Antiemetic and Antinausea Drugs



Antiemetic and Antinausea Drugs


Objectives


When you reach the end of this chapter, you will be able to do the following:



Drug Profiles



Key Terms


Antiemetic drugs Drugs given to relieve nausea and vomiting. (p. 846)


Chemoreceptor trigger zone (CTZ) The area of the brain that is involved in the sensation of nausea and the action of vomiting. (p. 845)


Emesis The forcible emptying or expulsion of gastric and, occasionally, intestinal contents through the mouth; also called vomiting. (p. 845)


Nausea Sensation often leading to the urge to vomit. (p. 845)


Vomiting center The area of the brain that is involved in stimulating the physiologic events that lead to nausea and vomiting. (p. 845)


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Anatomy, Physiology, and Pathophysiology Overview


Nausea and Vomiting


Nausea and vomiting are two gastrointestinal (GI) disorders that can be extremely unpleasant but also can lead to more serious complications if not treated promptly. Nausea is an unpleasant feeling that often precedes vomiting. If it does not subside spontaneously or is not relieved by medication, it can lead to vomiting. Vomiting, which is also called emesis, is the forcible emptying or expulsion of gastric and, occasionally, intestinal contents through the mouth. A variety of stimuli can induce nausea and vomiting, including foul odors or tastes, unpleasant sights, irritation of the stomach or intestines, and certain drugs (ipecac or antineoplastic drugs).


The vomiting center is an area in the brain that is responsible for initiating the physiologic events that lead to nausea and vomiting. Neurotransmitter signals are sent to the vomiting center from the chemoreceptor trigger zone (CTZ), another area in the brain involved in the induction of nausea and vomiting. These signals alert those areas of the brain to the existence of nauseating substances (noxious stimuli) that need to be expelled from the body. Once the CTZ and vomiting center are stimulated, they initiate the events that trigger the vomiting reflex. The neurotransmitters involved in this process and their respective receptors are listed in Table 52-1. The various pathways and the areas of the body that send the signals to the vomiting center are illustrated in Figure 52-1. Two specific types of nausea and vomiting, chemotherapy-induced and postoperative, produce much more intense symptoms and are treated much more aggressively than general nausea and vomiting.



PATIENT-CENTERED CARE: LIFESPAN CONSIDERATIONS FOR THE PEDIATRIC PATIENT


Syrup of Ipecac and Poisioning



• Since November 2003, the American Academy of Pediatrics (AAP) has strongly advised against the use of syrup of ipecac as an emetic when children swallow a poisonous substance. In a statement published in the November 2003 issue of Pediatrics, the AAP recommended that syrup of ipecac no longer be used as a home treatment for poisoning. Syrup of ipecac is still not recommended for use at this time. See http://www.healthychildren.org for more information.


• If a child has been exposed to a toxic substance, the caregiver must call the national poison control hotline at 800-222-1222. Calls are routed to the local poison control center or take to the nearest emergency department.


• Steps to follow to prevent accidental poisoning, as identified by the AAP, include the following: (1) Keep potential poisons out of sight and out of reach. (2) Always check to make sure containers are securely closed and the cabinets where they are stored are securely shut and locked after poisonous substances are used. (3) Never transfer a substance from its original to an alternate container. (4) Safely dispose of all unused and unneeded medications. (5) Never refer to medicines as “candy.”


• The AAP specifies that the following steps be implemented for the treatment of poisoning in young children: (1) If the poison has been ingested, first call the national poison control hotline at 800-222-1222. (2) If the poison has touched the skin or eyes, run tap water over the skin or eyes for 15 to 20 minutes. (3) If the poison has been inhaled, remove the child from the hazardous environment. (4) In all cases of poisoning, if the victim is conscious and alert, call the local poison control center. If the victim has collapsed or stopped breathing, call 911 for emergency transport to a hospital.


• There remains no indication for use of syrup of ipecac in any setting including health care settings.


Modified from American Academy of Pediatrics: Poison treatment in the home, Pediatrics 112:1061-1064, 2003; Tips for poison prevention and treatment, available at http://www.healthychildren.org.




Pharmacology Overview


Antiemetic Drugs


Drugs used to relieve nausea and vomiting are called antiemetic drugs. All antiemetic drugs work at some site in the vomiting pathways. There are six categories of such drugs with varying mechanisms of action. When drugs from different categories are combined, the antiemetic effectiveness is increased because more than one pathway becomes blocked. Some of the more commonly used antiemetics in the various categories are listed in Table 52-2. The sites at which antiemetics work in the vomiting pathway are shown in Figure 52-2.




Mechanism of Action and Drug Effects


Drugs used to prevent or treat nausea and vomiting have many different mechanisms of action. Most work by blocking one of the vomiting pathways, as shown in Figure 52-2. In doing so, they block the neurologic stimulus that induces vomiting. The mechanisms of action of the drugs in the six antiemetic drug categories are summarized in Table 52-3.



Anticholinergic drugs (see Chapter 21) have several uses. As antiemetics, they act by binding to and blocking acetylcholine (ACh) receptors in the vestibular nuclei, which are located deep within the brain. When ACh is prevented from binding to these receptors, nausea-inducing signals originating in this area cannot be transmitted to the chemoreceptor trigger zone (CTZ). Anticholinergics also block receptors located in the reticular formation so that nausea-inducing signals originating in this area cannot be transmitted to the vomiting center. Anticholinergics also tend to dry GI secretions and reduce smooth muscle spasms, both of which effects are often helpful in reducing acute GI symptoms, including nausea and vomiting.


Antihistamines (histamine 1 [H1] receptor blockers) act by inhibiting vestibular stimulation in a manner that is very similar to that of the anticholinergics. Although they bind primarily to H1 receptors, they also have potent anticholinergic activity, including antisecretory and antispasmodic effects. Thus, the antihistamines (see Chapter 36) prevent cholinergic stimulation in both the vestibular and reticular systems. Nausea and vomiting occur when these systems are stimulated. Note that these drugs are not to be confused with histamine 2 [H2] receptor blockers used for gastric acid control (see Chapter 50).


Antidopaminergic drugs, although they are traditionally used for their antipsychotic effects (see Chapter 16), also prevent nausea and vomiting by blocking dopamine receptors in the CTZ. Many of the antidopaminergics also have anticholinergic actions similar to those of anticholinergic drugs. In addition, antidopaminergic drugs calm the central nervous system (CNS).


Prokinetic drugs, in particular metoclopramide, act as antiemetics by blocking dopamine receptors in the CTZ, which desensitizes the CTZ to impulses it receives from the GI tract. Their primary action, however, is to stimulate peristalsis in the GI tract. This enhances the emptying of stomach contents into the duodenum, as well as intestinal movements.


Serotonin blockers work by blocking serotonin receptors located in the GI tract, CTZ, and vomiting center. There are many subtypes of serotonin receptors, and they are located throughout the body (CNS, smooth muscle, platelets, and GI tract). The receptor subtype involved in the mediation of nausea and vomiting is the 5-hydroxytryptamine 3 (5-HT3) receptor. These receptors are the site of action of the serotonin blockers such as ondansetron, granisetron, dolasetron, and palonosetron.


Tetrahydrocannabinol (THC), in a drug class by itself, is the major psychoactive substance in marijuana. Nonintoxicating doses in the form of the drug dronabinol are occasionally used as an antiemetic because of the drug’s inhibitory effects on the reticular formation, thalamus, and cerebral cortex. These effects cause an alteration in mood and in the body’s perception of its surroundings, which may be beneficial in relieving nausea and vomiting. Although this particular category of antiemetics is less commonly prescribed, there are occasionally patients who respond well to THC. Examples are patients being treated for cancer or acquired immunodeficiency syndrome (AIDS) who experience nausea and vomiting. In such patients, dronabinol may also stimulate the appetite, and nutritional wasting syndromes are common in both diseases. The drug also demonstrates some benefit in controlling the symptoms of glaucoma. There is a large, but highly controversial, political movement with participation of many cancer, AIDS, and glaucoma patients in favor of legalization of the marijuana plant for these uses.


Indications


The therapeutic uses of the antiemetic drugs vary depending on the drug category. There are several indications for the drugs in each class. These are listed in Table 52-2.


Contraindications


The primary contraindication for all antiemetics is known drug allergy. Other contraindications for various specific drugs are mentioned in the drug profiles.


Adverse Effects


Most of the adverse effects of the antiemetics stem from their nonselective blockade of various receptors. Some of the more common adverse effects associated with the various categories of antinausea drugs are listed in Table 52-4.



Interactions


The drug interactions associated with the antiemetic drugs are specific to the individual drug categories. Anticholinergics have additive drying effects when given with antihistamines and antidepressants. Increased CNS depressant effects are seen when antihistamine antiemetics are administered with barbiturates, opioids, hypnotics, tricyclic antidepressants, or alcohol. Increased CNS depression also occurs when alcohol or other CNS depressants are given together with antidopaminergic drugs. Combining metoclopramide with alcohol can result in additive CNS depression. Anticholinergics and analgesics can block the motility effects of metoclopramide. Serotonin blockers and THC have no significant drug interactions.


Dosages


For dosage information on selected antiemetic drugs, see the table on this page.


Drug Profiles


Antiemetics are used to treat nausea and vomiting in a variety of clinical situations, including chemotherapy-induced and postoperative nausea and vomiting, both of which can be especially difficult to treat. The ultimate goals of antiemetic therapy are minimizing or preventing fluid and electrolyte disturbances and minimizing deterioration of the patient’s nutritional status. Most of the antiemetics act by blocking receptors in the CNS, but some work directly in the GI tract. There are six major classes of antiemetic drugs, although there are other drugs that may also be used to treat nausea and vomiting, including corticosteroids such as dexamethasone (see Chapter 33) and anxiolytics such as lorazepam (see Chapter 16). Lorazepam is often used in the treatment and prevention of chemotherapy-induced nausea and vomiting. In addition to an antiemetic effect, it also helps to blunt the memory of the nausea and vomiting experience (especially with cancer chemotherapy). Dexamethasone,



DOSAGES
Selected Antiemetic and Antinausea Drugs























DRUG (PREGNANCY CATEGORY) PHARMACOLOGIC CLASS USUAL DOSAGE RANGE INDICATIONS/USES
Anticholinergics
scopolamine (Transderm-Scōp) (C) Anticholinergic, belladonna alkaloid Apply 1 patch to hairless area behind ear every 3 days (starting at least 4 hr before travel) Motion sickness prophylaxis
Antihistamines
♦ meclizine (Antivert, Bonine) (B) Anticholinergic, antihistamine

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  1. Pituitary Drugs
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  3. Acid-Controlling Drugs
  4. Analgesic Drugs

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May 9, 2017 | Posted by in NURSING | Comments Off on Antiemetic and Antinausea Drugs

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