Pharmacology
Cardiovascular
What is the mechanism of action of atropine in the management of bradyarrhythmias:
Answer:
Atropine antagonises the action of the parasympathetic neurotransmitter acetylcholine at muscarinic receptors. Therefore it blocks the effect of the vagus nerve on both the sinoatrial and atrioventricular node, increasing sinus automaticity and facilitating AV node conduction.Atropine
Pharmacology / Cardiovascular
Last Updated: 17th April 2019
Atropine is used in sinus, atrial or nodal bradycardia or AV block, when the haemodynamic condition of the patient is unstable because of the bradycardia. The dose in this case is 500 micrograms intravenously, repeated if necessary to a maximum of 3 mg. Doses greater than 3 mg can cause a paradoxical slowing of the heart rate.
Asystole during cardiac arrest is usually caused by primary myocardial pathology rather than excessive vagal tone and there is no evidence that routine use of atropine is beneficial in the treatment of asystole or PEA.
Mechanism of Action
Atropine antagonises the action of the parasympathetic neurotransmitter acetylcholine at muscarinic receptors. Therefore it blocks the effect of the vagus nerve on both the sinoatrial and atrioventricular node, increasing sinus automaticity and facilitating AV node conduction.
Contraindications
Antimuscarinics should be avoided in:
- Gastrointestinal obstruction, intestinal atony or paralytic ileus
- Myasthenia gravis
- Prostatic enlargement, significant bladder outflow obstruction or urinary retention
- Severe ulcerative colitis or toxic megacolon
Side Effects
Side effects of atropine are dose-related and include:
- Dilation of pupils with loss of accommodation
- Blurred vision
- Dry mouth
- Urinary retention
- Constipation
- Drowsiness
- Acute confusion
- Skin dryness and flushing
- Tachycardia, palpitations and arrhythmias
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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 |
