Pulmonary complications

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What is aspiration?

Aspiration is the passage of material from the pharynx into the trachea. Aspirated material can originate from the stomach, esophagus, mouth, or nose. The materials involved can be particulate matter (e.g., food), a foreign body, fluid (e.g., blood, saliva) or gastrointestinal contents. Aspiration can cause a pneumonitis or a pneumonia, with the former occurring most often as a complication on induction of anesthesia.

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What differentiates aspiration pneumonitis from aspiration pneumonia?

The primary pathophysiology of aspiration pneumonitis is acute inflammation because of chemical irritation of the tracheobronchial tree, caused by sterile, acidic, gastric contents containing digestive enzymes and bile acids. Aspiration pneumonia, however, is primarily infectious because of aspiration of bacteria in patients who are frail, elderly, and/or immunocompromised, and is associated with poor dentition and dysphagia. A key differentiating factor between aspiration pneumonitis and aspiration pneumonia is the acidity and source of the vomitus. Aspiration of acidic gastric contents not only irritates the tracheobronchial tree directly, but also activates digestive enzymes (i.e., pepsinogen), which may contribute to aspiration pneumonitis. The source of the vomitus in aspiration pneumonitis is the stomach, whereas aspiration pneumonia is often caused by bacteria from the oropharynx. Aspiration pneumonia, however, can be caused by the aspiration of gastric contents particularly if patients are on proton-pump inhibitors or histamine-2 (H2) antagonist, which increases the gastric pH, leading to gastric colonization of bacteria. The presentation of aspiration pneumonitis is more acute than aspiration pneumonia and is more commonly associated with anesthesia.

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How is aspiration pneumonitis and aspiration pneumonia treated?

Aspiration pneumonitis is treated with supportive care and aspiration pneumonia with antibiotics. It is important to note that there is a degree of overlap between aspiration pneumonia and aspiration pneumonitis, and some patients with aspiration pneumonitis can develop a pneumonia.

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What is Mendelson syndrome?

Mendelson syndrome was the first description of aspiration pneumonitis in the literature. An obstetrician named Curtis Mendelson described this syndrome as dyspnea, cyanosis, and tachycardia in obstetric patients who had aspirated while receiving general anesthesia. He also described the immediate complications of aspiration pneumonitis as asthma-like (bronchospasm, wheezing, hypercapnia, etc.), which occurred if the gastric contents were acidic, whereas if the aspiration volume was large and not acidic, the respiratory pathology was caused by obstruction of the airways leading to atelectasis and hypoxemia. He distinguished the pathology of aspiration pneumonitis as irritative as opposed to aspiration pneumonia, which is infectious. This landmark paper shaped our current preoperative fasting guidelines to reduce gastric volumes and improved our anesthetic techniques for patients at risk for aspiration, such as giving preoperative medications to neutralize gastric pH and performing rapid sequence induction and intubation (also known as rapid sequence induction [RSI]).

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What are the specific risk factors for a vomitus to cause aspiration pneumonitis?

The two primary risk factors for the development of aspiration pneumonitis are the following:

• 1)

  pH of gastric contents under 2.5

• 2)

  Gastric volumes over 25 mL

Aspiration of gastric contents containing small volumes (< 25 mL) and low acidity (pH > 2.5) are less likely to cause clinically significant aspiration pneumonitis.

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How often does aspiration occur with anesthesia, and what is the morbidity and mortality rate?

The incidence of significant aspiration is 1 per 10,000 anesthetics. Studies of anesthetics in children demonstrate about twice that occurrence. The average hospital stay after aspiration is 21 days, much of which may be in intensive care. Complications range from bronchospasm, pneumonia, and acute respiratory distress syndrome (ARDS), lung abscess, and empyema. The average mortality rate is 5%.

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What are risk factors for aspiration with anesthesia?

It is important to emphasize that aspiration risk is not binary and that a continuum exists between low and high risk. Risk factors for aspiration include the following:

• Extremes of age

• Emergency operations

• Type of surgery (most common in cases of esophageal, upper abdominal, or emergent laparotomy operations)

• Recent meal

• Delayed gastric emptying (narcotics, diabetes, trauma, pain, intraabdominal infections, and end-stage renal disease)

• Gastroesophageal reflux disease (GERD; decreased lower esophageal sphincter tone, hiatal hernia)

• Trauma

• Pregnancy

• Depressed level of consciousness (i.e., Glasgow Coma Scale < 8)

• Morbid obesity (higher incidence of hiatal hernia)

• Difficult airway

• Neuromuscular disease (impaired ability to protect their airway)

• Esophageal disease (e.g., scleroderma, achalasia, diverticulum, Zenker diverticulum, prior esophagectomy/gastrectomy)

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What precautions can be undertaken before anesthetic induction to prevent aspiration or mitigate its sequelae?

The main precaution is to recognize which patients are at risk. Patients having elective surgical procedures should be fasted per American Society of Anesthesiologists guidelines. Before anesthetic induction, oral nonparticulate antacids, such as sodium citrate can be administered to patients at risk for aspiration (e.g., severe uncontrolled GERD). This functions to raise the gastric pH and lessen the severity of the pneumonitis if aspiration were to occur. H2-receptor antagonists (e.g., cimetidine, ranitidine, and famotidine) can be used to raise the gastric pH as well, but must be administered approximately 30 to 60 minutes before induction of anesthesia to be effective. The use of proton-pump inhibitors in place of, or in concert with, H2 antagonists has not proven to be more efficacious. The use of orogastric or nasogastric drainage before induction is most effective in patients with intestinal obstruction. In situ nasogastric tubes should be suctioned before induction in this patient population.

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