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Flashcards in Antihistamines Deck (17):


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, especially high in skin, lungs, and GI tract, low content in plasma


(Synthesis/storage and release)

Synthesis/storage: mast cells and basophils, produced by neurons; release: allergic, nonallergic


Histamine receptors: H1

-Vasodilation: skin of the face and upper body, extensive: can cause hypotension
-Increased capillary permeability: edema
-Bronchoconstriction: does not cause asthma attack
-CNS effects: Role in cognition, memory, and sleep-waking cycles
-Other: itching, pain, secretion of mucus


H2 receptors

Secretion of gastric acid, act directly on parietal cells to promote acid release, dominant role in acid release


Histamine antagonists/antihistamines

Act primarily on H1 and H2 receptors; used primarily for two pathologic states: allergic disorders, peptic ulcer disease


Allergic responses

Mediated by histamine and other compounds: prostaglandins, leukotrienes, tryptase


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


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


Antihistamine: H1 antagonists

Produce selective blockade of H1 receptors, used for treatment of mild allergic disorders


Antihistamine: H2 antagonists

Produce selective blockade of H2 receptors, used for treatment of gastric and duodenal ulcers, not used for treatment of allergies


H1 Antagonists

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


H1 Antagonists: Pharmacologic effects

Peripheral effects: reduce localized flushing, reduce itching and pain
Effects on CNS:
-Therapeutic dose: CNS depression; second generation negligible CNS depression
Overdose: CNS stimulation, convulsions, very young children especially sensitive to CNS stimulation


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


H1 Antagonists: Adverse effects

Sedation (less with second and third generation); nonsedative CNS effects: dizziness, fatigue, coordination problems, confusion;
GI effects: nausea, vomiting, loss of appetite, constipation;
Anticholinergic effects: weak atropine-like effects
-Severe respiratory depression, severe local tissue injury, cardiac dysrhythmias (rare)


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


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