Chapter 17

• Diphenhydramine is one of a diverse group of drugs that competitively blocks H1 receptors. Many drugs with H1-receptor antagonist properties have considerable antimuscarinic, or atropine-like, activity (eg, dry mouth), or antiserotonergic activity (antiemetic).
• H2 blockers reduce the perioperative risk of aspiration pneumonia by decreasing gastric fluid volume and raising the pH of gastric contents.
• Metoclopramide increases lower esophageal sphincter tone, speeds gastric emptying, and lowers gastric fluid volume by enhancing the stimulatory effects of acetylcholine on intestinal smooth muscle.
• Ondansetron, granisetron, and dolasetron selectively block serotonin 5-HT3 receptors, with little or no effect on dopamine receptors. 5-HT3 receptors, which are located peripherally and centrally, appear to play an important role in the initiation of the vomiting reflex.
• Ketorolac is a parenterally administered nonsteroidal antiinflammatory drug that provides analgesia by inhibiting prostaglandin synthesis.
• Clonidine is a commonly used antihypertensive agent but in anesthesia it is used as an adjunct for epidural and peripheral nerve block anesthesia and analgesia. It is often used in the management of patients with chronic neuropathic pain to increase the efficacy of epidural opioid infusions.
• Dexmedetomidine is a parenteral selective α2 agonist with sedative properties. It appears to be more selective for the α2 receptor than clonidine.
• Selective activation of carotid chemoreceptors by low doses of doxapram stimulates hypoxic drive, producing an increase in tidal volume and a slight increase in respiratory rate. However, doxapram is not a specific reversal agent and should not replace standard supportive therapy (ie, mechanical ventilation).
• Naloxone reverses the agonist activity associated with endogenous or exogenous opioid compounds.
• Flumazenil is useful in the reversal of benzodiazepine sedation and the treatment of benzodiazepine overdose.
• Aspiration does not necessarily result in aspiration pneumonia. The seriousness of the lung damage depends on the volume and composition of the aspirate. Patients are at risk if their gastric volume is greater than 25 mL (0.4 mL/kg) and their gastric pH is less than 2.5.

Many drugs are routinely administered by anesthetists perioperatively to protect against aspiration pneumonitis, to prevent or reduce the incidence of perianesthetic nausea and vomiting, and to reverse respiratory depression secondary to narcotics or benzodiazepines. This chapter discusses these agents along with other unique classes of drugs that are often administered as adjuvants during anesthesia or analgesia.

Aspiration of gastric contents is a rare, potentially fatal, and often litigious event that can complicate anesthesia. Based on an animal study, it is often stated that aspiration of 25 mL of volume at a pH of less than 2.5 will be sufficient to produce aspiration pneumonia. Many factors place patients at risk for aspiration, including “full” stomach, intestinal obstruction, hiatal hernia, obesity, pregnancy, reflux disease, emergency surgery, and inadequate depth of anesthesia.

Many approaches are employed to reduce the potential for aspiration perioperatively. Many of these interventions, such as the holding of cricoid pressure (Sellick’s maneuver) and rapid sequence induction, may only offer limited protection. ...

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