NCLEX Review Guide: Respiratory Pharmacology - Antihistamines

First-Generation Antihistamines (H1 Antagonists)

• Diphenhydramine (Benadryl) and chlorpheniramine (Chlor-Trimeton) cross the blood-brain barrier, causing significant sedation and anticholinergic effects. These medications block histamine receptors in the CNS, leading to drowsiness, dry mouth, and blurred vision.
• Mechanism of action: Competitive antagonism of H1 receptors, preventing histamine-induced vasodilation, increased capillary permeability, and smooth muscle contraction in respiratory tract.

Second-Generation Antihistamines

• Loratadine (Claritin), cetirizine (Zyrtec), and fexofenadine (Allegra) are selective for peripheral H1 receptors and do not readily cross the blood-brain barrier. These cause minimal sedation and are preferred for long-term allergy management.
• Onset of action is typically 1-2 hours with duration of 12-24 hours, making them suitable for once-daily dosing in most patients.

Therapeutic Uses

• Allergic rhinitis, urticaria, and mild allergic reactions are primary indications for antihistamine therapy. These medications reduce sneezing, rhinorrhea, nasal congestion, and pruritus by blocking histamine-mediated inflammatory responses.
• First-generation antihistamines are often used for acute allergic reactions and as sleep aids due to their sedating properties, while second-generation are preferred for chronic conditions.

Common Pitfalls

• Do not confuse antihistamines with decongestants - antihistamines block histamine receptors while decongestants cause vasoconstriction of nasal blood vessels.
• Remember that antihistamines are preventive, not rescue medications for severe allergic reactions - epinephrine is the first-line treatment for anaphylaxis.

Assessment & Monitoring

1. Assess baseline respiratory status, including breath sounds, respiratory rate, and oxygen saturation before administration
2. Monitor for therapeutic effectiveness: decreased sneezing, rhinorrhea, and pruritus within 1-2 hours of administration
3. Evaluate for adverse effects: sedation level, anticholinergic symptoms, and cardiovascular effects in elderly patients
4. Document patient response and any side effects experienced during treatment

Patient Education Points

• Take medication before allergen exposure when possible for maximum effectiveness, as antihistamines work better prophylactically than therapeutically after symptoms develop.
• Avoid alcohol and other CNS depressants when taking first-generation antihistamines due to increased risk of excessive sedation and respiratory depression.
• Use sugar-free gum or frequent mouth rinses to combat dry mouth, and increase fluid intake unless contraindicated to prevent constipation from anticholinergic effects.