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Adv Pharm > Chapter 56: Antihisamines > Flashcards

Chapter 56: Antihisamines Flashcards

1
Q

Histamine

A

Endogenous, locally acting compound

Found in specialized cells

Important role in:
Allergic reaction
Regulation of gastric acid secretion

Histamine use limited to diagnostic procedures

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2
Q

Histamine distribution

A

Present in practically all tissues

Especially high in skin, lungs, and gastrointestinal (GI) tract

Low content in plasma

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3
Q

Histamine synthesis/storage

A

Mast cells and basophils

Produced by neurons in posterior hypothalamus with atonal projections to the frontal and temporal cortices and other areas of the brain

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4
Q

Histamine release

A

Allergic: Allergic release of histamine requires prior exposure to the allergen; an allergic reaction cannot occur during initial allergen exposure.

Nonallergic: Several agents (certain drugs, radiocontrast media, plasma expanders) can act directly on mast cells to trigger histamine release. With these agents, no prior sensitization is needed. Cell injury can also cause direct release.

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5
Q

H1 Receptors activation

A

Vasodilation
Skin of the face and upper body
Extensive vasodilation 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

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6
Q

H2 Receptors activation

A

Secretion of gastric acid

Act directly on parietal cells to promote acid release

Dominant role in acid release

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7
Q

Role of Histamine in Allergic Response: mild allergy

A

Rhinitis, itching, localized edema are caused largely by histamine, acting at H1 receptors

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8
Q

Role of Histamine in Allergic Response: severe anaphylactic reaction

A

Anaphylactic shock: Bronchoconstriction, hypotension, and edema of the glottis

Histamine plays a minor role

Leukotrienes: Principal mediators

Antihistamines are little help as treatment

Epinephrine is the drug of choice

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9
Q

Two Types of Antihistamines: H1 Antagonists and H2 Antagonists

A

The principal use of H1 blockers is the treatment of mild allergic disorders
Produce selective blockade of H1 receptors
Used for treatment of mild allergic disorders

The principal use of H2 blockers is the treatment of gastric and duodenal ulcers
Produce selective blockade of H2 receptors
Used for treatment of gastric and duodenal ulcers
Not used for treatment of allergies

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10
Q

H1 Antagonists: 2 groups

A

First-generation H1 antagonists (highly sedating)

Second-generation H1 antagonists

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11
Q

H1 antagonists MOA

A

Block the actions of histamine at H1 receptors

Do not block H2 receptors

Some bind to muscarinic receptors

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12
Q

H1 antagonists: pharmacologic effects

A

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

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13
Q

H1 antagonists: therapeutic uses

A

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

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14
Q

H1 antagonists ADR

A

Sedation
Less with second and third generation

Nonsedative CNS effects
Dizziness, fatigue, coordination problems, confusion

GI effects
Can cause nausea, vomiting, loss of appetite, constipation (give with food)

Anticholinergic effects
Weak atropine-like effects

Severe respiratory depression

Severe local tissue injury

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15
Q

1st generation H1 antagonist

A

OTC

Benadryl, Chlor-trimeton, phenergan, vistaril

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16
Q

H1 antagonists: second generations

A

OTC
Cetirizine -Zyrtec, Xyzal, Fexofenadine -Allegra, Loratadine -Claritin, Clarinex

17
Q

H1 antagonists drug interactions

A

CNS depressants

18
Q

H1 antagonists use during pregnancy and lactation

A

Unknown with pregnancy –only use when benefit outweigh risks, avoid late in 3rd trimester

Can be excreted in milk, present a risk to the infant

19
Q

Acute toxicity of H1 antagonists

A

Large margin of safety

Widespread availability of drugs

CNS and anticholinergic reactions: Dilated pupils, flushed face, hyperpyrexia, tachycardia, dry mouth, urinary retention

Children: CNS excitation

Extreme cases: Coma, cardiovascular collapse, death

20
Q

H1 antagonist acute toxicity tx

A

No specific antidote to antihistamine poisoning

Treatment directed at drug removal and managing symptoms
Activated charcoal
Cathartic

Convulsions: IV benzodiazepines (lorazepam, midazolam)

Hyperthermia: Ice packs or sponge baths

21
Q

H1 antagonists contraindications

A

Third trimester of pregnancy

Nursing mothers

Newborn infants

22
Q

H1 antagonists caution

A

Young children

Older adults

Patients whose conditions may be aggravated by muscarinic blockade

23
Q

H1 antagonists 2nd gen

A

Produce much less sedation than first-generation agents

Cross the blood-brain barrier poorly

Have a low affinity for H1 receptors of the CNS

Largely devoid of anticholinergic actions

24
Q

Fexofenadine [Allegra, Allegra Allergy, Allegra ODT]

A

Uses: Oral therapy of seasonal allergic rhinitis and for chronic idiopathic urticaria

Of second-generation antihistamines, offers best combination of efficacy and safety

Use with caution in patients with renal impairment

Do not take with citrus fruit juice
Decreases absorption of medication

25
Q

Cetirizine [Zyrtec]

A

Uses: Allergic rhinitis and chronic idiopathic urticaria

Food delays absorption

More sedating than other second-generation antihistamines but less sedating than first-generation drugs

26
Q

Levocetirizine [Xyzal]

A

Uses: Allergic rhinitis and chronic idiopathic urticaria

More sedating than other second-generation antihistamines but less sedating than first-generation agents

Most common side effects: Drowsiness, fatigue, muscle weakness, dry mouth

Avoid alcohol and other CNS depressants

27
Q

Loratadine [Claritin]

A

Use: Seasonal allergic rhinitis

Generally well tolerated

Food delays absorption

Use with caution in patients with significant hepatic and renal impairment

28
Q

Desloratadine [Clarinex]

A

Uses: Seasonal allergic rhinitis, perennial allergic rhinitis, and chronic idiopathic urticaria

Adv Pharm (76 decks)

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