Antihistamines, Decongestants, Antitussives, and Expectorants



Antihistamines, Decongestants, Antitussives, and Expectorants


Objectives


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



Drug Profiles



Key Terms


Adrenergics (sympathomimetics) Drugs that stimulate the sympathetic nerve fibers of the autonomic nervous system that use epinephrine or epinephrine-like substances as neurotransmitters. (p. 580)


Antagonists Drugs that exert an action opposite to that of another drug or compete for the same receptor sites. (p. 577)


Anticholinergics (parasympatholytics) Drugs that block the action of acetylcholine and similar substances at acetylcholine receptors, which results in inhibition of the transmission of parasympathetic nerve impulses. (p. 580)


Antigens Substances that are capable of inducing specific immune responses and reacting with the specific products of those responses, such as antibodies and specifically sensitized T lymphocytes. Antigens can be soluble (e.g., a foreign protein) or particulate or insoluble (e.g., a bacterial cell). (p. 577)


Antihistamines Substances capable of reducing the physiologic and pharmacologic effects of histamine. (p. 577)


Antitussive A drug that reduces coughing, often by inhibiting neural activity in the cough center of the central nervous system. (p. 582)


Corticosteroids Any of the hormones produced by the adrenal cortex, either in natural or synthetic drug form. They control many key processes in the body, such as carbohydrate and protein metabolism, the maintenance of serum glucose levels, electrolyte and water balance, and the functions of the cardiovascular system, skeletal muscle, kidneys, and other organs. (p. 580)


Decongestants Drugs that reduce congestion or swelling, especially of the upper or lower respiratory tract. (p. 580)


Empiric therapy A method of treating disease based on observations and experience, rather than a knowledge of the precise cause for the disorder. (p. 576)


Expectorants Drugs that increase the flow of fluid in the respiratory tract, usually by reducing the viscosity of secretions, and facilitate their removal by coughing. (p. 584)


Histamine antagonists Drugs that compete with histamine for binding sites on histamine receptors. (p. 577)


Influenza A highly contagious infection of the respiratory tract caused by a myxovirus and transmitted by airborne droplets. (p. 576)


Nonsedating antihistamines Medications that primarily work peripherally to block the actions of histamine and therefore do not generally have the central nervous system effects of many of the older antihistamines; also called second-generation antihistamines and peripherally acting antihistamines. (p. 580)


Reflex stimulation An irritation of the respiratory tract occurring in response to an irritation of the gastrointestinal tract. (p. 583)


Rhinovirus Any of about 100 serologically distinct ribonucleic acid (RNA) viruses that cause about 40% of acute respiratory illnesses. (p. 576)


Sympathomimetic drugs A class of drugs whose effects mimic those resulting from the stimulation of the sympathetic nervous system. (p. 581)


Upper respiratory tract infection (URI) Any infectious disease of the upper respiratory tract, including the common cold, laryngitis, pharyngitis, rhinitis, sinusitis, and tonsillitis. (p. 576)


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Anatomy, Physiology, and Pathophysiology Overview


Common colds result from a viral infection, most often infection with a rhinovirus or an influenza virus. These viruses invade the tissues (mucosa) of the upper respiratory tract (nose, pharynx, and larynx) to cause an upper respiratory tract infection (URI). The inflammatory response elicited by these viruses stimulates excessive mucus production. This fluid drips behind the nose, down the pharynx, and into the esophagus and lower respiratory tract, which leads to symptoms typical of a cold: sore throat, coughing, and upset stomach. Irritation of the nasal mucosa often triggers the sneeze reflex and also causes the release of several inflammatory and vasoactive substances, which results in the dilation of the small blood vessels in the nasal sinuses and leads to nasal congestion. Treatment of the common symptoms of URI involves the combined use of antihistamines, nasal decongestants, antitussives, and expectorants.


In 2008, the U.S. Food and Drug Administration (FDA) issued recommendations that over-the-counter (OTC) cough and cold products not be given to children younger than 2 years of age. This followed numerous case reports of symptoms such as oversedation, seizures, tachycardia, and even death in toddlers. There is also evidence that such medications are simply not effective in small children, and parents are advised to consult their pediatrician on the best ways to manage these illnesses. A 2010 study showed a dramatic decrease in young children emergency room visits since the FDA recommendation.


Many antihistamines, nasal decongestants, antitussives, and expectorants are available without prescription. However, these drugs can only relieve the symptoms of a URI. They can do nothing to eliminate the causative pathogen. Antiviral drugs are currently the only drugs that are effective; however, treatment with these mediations is often hampered by the fact that the viral cause cannot be readily identified. Because of this, the treatment rendered can only be based on what is believed to be the most likely cause, given the presenting clinical symptoms. Such treatment is called empiric therapy. Some patients seem to gain benefit from the use of herbal products and other supplements, such as vitamin C, to prevent the onset of cold signs and symptoms or at least to decrease their severity. Herbal products commonly used for colds are echinacea and goldenseal (see the Safety: Herbal Therapies and Dietary Supplements boxes on p. 577). There is limited research data regarding the efficacy of herbal products, and some can have significant drug-drug or drug-disease interactions.


Pharmacology Overview


Antihistamines


Histamine is a substance that performs many functions. It is involved in nerve impulse transmission in the central nervous system (CNS), dilation of capillaries, contraction of smooth muscle, stimulation of gastric secretion, and acceleration of the heart rate. There are two types of cellular receptors for histamine. Histamine 1 (H1) receptors mediate smooth muscle contraction and dilation of capillaries; histamine 2 (H2) receptors mediate acceleration of the heart rate and gastric acid secretion. The release of excessive amounts of histamine can lead to anaphylaxis and severe allergic symptoms and may result in any or all of the following physiologic changes:



• Constriction of smooth muscle, especially in the stomach and lungs


• Increase in body secretions





• Vasodilatation and increased capillary permeability, which results in the movement of fluid out of the blood vessels and into the tissues and thus causes a drop in blood pressure and edema


Antihistamines are drugs that directly compete with histamine for specific receptor sites. For this reason, they are also called histamine antagonists. Antihistamines that compete with histamine for the H2 receptors are called H2 antagonists or H2 blockers and include cimetidine (Tagamet), ranitidine (Zantac), famotidine (Pepcid), and nizatidine (Axid). Because they act on the gastrointestinal (GI) system, they are discussed in detail in Chapter 50. This chapter focuses on the H1 antagonists (also called H1 blockers); they are the drugs commonly known as antihistamines. They are very useful drugs, because approximately 10% to 20% of the general population is sensitive to various environmental allergens. Histamine is a major inflammatory mediator in many allergic disorders, such as allergic rhinitis (e.g., hay fever and mold, dust allergies), anaphylaxis, angioedema, drug fevers, insect bite reactions, and urticaria (itching).


H1 antagonists include drugs such as diphenhydramine (Benadryl), chlorpheniramine (generic), fexofenadine (Allegra), loratadine (Claritin), and cetirizine (Zyrtec). They are of greatest value in the treatment of nasal allergies, particularly seasonal hay fever. They are also given to relieve the symptoms of the common cold, such as sneezing and runny nose. In this regard they are palliative, not curative; that is, they can help alleviate the symptoms of a cold but can do nothing to destroy the virus causing it.


The clinical efficacy of the different antihistamines is very similar, although they have varying degrees of antihistaminic, anticholinergic, and sedating properties. The particular actions and indications for a particular antihistamine are determined by its specific chemical makeup. All antihistamines compete with histamine for the H1 receptors in the smooth muscle surrounding blood vessels and bronchioles. They also affect the secretions of the lacrimal, salivary, and respiratory mucosal glands, which are the primary anticholinergic actions of antihistamines. These drugs differ from each other in their potency and adverse effects, especially in the degree of drowsiness they produce. The antihistaminic, anticholinergic, and sedative properties of some of the more commonly used antihistamines are summarized in Figure 36-1. Because of their antihistaminic properties, they are indicated for the treatment of allergies. They are also useful for the treatment of problems such as vertigo, motion sickness, insomnia, and cough. Several classes of antihistamines are listed in Table 36-1, along with their various anticholinergic and sedative effects.




Mechanism of Action and Drug Effects


During allergic reactions, histamine and other substances are released from mast cells, basophils, and other cells in response to antigens circulating in the blood. Histamine molecules then bind to and activate other cells in the nose, eyes, respiratory tract, GI tract, and skin, producing the characteristic allergic signs and symptoms. For example, in the respiratory tract histamine causes extravascular smooth muscle (e.g., in the bronchial tree) to contract, whereas antihistamines cause it to relax. Also, histamine causes pruritus by stimulating nerve endings. Antihistamines can prevent or alleviate this itching.


Circulating histamine molecules bind to histamine receptors on basophils and mast cells. This stimulates further release of histamine stored within these cells. Antihistamine drugs work by blocking the histamine receptors on the surfaces of basophils and mast cells, thereby preventing the release and actions of histamine stored within these cells. They do not push off histamine that is already bound to a receptor but compete with histamine for unoccupied receptors. Therefore, they are most beneficial when given early in a histamine-mediated reaction, before all of the free histamine molecules bind to cell membrane receptors. The binding of H1 blockers to these receptors prevents the adverse consequences of histamine binding: vasodilation; increased GI, respiratory, salivary, and lacrimal secretions; and increased capillary permeability with resultant edema. The various drug effects of antihistamines are listed in Table 36-2.



Indications


Antihistamines are indicated for the management of nasal allergies, seasonal or perennial allergic rhinitis (e.g., hay fever), and some of the typical symptoms of the common cold. They are also useful in the treatment of allergic reactions, motion sickness, Parkinson’s disease (due to their anticholinergic effects), and vertigo. In addition, they are sometimes used as sleep aids.


Contraindications


Use of antihistamines is contraindicated in cases of known drug allergy. They are not to be used as the sole drug therapy during acute asthmatic attacks. In such cases, a rapidly acting bronchodilator such as albuterol, or in extreme cases epinephrine, is the most urgently needed medication. Other contraindications may include narrow-angle glaucoma, cardiac disease, kidney disease, hypertension, bronchial asthma, chronic obstructive pulmonary disease, peptic ulcer disease, seizure disorders, benign prostatic hyperplasia, and pregnancy. Fexofenadine is not recommended for those with renal impairment. Desloratadine is not recommended for pediatric patients. Loratadine is not recommended for children younger than 2 years of age. Antihistamines should be used with caution in patients with impaired liver function or renal insufficiency, as well as in lactating mothers.


Adverse Effects


Drowsiness is usually the chief complaint of people who take antihistamines, but the sedative effects vary from class to class (see Table 36-1). Fortunately, sedative effects are much less common, although still possible, with the newer “nonsedating” drugs. The anticholinergic (drying) effects of antihistamines can cause adverse effects such as dry mouth, changes in vision, difficulty urinating, and constipation. Reported adverse effects of the antihistamines are listed in Table 36-3.



Interactions


Drug interactions of antihistamines are listed in Table 36-4. An allergist will usually recommend discontinuation of antihistamine drug therapy at least 4 days prior to allergy testing.


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May 9, 2017 | Posted by in NURSING | Comments Off on Antihistamines, Decongestants, Antitussives, and Expectorants

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