The first reported anesthesia-related death was likely due to aspiration of gastric contents. James Simpson, an obstetrician, reported it in 1848, just 2 years after the first public demonstration of anesthesia. The patient, a teenage girl anesthetized for a minor (nonobstetric) procedure, aspirated either gastric contents or brandy administered by her physician, and likely died of this complication. A century later, Curtis Mendelson highlighted the risk among obstetric patients and also demonstrated in seminal animal experiments that acidic and particulate aspirates were particularly dangerous (1). Subsequently, he recommended parturients not eat or drink in labor, more frequent use of antacids and regional anesthesia, and administration of general anesthesia by competent and experienced practitioners. His recommendations became an established dogma in obstetric anesthesia for decades.

Today the incidence of pulmonary aspiration in obstetrics, and in particular at the time of induction of anesthesia for emergency surgery during labor, is vanishingly rare. Figure 25-1 shows the death rate from aspiration in the United Kingdom since the introduction of the Confidential Enquires into Maternal Deaths in the 1950s (2). An increasing cause of maternal death and the commonest in the United Kingdom in the period 2005 to 2007 was sepsis, an issue now so important that it should put the rare incidence of aspiration into perspective. Current evidence suggests that the incidence of aspiration in the United States is not dissimilar to that in the United Kingdom (3). The main difference between the United States and the United Kingdom (and other European countries) is that NPO policies are still more widely practiced in the United States than in the United Kingdom and the rest of Europe. Although maternal mortality may be slightly higher in the United States than in the United Kingdom (4), intrapartum deaths from aspiration are very rare in both countries.

While few present-day practitioners would disagree with the prudence of competent administration of regional anesthesia as the preferred technique for laboring patients, the suggestion that all pregnant women are at increased risk of aspiration is far more controversial, as are fasting guidelines for pregnant patients, particularly those in labor. In this chapter, we review the evidence surrounding this issue and rationale for present guidelines for oral intake in labor and aspiration prophylaxis for labor, delivery, and obstetric surgical procedures.

Pulmonary aspiration of gastric contents can be caused by solid or liquid material. Aspiration of solid material may cause death by asphyxiation, whereas in pulmonary injury caused by liquid aspiration a distinction needs to be made between aspiration pneumonitis and aspiration pneumonia (5). Aspiration pneumonia is an infection of the respiratory tract caused by the inhalation of oropharyngeal material colonized by organisms such as gram-positive, gram-negative, and anaerobic bacteria. It is most often seen in the elderly and is often associated with dysphagia and/or abnormal gastric motility. Usually, patients first present with the typical signs and symptoms of pneumonia.

Aspiration pneumonitis is usually an acute lung injury (ALI) caused by the inhalation of acidic and/or particulate gastric contents. It is usually associated with a depressed level of consciousness due to anesthesia, sedation, seizures, or drug overdose. Aspiration injury models indicate that the inflammatory response is similar and most pronounced after aspiration of acidic aspirates and aspirates containing small particulate matter (6,7). Clinically, the most severe lung injury is observed in patients who aspirate acidic gastric contents with particulate matter (8). The aspirate induces a chemical burn, often associated with bronchospasm, which results in an alveolar exudate composed of edema, albumin, fibrin, cellular debris, and red blood cells. Ultimately there is an increase in intra-alveolar water and protein with a loss of lung volume leading to a reduction in lung compliance with intrapulmonary shunting of blood. This results in hypoxemia and an increase in pulmonary vascular resistance. After the initial injury there is an intense inflammatory response with the release of cytokines, interleukins, and tumor necrosis factor. Further amplification of the inflammatory process may result in ALI or acute respiratory distress syndrome (ARDS) (9,10). Most patients will have an abnormal chest x-ray but this may take several hours before it becomes apparent (Fig. 25-2) (11). Several detailed reviews of the physiology of pulmonary aspiration of gastric contents in the setting of anesthesia have been published (12,13,14,15).

Classically aspiration in obstetrics was described as occurring at the time of induction of anesthesia, with the anesthetist observing the passage of gastric contents into the tracheobronchial tree. In particular, aspiration in obstetrics was often associated with repeated attempts at intubation in women with a difficult airway or in women in whom the airway had become distorted due to misplaced cricoid pressure. Such events are now rarer and aspiration seems to be as or more likely to occur at the end of surgery when the woman is being extubated (16). In a review of 183 cases of aspiration in the perianesthetic period, 85% presented as observed regurgitation, but the remainder was first manifest by respiratory problems observed later (17).

If pulmonary aspiration is observed, the tracheobronchial tree should be suctioned and bronchoscopy may be required
to remove large particles of food. Bronchospasm should be treated as indicated. Bronchial or bronchoalveolar lavage is not recommended due to the possibility of further spreading particulate matter deeper into the lung. Prophylactic antibiotic therapy is not indicated in aspiration unless/until the clinical course suggests infection (clinical deterioration, fever, leukocytosis, deterioration of chest x-ray). However, antibiotics may be indicated if bacterial colonization of the gastric aspirate is suspected (rarely in obstetrics) or if the clinical condition is not improving at 48 hours (12,13,14,15).

Exudation of fluid into the alveoli, alteration of surfactant, and intrapulmonary shunting can all lead to hypoxemia. CPAP or protective ventilation strategies may be required while the lung injury resolves. The routine use of corticosteroids was, for many years, a standard practice in the management of aspiration. Thereafter, their use became controversial and current evidence no longer supports their use in aspiration syndromes (5).

In severe cases pulmonary aspiration can ultimately cause an ALI or ARDS. In such cases protective ventilatory strategies
have been shown to improve outcome; a tidal volume of 6 mL/kg with a plateau pressure less than 30 cm H₂O has been shown to reduce mortality when compared to the use of a tidal volume of 12 mL/kg and a plateau pressure of 50 cm H₂O (18,19). In a randomized controlled trial of 1,000 patients with ARDS, it has been demonstrated that the patients who received conservative as opposed to liberal fluid management, guided by central venous pressure and/or pulmonary capillary wedge pressure measurements, had better lung function and a shorter duration of mechanical ventilation (20). As in uncomplicated aspiration syndromes the use of corticosteroids also does not appear to improve lung function or recovery in patients with ALI and ARDS (21,22). Minimizing the risk of sepsis and the use of prophylaxis against gastrointestinal bleeding and thromboembolic events are considered a basic standard of care in any critically ill patient; adequate thromboprophylaxis is particularly important in the higher-risk obstetric patient.

Despite being one of the most feared and debated complications of general anesthesia, pulmonary aspiration in anesthetic practice is actually quite rare. Several observational studies based on quality assurance data or electronic medical records have estimated the overall risk in the general surgical population to be 3 to 5 per 10,000, or 1:2,000 to 1:3,500. In 1986, Olsson et al. analyzed 185,358 computerized anesthetic records at the Karolinska Hospital and concluded that the incidence of aspiration was 4.7 in 10,000 anesthetics, or 1:2,131 (23). Similarly, Warner et al. analyzed 215,488 anesthetics at the Mayo Clinic taking place in the late 1980s. They found the incidence of aspiration to be 1:3,216, or 3.1/10,000 (24). Two large studies from the United Kingdom and France each concluded the risk of aspiration was approximately 1:14,000 (25,26), while a Canadian report found a higher incidence of 1:1,116 among 112,000 patients treated in the 1970s at a single teaching hospital (27). Sakai et al. more recently reported results from analysis of 99,441 anesthetics at the University of Pittsburgh between 2001 and 2004; they found an incidence of 1:7,103 (or 1.4/10,000) (28). Nearly every study has demonstrated considerably higher incidence in emergency surgery, as well as trauma surgery and upper gastrointestinal, thoracic, and esophageal procedures (15).

The incidence of aspiration in pregnant patients is likely higher than in general surgical patients, but some caution is in order in interpreting the statistics. Mendelson, who first characterized the acid aspiration syndrome in laboring patients, found an overall incidence of 15/10,000 (1), though this was in the 1940s and in an era of frequent use of mask inhalation anesthesia, limited use of regional analgesia, and before any limitations on oral intake or use of antacids. Krantz and Edwards (29) reported an overall incidence of 1.6/10,000 among vaginal deliveries between 1962 and 1965. Nurses with no formal anesthesia training administered inhalation mask anesthesia. The incidence fell to 0.9/10,000 in the period from 1965 to 1972. Conversely, the incidence in cesarean delivery was higher, 1:430 (23.3/10,000). Dindelli et al. evaluated 12,380 cesarean deliveries under general anesthesia in an Italian hospital between 1977 and 1992. They found the incidence of aspiration to be 1:1,547, or 6.4/10,000 (30). Nearly all cases were associated with emergency surgery or difficult intubation. Soreide et al. studied cases in two Norwegian hospitals over 4 years and found an incidence of aspiration during cesarean delivery of 11/10,000 (31), but all cases were felt to have other risk factors.

Most recently, several investigators have reported general anesthesia without tracheal intubation in peripartum patients with low incidence of aspiration. Ezri et al. studied 1,870 patients undergoing general anesthesia without antacid therapy, cricoid pressure, or tracheal intubation. No cesarean deliveries were included, and most patients underwent procedures in the immediate postpartum period for conditions such as laceration repair and extraction of retained placenta. One case of mild aspiration was reported (1:1,870, or 5.3/10,000) (32). Han et al. reported 1,067 elective cesarean deliveries performed with the laryngeal mask airway (LMA) in fasted patients with no other risk factors for aspiration. Antacid and ranitidine premedication was used, and cricoid pressure was maintained until delivery. There were no cases of aspiration or regurgitation, and no surrogate indicators such as broncho- or laryngospasm, bile-stained LMA, or unexplained hypoxemia (33). Finally, Halaseh et al. (34) and Yao et al. (35) each reported series of 3,000 and 700 cesarean deliveries (some of which were urgent) in fasted patients with the Proseal® LMA and LMA Supreme® respectively. These devices allow gastric suctioning through an integrated port, and the stomach contents were aspirated after securing the airway. No cases of aspiration were reported in either study.

In summary, some older series of pregnant patients undergoing general anesthesia demonstrated an increased risk of aspiration, but many were characterized by outdated anesthetic techniques and studied patients with other risk factors besides pregnancy. In particular, emergency surgery is a known risk factor for aspiration and this likely increased the risk, even in contemporary series. In properly fasted pregnant patients undergoing elective cesarean delivery, and even in peripartum surgery following vaginal delivery, the risk of aspiration appears extremely low.