Antihistamines and Decongestants Flashcards
Two receptors of histamines
- H1 receptors: mediate smooth muscle contraction and dilation of capillaries
- H2 Receptors: mediate acceleration of HR and gastric acid secretion
Excessive histamine – anaphylaxis, allergic symptoms causing:
- Constriction of smooth muscle esp. lung and stomach
- Increased body secretions
- Vasodilation and increased capillary permeability – fluid moves out of blood
vessels and into the tissues, causing a drop in BP and edema
Classification of antihistamines
- Histamine antagonists
- Directly compete with histamine for specific receptor sites
- H1 antagonists – H1 blockers – commonly known as antihistamines
- Diphenhydramine (Benadryl), fexofenadine (Allegra), desloratadine (Aerius), loratatinde (Claritin), ceterizine, (Claritin)
- They treat symptoms but are not curative
- H2 antagonists – H2 blockers (cimetidine, ranitidine [Zantac], famotidine [Pepcid AC] – act on GI system (Lilley et al, 2017 – chapter 39)
Movement of antihistamines
- Onset, peak, half-life, duration vary depending on particular antihistamine
- Nonsedating antihistamines (longer) vs. traditional (shorter)
- This has an impact on adverse effect profile
Action of antihistamines
- Directly compete with histamine for specific receptor sites
- Block histamine receptors (on basophils and mast cells) and prevent the release and actions of histamine
Efficacy vs. Adverse Effects between Traditional and nonsedative antihistamines
Traditional:
• More anticholinergic and sedative effects
Nonsedating:
• Less anticholinergic and sedative effects
Antihistamine contraindacations
Allergy, acute angle closure glaucoma, HTN, heart disease, kidney disease, bronchial asthma, COPD, PUD, seizure disorder, BPH, seizure disorders
Antihistamine Adverse Effects
Anticholinergic properties – also Table 37-3
Antihistamine Patient Education
- Sedating effects of traditional antihistamines
* Take with food
Antihistamine Indication
- Nasal allergies, seasonal/perennial allergic rhinitis (hay fever), some symptoms of common cold (rhinorrhea)
- Allergic reactions
- Motion sickness
- Parkinson’s disease (d/t anticholinergic effects)
- Vertigo
- Sleep aides (sad)
Antihistamine route
- Most – only PO; also IV/IM (diphenhydramine)
- Give the right drug!
- diphenhydrmaine (Benadryl) vs. dimenhydrinate (Gravol)
- Diphenhydramine is often given for antipruritic properties that are an adverse effect of opioids
- Reduced doses for elderly
Drug Interactions
Table 37-4
Classifications of decongestants
- Adrenergic (sympathomimetics) – largest group
- Anticholinergics (parasympathomimetics) – used less commonly
- Corticosteroids – intranasal (see list of drugs p. 709)
• Many OTC options
Movement of decongestants
- Onset, peak, half-life, duration vary depending on particular decongestant
- Oral route – prolonged effect, onset slow, less potent action
- Topical (inhaled) – rapid absorption – rapid action – rapid decline in action
- Overuse of topical (inhaled) OTC adrenergic decongestants can lead to rebound congestion (which leads to more use and dependence)
- Rhinitis medicamentosa
Action of decongestants
• Shrink engorged nasal mucous membranes and relieve nasal stuffiness
• Adrenergic – constrict blood vessels - allows secretions in to drain (nasal, or blood/lymph circulation)
• Nasal steroids – exert anti-inflammatory effects on cells that mediate the
immune system (goal is not immunosuppression but reduction of inflammatory symptoms to promote comfort)
