- Diphenhydramine is an example 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
- 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
- Clonidine is a commonly used antihypertensive
agent but in anesthesia it is used as an adjunct for epidural infusions
in pain management. It is most useful in the management of patients
with neuropathic pain who become increasingly resistant to epidural
- Dexmedetomidine is a parenteral selective α2-agonist
with sedative properties. It appears to be more selective for the α2-receptor
- 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.
This final pharmacology chapter describes several drugs of particular
interest to the anesthesiologist. Because some of these are histamine-receptor
antagonists, the physiology of histamine is briefly reviewed. Diphenhydramine
represents the classic antihistaminic drug. Cimetidine, ranitidine,
and famotidine are helpful in the preoperative preparation of
patients at risk for aspiration pneumonia. The chapter reviews other
drugs (eg, metoclopramide, antacids, and proton pump inhibitors)
that may be used to decrease the risk of aspiration as well as serotonin
antagonists, which have proved to be potent antiemetics. The α2-adrenergic
agonists clonidine and dexmedetomidine are also reviewed. The chapter
concludes with a discussion of a respiratory stimulant (doxapram),
an opioid antagonist (naloxone), and a benzodiazepine antagonist
Histamine is found in the central nervous system, in the gastric
mucosa, and in other peripheral tissues. It is synthesized by decarboxylation
of the amino acid histidine. Histaminergic neurons are primarily
located in the posterior hypothalamus but have wide projections
in the brain. Histamine ...