Minimizing the Risk

Anesthesiologists should first take appropriate steps to reduce the likelihood of aspiration but, should it occur, management strategies to reduce the risk of complications from the aspiration should be initiated. Because the complications of aspiration are rare and many patients may have silent aspirations that are neither witnessed nor apparent, the incidence of aspiration is probably significantly underestimated and its relationship to the patient’s postoperative course underappreciated. Even when aspiration is witnessed, the risk of complications associated with it varies considerably; for many patients there are no significant consequences.

A number of approaches have been recommended to reduce both the risk of aspiration and the physiologic consequences of aspiration should it occur.

The primary method for reducing the risk of aspiration is to ensure that the patient has an empty stomach before induction of anesthesia. Fasting is the recommended approach for reducing the quantity of gastric contents. Although no clear-cut data define the exact duration of fasting that is required, a number of recommendations have been proposed related to the duration of fasting and the type of foods that should be avoided. Based on a review of the evidence related to the risk of aspiration associated with increased gastric volumes and our current state of knowledge, practice guidelines have been developed to define the most appropriate duration of fasting for adults and children. The guidelines suggest a minimum fasting period of 2 hours after ingesting clear liquids. For adult patients, fasting for at least 6 hours after a light meal is recommended. Children taking breast milk or infant formulas should fast for 4 hours before elective surgical procedures for which anesthesia will be provided. Unfortunately, even with fasting for these time periods or longer, patients can still have significant gastric volumes due to reduced gastric emptying as well as increased gastric secretions. As a result, although fasting is recommended and appropriate, the anesthesiologist must still be cognizant of the potential risk of aspiration even after a patient has been fasting for an extended period of time.

To reduce the volume and acidity of gastric secretions, a number of pharmacologic agents such as gastrointestinal stimulants, gastric acid secretion blockers, and antacids are commonly used by many clinicians. Although of theoretic value, there is, unfortunately, not much data to support their routine use. As a result, the use of any of these agents is not recommended, except in patients with a high likelihood of delayed gastric emptying, such as obese, pregnant, or diabetic patients. For this select group of patients at higher risk of aspiration, if antacids are to be used, only nonparticulate agents should be administered. The routine use of other agents, such as antiemetics or anticholinergics, has not been demonstrated to reduce the risk of pulmonary aspiration, although they may be of value in select patients at high risk of aspiration or in patients with known gastroesophageal reflux, including some elderly patients.

Cricoid pressure has also been advocated as a way to reduce the risk of regurgitation and aspiration, particularly as part of the “rapid sequence induction” technique. Although it is commonly used to reduce regurgitation and aspiration, there is not much objective data to support its value. In addition, it is difficult to confirm proper application of the cricoid pressure, and, in some cases, the cricoid pressure interferes with airway management.

Management Strategies

If a patient is identified as having aspirated, the primary therapeutic interventions are supportive. No specific therapies directed toward the aspiration itself are generally required unless there is clinical evidence to suggest airway obstruction due to particulate or foreign body aspiration. First, gas exchange must be assured. Supplemental oxygen should be provided to maintain adequate oxygenation. Routine bronchopulmonary hygiene, including suctioning of pulmonary secretions, and other supportive measures are the only additional approaches that have been demonstrated to be effective. There are no data that support the empiric initiation of other therapies immediately after a witnessed or suspected aspiration.

In the event that aspiration is witnessed, careful assessment of the oropharynx should be performed. If necessary, the removal of debris from the oropharynx should be done with the use of a Yankauer suction catheter. If regurgitation or vomiting is ongoing, the patient should be placed in the head down lateral decubitus position to minimize the risk of further aspiration into the airway. Placement of a nasogastric tube may be required to remove additional gastric contents and prevent ongoing aspiration, although leaving a nasogastric tube in place may increase the risk of reflux. Bronchodilator therapy with beta-agonists is indicated if bronchospasm is triggered by the aspiration. The bronchodilatory therapy will not only improve the wheezing but might also improve mucociliary function and facilitate clearance of secretions in the postoperative period.

For some patients with large-volume aspiration or those known to have aspirated particulate material or material with a low pH, additional interventions may be required. Bronchoalveolar lavage is not indicated in these situations as it can cause the aspirate to move more distally into the smaller airways rather than facilitate clearance of the aspirate. Lavage does not reduce the likelihood of pneumonitis. Bronchoscopy can be used to facilitate removal of particulate aspirate, particularly if a foreign body is identified in the larger airways. If the patient develops further complications from the aspiration, including systemic inflammation and sepsis, additional therapeutic interventions may be necessary, including vasopressors and appropriate fluid resuscitation for optimization of intravascular volume.

For most patients who aspirate, antibiotic therapy is not required and should be withheld. Unless there is documented evidence of infection, the early administration of antibiotics may simply increase the risk of antibiotic-resistant infection. In general, antibiotics should be administered on the basis of documented clinical infection with a positive sputum Gram stain, positive cultures, or a focal persistent infiltrate associated with fever and an elevated white blood cell count. For the patient who requires continued ventilator support, bronchoalveolar lavage can be used to obtain a specimen for culture. In addition, later in the patient’s postoperative course, if a pulmonary infiltrate persists or the sputum culture becomes positive, antibiotic coverage directed toward the offending organism should be initiated. Early administration of antibiotics may be appropriate in some select clinical situations. For example, if a patient has known bowel obstruction or the aspirated material is feculent, antibiotic therapy that provides adequate gram-negative bacterial coverage should be initiated.

Incidence of Clinically Significant Aspiration

Although aspiration is of concern to every anesthesiologist, the incidence of aspiration in patients receiving anesthesia is difficult to define. It has been found to occur in 1 per 2000 to 3000 adult patients undergoing elective surgery and in 1 per 1200 to 2600 anesthetic procedures in children. During emergency procedures, the incidence may be three to four times higher than it is during elective procedures. One of the difficulties in evaluating information obtained from published studies of the risk of aspiration is that the diagnosis is difficult to make and the frequency varies considerably by patient population and approaches to airway management. In some cases the aspiration may be silent and unrecognized. In addition, most patients who aspirate demonstrate no evidence of complications from the aspiration. Even those patients who have a witnessed aspiration often have minimal, if any, sequelae. As a result, the diagnosis may be missed because it is based primarily on the complications that result from the aspiration rather than on observation of aspiration itself.

The incidence of aspiration reported in the literature is variable, influenced in large part by the definition of aspiration. One of the reasons for the lack of consensus about the definition of aspiration is that the clinical manifestations vary considerably, based in part on the volume of aspirate and in part on the characteristics of the aspirate itself. For example, the patient who loses the normal cough reflex during deep sedation or induction of anesthesia may aspirate small amounts of oral secretions with no obvious clinical manifestations and no clinical consequence. On the other hand, the patient who regurgitates gastric contents, such as a recently completed large meal, and aspirates the material into the lungs may have significant clinical manifestations, including laryngospasm, bronchospasm, gas trapping, gas exchange abnormalities (both acute and extended), pneumonitis, pneumonia, or pulmonary abscess formation.

Differentiating Aspiration Pneumonitis from Aspiration Pneumonia

Because of the overlapping clinical findings, the consequences of aspiration are difficult to characterize. One of the clinical challenges in the patient with documented pulmonary aspiration is to differentiate pneumonitis from pneumonia. The definitive differentiation is difficult to confirm because there are no obvious markers, and for some patients, inflammation associated with the aspiration causes pneumonitis, which may progress to pneumonia. In general, the diagnosis is made based on the clinical presentation and clinical signs and symptoms. Aspiration pneumonitis often gives rise to an infiltrate, but it is usually fleeting, lasting only a few hours. In fact, many patients with witnessed aspiration will have an infiltrate on chest radiography, but it will generally clear within hours of the aspiration without therapy. On the other hand, an aspirate that is acidic can cause chemical pneumonitis resulting in the exudation of fluid into the lung parenchyma. The risk of chemical pneumonitis is greatest if the pH of the aspirate is less than 2.5 or if the quantity of aspirate is large or particulate. If blood is aspirated, there may be an infiltrate immediately after the aspiration, but it usually clears rapidly with minimal consequences.

The greatest concern in the patient who aspirates is the risk that the aspiration will progress to pneumonia. Although the clinical features of pneumonitis and pneumonia overlap, if the patient has a persistent fever that cannot be attributed to a wound infection or other cause or develops other clinical evidence of infection or sepsis, a pulmonary infection must be considered. An elevated white blood cell count, purulent sputum, and worsening clinical status are most likely associated with pneumonia after aspiration rather than inflammation (pneumonitis) alone.

Risk Factors for Aspiration

The largest body of evidence related to the diagnosis and management of aspiration has concentrated on identification of patients at increased risk, particularly in the setting of anesthesia and surgery. Unfortunately, these studies do not rigidly or consistently define pulmonary aspiration, making the estimation of risk and analysis of the natural history of aspiration difficult.

Despite the difficulty in identifying specific risk factors, a number of factors have been associated with an increased likelihood of aspiration. Trauma patients and any patient with impaired gastric emptying are at risk of aspiration when rendered unconscious. Many trauma patients have recently eaten, so their stomachs may be full; pain and discomfort will also delay gastric emptying. In addition, the trauma patient may have an altered level of consciousness due to the injury, compromising the ability to protect the airway before tracheal intubation. The same is true for the patient experiencing severe pain and those who have recently received narcotic analgesics that reduce gastric emptying. Other patients at risk of aspiration include those with pre-existing airway abnormalities, those with esophageal disease, motility disorders, and altered gastroesophageal sphincter tone. The obese patient and the pregnant patient are also at increased risk of aspiration because of delayed gastric emptying and, in some cases, the lower pH of gastric contents.

In addition to the increased risk of aspiration in select patient populations, the likelihood of developing aspiration pneumonitis also varies by patient population. The primary problem for the clinician is to understand which patients are vulnerable to the more serious sequelae of aspiration, such as pneumonitis and pneumonia, versus those who aspirate without physiologic consequences. For instance, aspiration pneumonitis is a well-known complication after drug overdose, seizure, and cerebrovascular accident; it is also associated with general anesthesia. Aspiration has long been considered the most common cause of death in patients with dysphagia and a compromised coughing reflex, as may occur in neurologic disease. It has been estimated that 5% to 15% of community-acquired pneumonia is secondary to aspiration. This complication is probably most common in elderly patients who reside in nursing homes.

In a study evaluating the significance of pulmonary aspiration during the perioperative period, pulmonary aspiration was defined by the presence of bilious secretions or particulate matter in the tracheobronchial tree or new pulmonary infiltrates on postoperative chest radiography in patients without any clinical findings on preoperative examination. Clearly, this definition may mistakenly include patients with postoperative pulmonary edema, acute respiratory distress syndrome (ARDS), or pre-existing pneumonia that went undetected.

Some general conditions are associated with increased risk of aspiration. They include higher American Society of Anesthesiologists (ASA) physical status and patients undergoing emergency procedures. Many other conditions thought to be associated with aspiration were not found to be independent risk factors. Some of those include age, gender, obesity, ingestion of a meal within 3 hours, experience and type of anesthesia provider, and type of surgical procedure. It is also interesting that no pulmonary aspiration was detected in those patients undergoing cesarean sections under general anesthesia. The most common predisposing conditions associated with aspiration for patients undergoing elective procedures are gastrointestinal obstruction, lack of coordination of swallowing, depressed level of consciousness, and having eaten a recent meal.

Data from both animal and human studies suggest that a primary determinant in the development of aspiration pneumonitis is the pH of the aspirate. A pH of less than 2.5 in the aspirate is necessary to cause clinically significant aspiration pneumonitis. The volume of aspirate also contributes to the likelihood of pneumonitis. A number of studies indicate that the critical volume is 25 mL or 0.4 mL/kg for causing pneumonitis. Particulate antacids may increase the gastric pH but may also cause pulmonary problems if the particulate matter is aspirated. Nonparticulate antacids, often administered to reduce the pH of the gastric contents, on the other hand, may contribute to the risk of pneumonitis because they increase residual gastric volume.

The combined impact of the pH and volume on the risk of aspiration pneumonitis is not clearly defined. In at least one study evaluating volume and pH implications, 80% of rats survived aspiration of volumes exceeding 2.0 mL/kg as long as the pH was greater than 2.5. Other studies support this conclusion, suggesting that the administration of a nonparticulate antacid is appropriate for the patient at increased risk of aspiration in spite of its effect on intragastric volume.

Anesthetic Induction Strategies in Patients at Risk

For those patients at risk of aspiration, including those with a full stomach or delayed gastric emptying (e.g., the diabetic patient or the obese patient), the airway must be secured with extreme caution. Although the data on the value of nonparticulate antacid are limited, it is probably prudent to administer it before induction of anesthesia. Cricoid pressure should be considered and is generally applied when the patient’s normal protective reflexes are compromised or the patient is suspected of having a full stomach, although the value of this procedure has also not been proven. These patients should also be placed in the head-up position, if clinically feasible, although positioning will be dictated by the overall clinical needs of the patient.

The best airway management technique to be used for the patient at risk of aspiration is not known. A cuffed endotracheal tube should be used for most patients at risk of aspiration, but the presence of a cuff alone may not protect the patient from aspiration of fluids around the cuff, particularly if the patient has increased gastric pressure or volume of secretions and is in the supine position. Nonetheless, the cuffed endotracheal tube will protect against aspiration of larger particulate matter. There are now some case reports suggesting that endotracheal tubes with low volume, low pressure cuffs may reduce the risk of aspiration. Other studies have suggested that endotracheal tubes with subglottic suction ports may allow better suctioning of secretions above the cuff of the endotracheal tube and thus may minimize the risk of aspiration while the patient is intubated ProSeal laryngeal mask airways have also been shown to protect adult and pediatric patients from large-volume aspiration, although no studies have confirmed that these airways are as effective as cuffed endotracheal tubes at reducing the risk of aspiration.

Documentation of Aspiration

Aspiration of clear liquids of high pH and limited quantity is generally tolerated with minimal sequelae. However, it is difficult to predict whether an individual patient will develop clinically significant pneumonitis, pneumonia, or ARDS after aspiration. The underlying clinical condition of the patient, the physiologic status of the patient at the time of the aspiration, and other factors will influence the subsequent course. If aspiration is suspected, the patient should be observed in a monitored setting for several hours after the aspiration so that appropriate management is ensured. A chest radiograph should be obtained and reviewed for evidence of aspiration or pulmonary infiltrate.