Pharmacology
Respiratory
Which of the following is NOT an indication for a H1 receptor antihistamine:
Answer:
Antihistamines are competitive inhibitors at the H1-receptor (in contrast to H2 receptor antagonists used to decrease gastric acid secretion in gastroesophageal reflux disease). Indications:- Allergic rhinitis and conjunctivitis
- Urticarial rashes, pruritus, insect bites and stings
- Angioedema
- Nausea/vomiting and prevention of motion sickness
- Insomnia
Antihistamines
Pharmacology / Respiratory
Last Updated: 21st November 2021
Antihistamines are competitive inhibitors at the H1-receptor (in contrast to H2 receptor antagonists used to decrease gastric acid secretion). They act to relax histamine-induced bronchoconstriction, block the vasodilator effect of histamine, inhibit histamine-induced increases in capillary permeability and block mucus secretion and sensory nerve stimulation.
Pharmacokinetics
Histamine (H1)-receptor antagonists are well absorbed after oral administration. The effects of these agents are usually seen in 30 minutes (with maximal effects at 1 – 2 h); the duration of action is 3 – 8 hours for first-generation compounds and 3 – 24 hours for second-generation compounds.
H1-receptor antagonists are metabolised in the liver; many induce microsomal enzymes and alter their own metabolism and that of other drugs.
Indications
- Allergic rhinitis and conjunctivitis
- Urticarial rashes, pruritus, insect bites and stings
- Angioedema
- Nausea/vomiting and prevention of motion sickness
- Insomnia
Chlorphenamine
All older antihistamines cause sedation but alimemazine tartrate and promethazine may be more sedating whereas chlorphenamine maleate and cyclizine may be less so. This sedating activity is sometimes used to manage the pruritus associated with some allergies or used to manage occasional insomnia. There is little evidence that any one of the older, 'sedating' antihistamines is superior to another and patients vary widely in their response.
Cetirizine
The newer antihistamines e.g. cetirizine cause less sedation and psychomotor impairment than the older antihistamines because they are much less lipid soluble and penetrate the blood brain barrier only to a slight extent.
Cautions
Antihistamines should usually be avoided in acute porphyrias (although some antihistamines are thought to be safe).
Antihistamines should be used with caution in epilepsy, prostatic hypertrophy, urinary retention, hepatic impairment and susceptibility to angle-closure glaucoma.
Side Effects (significantly reduced with second-generation agents)
Elderly patients and children are more susceptible to side effects.
Common side effects of antihistamines may include:
- Anticholinergic effects (blurred vision, dry mouth, urinary retention)
- Headache
- Gastrointestinal disturbances
- Psychomotor impairment (sedation, dizziness and loss of appetite)
Drowsiness may affect performance of skilled tasks (e.g. cycling or driving); sedating effects are enhanced by alcohol and opioid analgesics.
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- Biochemistry
- Blood Gases
- Haematology
| Biochemistry | Normal Value |
|---|---|
| Sodium | 135 – 145 mmol/l |
| Potassium | 3.0 – 4.5 mmol/l |
| Urea | 2.5 – 7.5 mmol/l |
| Glucose | 3.5 – 5.0 mmol/l |
| Creatinine | 35 – 135 μmol/l |
| Alanine Aminotransferase (ALT) | 5 – 35 U/l |
| Gamma-glutamyl Transferase (GGT) | < 65 U/l |
| Alkaline Phosphatase (ALP) | 30 – 135 U/l |
| Aspartate Aminotransferase (AST) | < 40 U/l |
| Total Protein | 60 – 80 g/l |
| Albumin | 35 – 50 g/l |
| Globulin | 2.4 – 3.5 g/dl |
| Amylase | < 70 U/l |
| Total Bilirubin | 3 – 17 μmol/l |
| Calcium | 2.1 – 2.5 mmol/l |
| Chloride | 95 – 105 mmol/l |
| Phosphate | 0.8 – 1.4 mmol/l |
| Haematology | Normal Value |
|---|---|
| Haemoglobin | 11.5 – 16.6 g/dl |
| White Blood Cells | 4.0 – 11.0 x 109/l |
| Platelets | 150 – 450 x 109/l |
| MCV | 80 – 96 fl |
| MCHC | 32 – 36 g/dl |
| Neutrophils | 2.0 – 7.5 x 109/l |
| Lymphocytes | 1.5 – 4.0 x 109/l |
| Monocytes | 0.3 – 1.0 x 109/l |
| Eosinophils | 0.1 – 0.5 x 109/l |
| Basophils | < 0.2 x 109/l |
| Reticulocytes | < 2% |
| Haematocrit | 0.35 – 0.49 |
| Red Cell Distribution Width | 11 – 15% |
| Blood Gases | Normal Value |
|---|---|
| pH | 7.35 – 7.45 |
| pO2 | 11 – 14 kPa |
| pCO2 | 4.5 – 6.0 kPa |
| Base Excess | -2 – +2 mmol/l |
| Bicarbonate | 24 – 30 mmol/l |
| Lactate | < 2 mmol/l |
