Management of Patients at Risk for Aspiration Pneumonia
A 58-year-old man is scheduled for elective inguinal hernia repair. His past history reveals a persistent problem with heartburn and passive regurgitation of gastric contents into the pharynx. He has been told by his internist that these symptoms are due to a hiatal hernia.
Why would a history of hiatal hernia concern the anesthesiologist?
Perioperative aspiration of gastric contents (Mendelson’s syndrome) is a potentially fatal complication of anesthesia. Hiatal hernia is commonly associated with symptomatic GERD, which is considered a predisposing factor for aspiration. Mild or occasional heartburn may not significantly increase the risk of aspiration. In contrast, symptoms related to passive reflux of gastric fluid, such as acid taste or sensation of refluxing liquid into the mouth, should alert the clinician to a high risk of pulmonary aspiration. Paroxysms of coughing or wheezing, particularly at night or when the patient is flat, may be indicative of chronic aspiration. Aspiration can occur on induction, during maintenance, or upon emergence from anesthesia.
Which patients are predisposed to aspiration?
Patients with altered airway reflexes (eg, drug intoxication, general anesthesia, encephalopathy, neuromuscular disease) or abnormal pharyngeal or esophageal anatomy (eg, large hiatal hernia, Zenker’s diverticulum, scleroderma, pregnancy, obesity) are prone to pulmonary aspiration.
Does aspiration consistently result in aspiration pneumonia?
Not necessarily. The seriousness of the lung damage depends on the volume and composition of the aspirate. Traditionally, patients are considered to be at risk if their gastric volume is greater than 25 mL (0.4 mL/kg) and their gastric pH is less than 2.5. Some investigators believe that controlling acidity is more important than volume and that the criteria should be revised to a pH less than 3.5 with a volume greater than 50 mL.
Patients who have eaten immediately prior to emergency surgery are obviously at risk. Traditionally, “NPO after midnight” implied a preoperative fast of at least 6 h. Current opinion allows clear liquids until 2-4 h before induction of anesthesia, although solids are still taboo for 6 h in adult patients. Some patients who have fasted for 8 h or more before elective surgery also meet the at-risk criteria, however. Certain patient populations are particularly likely to have large volumes of acidic gastric fluid: patients with an acute abdomen or peptic ulcer disease, children, the elderly, diabetic patients, pregnant women, and obese patients. Furthermore, pain, anxiety, or opioid-agonists may delay gastric emptying. Note that pregnancy and obesity place patients in double jeopardy by increasing the chance of aspiration (increased intraabdominal pressure and distortion of the lower esophageal sphincter) and the risk of aspiration pneumonia (increased acidity and volume of gastric contents). Aspiration is more common in patients undergoing esophageal, upper abdominal, or emergency laparoscopic surgery.
Which drugs lower the risk of aspiration pneumonia?
H2-Receptor antagonists decrease gastric acid secretion. Although they will not affect gastric contents already in the stomach, they will inhibit further acid production. Both gastric pH and volume are affected. In addition, the long duration of action of ranitidine and famotidine may provide protection in the recovery room.
Metoclopramide shortens gastric emptying time and increases lower esophageal sphincter tone. It does not affect gastric pH, and it cannot clear large volumes of food in a few hours. Nonetheless, metoclopramide with ranitidine is a good combination for most at-risk patients. Antacids usually raise gastric fluid pH, but, at the same time, they increase gastric volume. Although antacid administration technically removes a patient from the at-risk category, aspiration of a substantial volume of particulate matter will lead to serious physiological damage. For this reason, clear antacids (eg, sodium citrate) are strongly preferred. In contrast to H2 antagonists, antacids are immediately effective and alter the acidity of existing gastric contents. Thus, they are useful in emergency situations and in patients who have recently eaten.
Anticholinergic drugs, particularly glycopyrrolate, decrease gastric secretions if large doses are administered; however, lower esophageal sphincter tone is reduced. Overall, anticholinergic drugs do not reliably reduce the risk of aspiration pneumonia and can reverse the protective effects of metoclopramide. Proton pump inhibitors are generally as effective as H2 antagonists.
What anesthetic techniques are used in full-stomach patients?
If the full stomach is due to recent food intake and the surgical procedure is elective, the operation should be postponed. If the risk factor is not reversible (eg, large hiatal hernia) or the case is emergent, proper anesthetic technique can minimize the risk of aspiration pneumonia. Regional anesthesia with minimal sedation should be considered in patients at increased risk for aspiration pneumonia. If local anesthetic techniques are impractical, the patient’s airway must be protected. Delivering anesthesia by mask or laryngeal mask airway is contraindicated. As in every anesthetic case, the availability of suction must be confirmed before induction. If there are signs suggesting a difficult airway, intubation should precede induction. Otherwise, a rapid-sequence induction is indicated.
How does a rapid-sequence induction differ from a routine induction?
• The patient is always preoxygenated prior to induction. Patients with lung disease require 3-5 min of preoxygenation.
• Prior curarization with a nondepolarizing muscle relaxant may prevent the increase in intraabdominal pressure that accompanies the fasciculations caused by succinylcholine. This step is often omitted, however, as it can decrease lower esophageal sphincter tone. If rocuronium has been selected for relaxation, a small priming dose (0.1 mg/kg) given 2-3 min prior to induction may speed its onset of action.
• A wide assortment of blades, video laryngoscopes, and endotracheal tubes are prepared in advance.
• An assistant may apply firm pressure over the cricoid cartilage prior to induction (Sellick’s maneuver). Because the cricoid cartilage forms an uninterrupted and incompressible ring, pressure over it is transmitted to underlying tissue. The esophagus is collapsed, and passively regurgitated gastric fluid cannot reach the hypopharynx. Excessive cricoid pressure (beyond what can be tolerated by a conscious person) applied during active regurgitation has been associated with rupture of the posterior wall of the esophagus. The effectiveness of Sellick’s maneuver has been questioned.
• A propofol induction dose is given as a bolus. Obviously, this dose must be modified if there is any indication that the patient’s cardiovascular system is unstable. Other rapid-acting induction agents can be substituted (eg, etomidate, ketamine).
• Succinylcholine (1.5 mg/kg) or rocuronium (0.9-1.2 mg/kg) is administered immediately following the induction dose, even if the patient has not yet lost consciousness.
• The patient is not artificially ventilated, to avoid filling the stomach with gas and thereby increasing the risk of emesis. Once spontaneous efforts have ceased or muscle response to nerve stimulation has disappeared, the patient is rapidly intubated. Cricoid pressure is maintained until the endotracheal tube cuff is inflated and tube position is confirmed. A modification of the classic rapid-sequence induction allows gentle ventilation as long as cricoid pressure is maintained.
• If the intubation proves difficult, cricoid pressure is maintained and the patient is gently ventilated with oxygen until another intubation attempt can be performed. If intubation is still unsuccessful, spontaneous ventilation should be allowed to return and an awake intubation performed.
• After surgery, the patient should remain intubated until airway reflexes have returned and consciousness has been regained.
What are the relative contraindications to rapid-sequence inductions?
Rapid-sequence inductions are usually associated with increases in intracranial pressure, arterial blood pressure, and heart rate. Contraindications to succinylcholine also apply (eg, thermal burns).
Describe the pathophysiology and clinical findings associated with aspiration pneumonia.
The pathophysiological changes depend on the composition of the aspirate. Acid solutions cause atelectasis, alveolar edema, and loss of surfactant. Particulate aspirate will also result in small-airway obstruction and alveolar necrosis. Granulomas may form around food or antacid particles. The earliest physiological change following aspiration is intrapulmonary shunting, resulting in hypoxia. Other changes may include pulmonary edema, pulmonary hypertension, and hypercapnia.
Wheezing, rhonchi, tachycardia, and tachypnea are common physical findings. Decreased lung compliance can make ventilation difficult. Hypotension signals significant fluid shifts into the alveoli and is associated with massive lung injury. Chest roentgenography may not demonstrate diffuse bilateral infiltrates for several hours after the event. Arterial blood gases reveal hypoxemia, hypercapnia, and respiratory acidosis.
What is the treatment for aspiration pneumonia?
As soon as regurgitation is suspected, the patient should be placed in a head-down position so that gastric contents drain out of the mouth instead of into the trachea. The pharynx and, if possible, the trachea should be thoroughly suctioned. The mainstay of therapy in patients who subsequently become hypoxic is positive-pressure ventilation. Intubation and the institution of positive end-expiratory pressure or continuous positive airway pressure are often required. Bronchoscopy and pulmonary lavage are usually indicated when particulate aspiration has occurred. Use of corticosteroids is generally not recommended and antibiotics are administered depending upon culture results.