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Flashcards in Antihistamines Deck (17):
0

Histamine

Endogenous compound; found in specialized cells
-Important role in: allergic reaction, regulation of gastric acid secretion. Histamine use limited to diagnostic procedures

1

Histamine distribution

Present in practically all tissues, especially high in skin, lungs, and GI tract, low content in plasma

2

Histamine
(Synthesis/storage and release)

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

3

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

4

H2 receptors

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

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Histamine antagonists/antihistamines

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

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Allergic responses

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

7

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

8

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

9

Antihistamine: H1 antagonists

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

10

Antihistamine: H2 antagonists

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

11

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

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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

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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

14

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)

15

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

16

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