Aspiration Pneumonitis Prophylaxis
Definition
Aspiration pneumonitis, although uncommon, is a potentially fatal condition that occurs as a result of regurgitation and aspiration of gastric contents. Aspiration of undigested or semidigested gastric contents into the respiratory tract can cause obstruction and an inflammatory response. Acute chemical pneumonitis and subsequent acute lung injury (aspiration pneumonitis) can result from aspiration of acidic gastric secretions.11 Aspiration of gastric contents is also an important risk factor for the development of adult respiratory distress syndrome (ARDS).12 Historically, a gastric pH of <2.5 and a gastric volume >25 mL (0.4 mL/kg) have been accepted as the cutoff values that place the patient at greater risk for severe pneumonitis should aspiration occur. However, in clinical practice, the values commonly used are a gastric pH <3.5 and gastric volume >50 mL, with pH appearing to be a greater determinant of morbidity than volume.13
Risk Factors
Patients at greatest risk for regurgitation and aspiration include those with increased gastric acid, elevated intragastric pressure, gastric or intestinal hypomotility, digestive structural disorders, neuromuscular incoordination, and depressed sensorium. These can include pregnant patients, obese patients, and trauma patients, as well as patients with a hiatal hernia, gastroesophageal reflux, esophageal motility disorders, or peptic ulcer disease.14 Diabetic patients with reflux symptoms or poor glucose control may also benefit from pharmacologic prophylaxis.15 In addition to having delayed gastric emptying, obese patients will often present with increased abdominal pressure and an abnormal airway; both factors predispose these individuals to aspiration. Hormonal changes in pregnant patients account for delayed gastric emptying and relaxation of the lower esophageal sphincter. An increase in intra-abdominal pressure is also seen in pregnant patients. Labor can increase gastrin levels, increasing gastric volume and acidity as well as delaying gastric emptying. Patients undergoing emergency surgery frequently have full stomachs because they have not had time to fast appropriately.
Rapid sequence induction, effective application of cricoid pressure, maintaining a patent upper airway, avoiding inflation of the stomach with anesthetic gases, inserting a large-bore gastric tube once the airway has been secured, as well as the use of regional anesthesia when possible, are probably the most important measures the anesthesia provider can take to reduce the patient's risk of aspiration.16,17 Administration of pharmacologic aspiration prophylaxis is not cost effective and does not reduce morbidity or mortality in healthy patients undergoing elective surgery. Administration of pharmacologic aspiration prophylaxis should, however, be considered to prevent morbidity (e.g., ARDS) in patients at risk for aspiration.
Medications
Many medications (e.g., antacids, gastric motility stimulants, H2-receptor antagonists) can reduce the risk of pneumonitis if aspiration occurs. These drugs, with the possible exception of metoclopramide, are relatively free of adverse effects and have a favorable risk-benefit profile.
Antacids
Antacids, effective in raising gastric pH to >3.5,1 should be given as a single dose (30 mL) approximately 15 to 30 minutes before induction of anesthesia. Nonparticulate antacids (e.g., sodium citrate/citric acid [Bicitra]) are the agents of choice because the suspension particles in particulate antacids can act as foci for an inflammatory reaction if aspirated and increase the risk of pulmonary damage.18 Antacids have two major advantages when used for aspiration pneumonitis prophylaxis; there is no “lag time” for onset of activity, and antacids are effective on the fluid already in the stomach. Their major disadvantages are (a) a short-acting buffering effect that is not likely to last as long as the surgical procedure (sodium citrate must be administered no more than 1 hour before induction of anesthesia, with its duration possibly dependent on gastric emptying); (b) the potential for emesis (due to their lack of palatability); (c) the possibility of incomplete mixing in the stomach; and (d) their administration adds fluid volume to the stomach.19,20
Gastric Motility Stimulants
The gastric motility stimulant, metoclopramide (Reglan), has no effect on gastric pH or acid secretion. This agent reduces gastric volume in predisposed patients (e.g., parturients, obese patients) by promoting gastric emptying. Preoperative metoclopramide increases lower esophageal sphincter pressure and reduces gastric volume.3,21 Metoclopramide should be administered 60 minutes before induction of anesthesia when given orally. When given by the IV route, metoclopramide should be administered 15 to 30 minutes before induction of anesthesia. The effects of metoclopramide on gastric emptying have been variable, especially when used with other agents. For example, the concomitant administration of anticholinergics (e.g., glycopyrrolate, atropine), or prior administration of opioids, can reduce lower esophageal sphincter pressure, which can offset the effects of metoclopramide on the upper gastrointestinal (GI) tract.22
H2-Receptor Antagonists
H2-receptor antagonists reduce gastric acidity and volume by decreasing gastric acid secretion. Unlike antacids, the H2-receptor antagonists do not produce immediate effects. Onset time for these agents when administered orally is 1 to 3 hours; good effects will be seen in 30 to 60 minutes when administered intravenously.3 Oral doses of the H2-receptor antagonists should not be crushed and given via a nasogastric tube at the time of surgery. As already mentioned, onset of action is not immediate. Furthermore, administration of tablets introduces particulate matter into the stomach, which can be detrimental if aspirated. Duration of action of H2-receptor antagonists is also important because the risk of aspiration pneumonitis extends through emergence from anesthesia. After IV administration, the cimetidine (Tagamet) dose should be repeated in 6 hours if necessary, whereas therapeutic concentrations of ranitidine (Zantac) and famotidine (Pepcid) persist for 8 and 12 hours, respectively.3 Although cimetidine is associated with more adverse reactions than famotidine or ranitidine, this is probably not clinically significant because only one or two doses of the agent are given.1
Proton-Pump Inhibitors
Proton-pump inhibitors (PPIs) (omeprazole, rabeprazole, lansoprazole, esomeprazole, and pantoprazole) act at the final site of gastric acid secretion, making these agents very effective in suppressing acid secretion. When the effects of preoperative IV pantoprazole on gastric pH and volume were compared with IV ranitidine and placebo, both pantoprazole and ranitidine significantly reduced the volume and increased the pH of gastric contents when compared to placebo (saline). There was no difference, however, between the pantoprazole and ranitidine groups.23 Therefore, there appears to be no need to use the more expensive PPIs in patients at risk for pulmonary aspiration.
Choice of Agent
2. D.W., a 5′4′′, 95-kg, 38-year-old woman, ASA-II, is scheduled to undergo a laparoscopic cholecystectomy under general anesthesia. D.W. has type 2 diabetes. Physical examination is normal except for an abnormal airway, which is anticipated to complicate intubation. Her medications include glipizide and an antacid for dyspepsia. The procedure is scheduled as a same-day surgery. What factors predispose D.W. to aspiration, and what premedication, if any, should D.W. receive for aspiration prophylaxis?
D.W. has several factors that place her at risk for aspiration. She is obese with an abnormal airway. She also has diabetes and reports symptoms of dyspepsia that are relieved by antacids. These conditions will predispose D.W. to increased abdominal pressure, delayed gastric emptying, and increased risk of regurgitation. Her abnormal airway may delay intubation, increasing the amount of time D.W. is susceptible to aspiration. Therefore, aspiration prophylaxis with medications that buffer gastric acid and reduce gastric volume is prudent for D.W. Because D.W.'s surgery is scheduled as a same-day surgery, D.W. will arrive at the hospital or surgical center approximately 90 minutes before the start of surgery. Although oral agents, in general, are less expensive than their parenteral counterparts, cimetidine 300 mg (Tagamet), famotidine 40 mg (Pepcid), or ranitidine 150 mg (Zantac) should be administered approximately 1 to 2 hours before induction of anesthesia to effectively decrease gastric acidity. Hence, due to time constraints, cimetidine 300 mg, famotidine 20 mg, or ranitidine 50 mg should be administered intravenously 30 to 60 minutes before induction of anesthesia in D.W. A nonparticulate antacid such as sodium citrate/citric acid solution (Bicitra) 30 mL PO can be administered to D.W. immediately before entering the OR rather than, or in addition to, an H2-receptor antagonist.
3. C.T., a 28-year-old woman, ASA-I, is admitted for an emergency cesarean section under general anesthesia. She is otherwise healthy and currently taking no medications. Why is C.T. susceptible to aspiration pneumonitis, and what preoperative medications would be appropriate to help prevent this adverse event from occurring in her?
C.T. is at an increased risk for aspiration and the possible development of pneumonitis because she is pregnant and about to undergo emergency surgery. In the obstetric patient, preoperative administration of the nonparticulate antacid, sodium citrate, or a H2-receptor antagonist can effectively reduce gastric acidity. Furthermore, the administration of a nonparticulate antacid before C-section can reduce maternal complications. Metoclopramide, however, can reduce peripartum nausea and vomiting. Therefore, an appropriate regimen for C.T. would include both sodium citrate/citric acid solution (Bicitra) 30 mL PO and metoclopramide 10 mg IV. Famotidine (or another H2-receptor antagonist) can be given instead of, or in addition to, the sodium citrate/citric acid solution. There is not sufficient time to administer famotidine and metoclopramide orally because of their slower onsets of action when administered by this route. Sodium citrate/citric acid solution provides immediate protection by raising gastric pH, metoclopramide will help reduce the increased gastric volume commonly seen in pregnant patients, and famotidine will provide sustained coverage throughout the surgery. These agents have not been shown to have detrimental effects on the fetus.24
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