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Chapter 70

Chapter 70. Antihistamines. Histamine. Endogenous compound Found in specialized cells Important role in: Allergic reaction Regulation of gastric acid secretion Histamine use limited to diagnostic procedures. Histamine. Distribution Present in practically all tissues

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Chapter 70

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  1. Chapter 70 Antihistamines

  2. Histamine • Endogenous compound • Found in specialized cells • Important role in: • Allergic reaction • Regulation of gastric acid secretion • Histamine use limited to diagnostic procedures

  3. Histamine • Distribution • Present in practically all tissues • Especially high in skin, lungs, and GI tract • Low content in plasma

  4. Histamine • Synthesis/storage • Mast cells and basophils • Produced by neurons • Release • Allergic • Nonallergic

  5. Fig.70–1. Release of histamine by allergen-antibody interaction. (IgE = immunoglobulin E.)

  6. Histamine Receptors • H1 receptor • Vasodilation • Increased capillary permeability • Bronchoconstriction • CNS effects • Other

  7. H1 Receptors • Vasodilation • Skin of the face and upper body • Extensive: can cause hypotension • Increased capillary permeability • Edema • Bronchoconstriction • Not the cause of asthma attack • CNS effects • Role in cognition, memory, and sleep-waking cycles • Other • Itching, pain, secretion of mucus

  8. H2 Receptors • Secretion of gastric acid • Act directly on parietal cells to promote acid release • Dominant role in acid release

  9. Histamine Antagonists/Antihistamines • Act primarily on H1 and H2 receptors • Used primarily for two pathologic states • Allergic disorders • Peptic ulcer disease

  10. Allergic Responses • Mediated by histamine and other compounds • Prostaglandins • Leukotrienes • Tryptase

  11. Mild Allergic Responses • Caused largely by histamine acting at H1 receptors • Rhinitis, itching, localized edema • Hay fever, mild transfusion reaction, acute urticaria • Usually responsive to antihistamine therapy

  12. Severe Allergic Responses • Anaphylaxis • Anaphylactic shock • Bronchoconstriction, hypotension, edema of the glottis • Histamine plays a minor role • Leukotrienes are the principal mediators • Antihistamines are of little use in treatment • Epinephrine is the drug of choice for treatment (see Chapter 17)

  13. Two Types of Antihistamines • H1 antagonists • Produce selective blockade of H1 receptors • Used for treatment of mild allergic disorders • H2 antagonists • Produce selective blockade of H2 receptors • Used for treatment of gastric and duodenal ulcers • Not used for treatment of allergies

  14. H1 Antagonists • H1 antagonists are divided into two major groups: • First-generation H1 antagonists (highly sedating) • Second-generation H1 antagonists • Mechanism of action • Block the actions of histamine at H1 receptors • Do not block H2 receptors • Some bind to muscarinic receptors

  15. H1 Antagonists • Pharmacologic effects • Peripheral effects • Reduce localized flushing • Reduce itching and pain • Effects on the CNS • Therapeutic dose: CNS depression • Second-generation negligible CNS depression • Overdose • CNS stimulation • Convulsions • Very young children especially sensitive to CNS stimulation

  16. H1 Antagonists • Therapeutic uses • Mild allergy • Severe allergy • Adjunct only, benefits may be limited • Motion sickness • Promethazine, dimenhydrinate • Insomnia • Common cold • May decrease rhinorrhea through anticholinergic properties, not H1 blockade

  17. H1 Antagonists • Adverse effects • Sedation • Less with second and third generation • Nonsedative CNS effects • Dizziness, fatigue, coordination problems, confusion • Gastrointestinal effects • Can cause nausea, vomiting, loss of appetite, constipation (give with food) • Anticholinergic effects • Weak atropine-like effects

  18. H1 Antagonists • Adverse effects • Severe respiratory depression • Severe local tissue injury • Cardiac dysrhythmias • Rare

  19. H1 Antagonists • Drug interactions • CNS depressants • Use of pregnancy and lactation • Acute toxicity • Large margin of safety • Widespread availability of drugs • CNS and anticholinergic reactions

  20. High-Risk Patients • Antihistamines contraindicated • During third trimester of pregnancy • Nursing mothers • Newborn infants • Use antihistamines with caution in: • Young children • Older adults • Patients whose conditions may be aggravated by muscarinic blockade

  21. H1 Antagonists: First Generation

  22. H1 Antagonists: Second Generation • Second generation (nonsedating) • Fexofenadine (Allegra) • Cetirizine (Zyrtec) • Levocetirizine (Xyza) • Loratadine (Claritin, Tavist ND, Alavert) • Desloratadine (Clarinex)

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