36 hours without the use of antibiotic treatment.

Mendelson’s study led to several recommendations that still are in use in the obstetric population to this day. Pregnant women are treated as though they have a “full stomach” regardless of their last meal, and inhalational anesthesia without intubation is strictly avoided. Opaque rubber masks that can conceal regurgitation and vomitus have been replaced with clear plastic masks and ingestion of solid food has been discouraged during labor. Two American anesthesiologists, Paleul Flagg, and James Miller suggested that experienced anesthesiologists could help avoid the complications that Mendelson described. Miller reported over 26000 deliveries at Hartford Hospital with no mortality secondary to asphyxia, partially attributed to expert anesthesiology staff. The argument was that safely administered general anesthesia could reduce the risk of aspiration in parturients.

Although Mendelson’s contributions to obstetric anesthesia should not be understated, modern obstetrics has evolved considerably since then. During the last thirty years, aspiration in pregnant women has markedly declined, primarily due to advances in obstetrical anesthesia. The standard use of regional anesthesia for most laboring women and increased awareness among anesthesia providers regarding the high risk of aspiration and potential difficult airway management in parturients has increased the safety in this population. Care has also improved by advances in difficult airway management devices including video laryngoscopes, endotracheal tube introducers, optical stylets, and flexible endoscopes. The routine utilization of pulse oximetry, capnography, and difficult airway algorithms have also helped to mitigate the risks associated with general anesthesia in parturients.