Antihistamines Flashcards
Histamine
Amine-organic compount
- Dreived from ammonia by replacement of one or more hydrogen atoms by organic groups
What is Histamine?
chemical messenger that mediates
several cellular responses:
○ Inflammatory reactions
○ Allergic reactions
○ Gastric acid secretion
○ Limited neurotransmitter
action in the brain
There are especially high
concentrations of histamine
receptors on cells found in the____
○ Lungs
○ Skin
○ Blood vessels
○ GI tract
Histamine is one of many
important mediators
released by cells in response
to _____
noxious stimuli
Histamine is stored in ____ in mast cells throughout the body
granules
Histamine receptors
○ H1 and H2
○ H3 and H4 (not clinically
applicable)
H1 receptor binding occurs where:
○ Peripheral sensory neurons
○ Intestinal smooth muscle
○ Secretory mucosa
○ Pulmonary smooth muscle
H1 AND H2 receptor binding occurs where:
○ Cardiovascular effects (Triggers release of Nitrous Oxide)
○ Dermatologic effects
“Triple Response” seen on the skin
○ Reddening of the skin
○ Wheal formation
○ Irregular “halo” flare
H2 receptor binding
○ Gastric effects: Stimulate gastric acid
secretion by activation of H2 receptors located on gastric parietal cells in the lining of the stomach
When histamine is released (unbound), it is rapidly degraded and inactivated by ____
by diamine oxidase
If the histamine release is widespread or very rapid, it may spread through the body via the blood before inactivation, potentially leading to _____
anaphylaxis.
If severe hypotension develops in
anaphylactic reaction, this is
sometimes called _____
Histamine Shock
Anaphylaxis Tx
○ Tx is Epinephrine IM (vasodilate
lungs/vasoconstrict arteries)
○ Diphenhydramine/Ranitidine IV second
line
○ Albuterol, Steroids (limited data)
MCAS
Mast Cell Activation Syndrome: Caused by excessive dumping of histamine
by mast cells, Multiple severe anaphylactic like events
● Involving more than one organ system
First-generation H1
antihistamines(inexpensive/effective
○ Diphenhydramine (Benadryl)
○ Chlorpheniramine (Chlor-Trimeton)
○ Doxylamine (Unisom)
○ Hydroxyzine (Vistaril)
○ Meclizine (Antivert)
○ Promethazine (Phenergan)
Newer-generation H1 antihistamines
○ Cetirizine (Zyrtec) 2nd gen
○ Loratadine (Claritin) 2nd gen
○ Desloratadine (Clarinex) 3rd gen
○ Fexofenadine (Allegra) 3rd gen
○ Levocetirizine (Xyzal) 3rd gen
Other H1 antihistamines
○ Azelastine (Optivar, Astelin)
○ Olopatadine (Pataday, Patanase)
H2 antihistamines (“H2 blockers”)
○ Famotidine (Pepcid)
○ Ranitidine (Zantac)
○ Cimetidine (Tagamet)
○ Nizatidine (Axid)
The prototype antihistamine
Diphenhydramine (Benadryl)
1st Generation H1 receptor blockers MOA
○ Binds to H1 histamine receptors, blocking the receptor-
mediated response of a target tissue.
○ Competitively antagonizes
○ CNS penetration
○ 1st generation H1 blockers
also antagonize cholinergic,
serotonin, and some alpha
adrenergic receptors
1st Generation H1 receptor blockers indications
○ Allergic rhinitis:
○ Allergic reactions and Urticaria:
○ Insomnia (short term)
○ Sedation:
○ Extrapyramidal symptoms
○ Motion sickness prevention:
○ Vertigo treatment:
○ Nausea/vomiting
○ Hydroxyzine is used frequently in Anxiety and Pruritus
1st Generation H1 receptor blockers contraindications
○ Patients under 2 years of age (PP age 3)
BBWs- Promethazine
■ Contraindicated in patients under 2 YOA due to sometimes fatal respiratory depression.
■ Severe chemical irritation and
tissue damage may result with IV/IM administration.
