McKenna's Pharmacology for Nursing, 2e

930 P A R T 1 1  Drugs acting on the gastrointestinal system O ne of the most common and most uncomfortable com- plaints encountered in clinical practice is that of nausea and vomiting. Vomiting is a complex reflex reaction to various stimuli (see Chapter 56). In some cases of overdose or poisoning, it may be desirable to induce vomiting to rapidly rid the body of a toxin. This can be accomplished by physical stimuli, often to the back of the throat. In some cases, gastric lavage is used to clear the contents of the stomach. In many clinical conditions, the reflex reaction of vomiting is not beneficial in ridding the body of any toxins but is uncomfortable and even clinically haz- ardous to the person’s condition. In such cases, an antiemetic is used to decrease or prevent nausea and vomiting. Antiemetic agents can be centrally acting or locally acting, and they have varying degrees of effec- tiveness. See Figure 59.1 for sites of action of antiemetics. Box 59.1 highlights important considerations related to use of antiemetics across the lifespan.

ANTIEMETIC AGENTS Drugs used in managing nausea and vomiting are called antiemetics (Table 59.1). All of them work by reducing the hyperactivity of the vomiting reflex in one of two ways: locally, to decrease the local response to stimuli that are being sent to the medulla to induce vomiting, or centrally, to block the chemoreceptor trigger zone (CTZ) or suppress the vomiting centre directly. The locally acting antiemetics may be antacids, local anaesthetics, adsorbents, protective drugs that coat the GI mucosa, or drugs that prevent distension and stretch stimulation of the GI tract. These agents are often reserved for use in mild nausea. Many of these drugs are discussed in Chapter 57. Centrally acting antiemetics can be classified into several groups: phenothiazines, non-phenothiazines, anticholinergics/antihistamines, serotonin (5-HT 3 ) receptor blockers and substance P/neurokinin 1 receptor antagonists.

5-HT 3 receptor blockers work here: dolasetron, granisetron, ondansetron, palonosetron and tropisetron

Phenothiazine antiemetics work here: chlorpromazine, prochlorperazine Non-phenothiazines

also work here: metoclopramide, domperidone

Dronabinol, nabilone work here

CTZ

Substance P/neurokinin 1 receptor antagonists work here: aprepitant, fosaprepitant

FIGURE 59.1  Sites of action of emetics/ antiemetics, CTZ, chemoreceptor trigger zone.