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Pharmacology

Cardiovascular

Question 135 of 180

The dose of adenosine must be quartered if given concomitantly in a patient taking which of the following drugs:

Answer:

The effects of adenosine are potentiated by dipyridamole, therefore if it is essential to give adenosine in a patient taking dipyridamole the dose should be quartered.

Adenosine is usually the treatment of choice for terminating paroxysmal supraventricular tachycardia.

Mechanism of Action

Adenosine stimulates A1-adenosine receptors and opens acetylcholine sensitive K+ channels, increasing K+ efflux. This hyperpolarises the cell membrane in the atrioventricular node and, by inhibiting the calcium channels, slows conduction in the AVN. As it has a very short duration of action (half-life only about 8 - 10 seconds), most side effects are short lived.

Administration

For a regular narrow-complex tachycardia the first step is to attempt vagal manoeuvres. If this is unsuccessful and the tachyarrhythmia persists, 6 mg intravenous adenosine should be administered into a central/large vein over 2 seconds, followed by 12 mg after 1 - 2 minutes if required, then a further 18 mg after 1 - 2 minutes if required (max 36 mg, as per the 2021 ALS Guidelines).

The effects of adenosine are potentiated by dipyridamole, therefore if it is essential to give adenosine in a patient taking dipyridamole the dose should be quartered.

The patient should be warned that they will feel unwell and may experience chest discomfort for a few seconds following the injection. An ECG (preferably multi-lead) should be recorded during the injection. If adenosine is contraindicated, or fails to terminate a regular narrow-complex tachycardia, the administration of verapamil 2.5 - 5 mg IV over 2 mins should be considered.

Contraindications

Adenosine is contraindicated in:

  • Asthma and COPD (can cause bronchospasm)
  • Decompensated heart failure
  • Long QT syndrome
  • Second- or third-degree AV block and sick sinus syndrome (unless pacemaker fitted)
  • Severe hypotension

Side Effects

Common side effects of adenosine include:

  • Apprehension
  • Dizziness, flushing, headache, nausea, dyspnoea
  • Angina (discontinue)
  • AV block, sinus pause and arrhythmia (discontinue if asystole or severe bradycardia occur)

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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

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