Adenosine

Adenosine is an endogenous nucleoside given by rapid intravenous injection. It briefly blocks atrioventricular nodal conduction, ending supraventricular tachycardia that uses the node in the circuit, and it unmasks atrial flutter or fibrillation by slowing the ventricular rate for a moment. Its half life is under 10 seconds, so the effects are short but can be alarming to the patient. It is available in many East African hospitals, although verapamil is the common alternative.

Classification. Antiarrhythmic (purine nucleoside); also a pharmacological stress agent.

Mechanism. Activates A1 adenosine receptors, opening potassium channels in the sinoatrial and atrioventricular nodes, which hyperpolarises the cells and causes transient atrioventricular block. A2A receptor activation causes vasodilation.

Pharmacodynamics. Onset in 10 to 30 seconds; duration of atrioventricular block 10 to 30 seconds. Causes flushing, chest discomfort, dyspnoea, and bronchospasm in asthma.

Pharmacokinetics. Taken up quickly by red cells and vascular endothelium; broken down by adenosine deaminase. Half life under 10 seconds. Methylxanthines (theophylline, caffeine) block its receptors; dipyridamole blocks its uptake and prolongs the effect.

Supraventricular tachycardia, adult (IV).

• 6 mg rapid IV bolus over 1 to 2 seconds into a large vein (antecubital), immediately followed by a 20 mL 0.9% sodium chloride flush with the arm raised.

• If no response in 1 to 2 minutes, 12 mg; repeat 12 mg once more if needed.

• Maximum 30 mg (6 mg, 12 mg, 12 mg).

• Use 3 mg as the first dose via a central line, after a heart transplant, or in a patient taking carbamazepine; reduce to 0.5 to 1 mg first dose with dipyridamole; larger doses with theophylline.

Diagnosis of wide or narrow complex tachycardia, adult. Give 6 mg, then 12 mg, then 12 mg by rapid IV bolus with continuous ECG recording.

Pharmacological stress imaging, adult (specialist). 140 micrograms/kg/minute by infusion over 4 to 6 minutes.

Children. 0.1 mg/kg IV rapid bolus (maximum 6 mg), then 0.2 mg/kg (maximum 12 mg), then 0.3 mg/kg (maximum 12 mg) if needed. Neonates: 150 micrograms/kg, increased in 50 microgram/kg steps.

Contraindications. Second or third degree atrioventricular block or sick sinus syndrome without a pacemaker; long QT syndrome; severe hypotension; decompensated heart failure; asthma with active wheeze or severe chronic obstructive disease; hypersensitivity; acute myocardial infarction with ongoing instability (caution).

Warnings.

• Transient asystole or severe bradycardia (have atropine or a defibrillator ready).
• Chest tightness, flushing and fear (warn the patient).
• Bronchospasm in asthma.
• Atrial fibrillation with a Wolff Parkinson White accessory tract can speed conduction, causing ventricular fibrillation.
• Seizures (rare).
• Triggers atrial fibrillation in a minority of patients.
• Give with resuscitation equipment present.

Adverse effects. Facial flushing, chest pain or tightness, shortness of breath, nausea, headache, light headedness, transient arrhythmias (bradycardia, asystole, atrial fibrillation, ventricular ectopy), bronchospasm.

Interactions. Theophylline, aminophylline, caffeine: block effect (raise dose or stop). Dipyridamole: effect prolonged and intensified (reduce dose). Carbamazepine: higher risk of heart block. Beta blockers and verapamil: additive nodal block. Digoxin: ventricular arrhythmia (rare reports). Nicotine and cocaine: variable effects.

Special groups.

• Pregnancy: used safely for supraventricular tachycardia in all trimesters.

• Breastfeeding: short half life; compatible.

• Patients of African ancestry: no dose change.

• Older adults: no change; monitor blood pressure.

• Kidney impairment: no change.

• Liver impairment: no change.

Formulations. Injection 3 mg/mL in 2 mL vials (6 mg) and 4 mL vials (12 mg).

Administration. Rapid IV push through a large proximal vein with a three way tap and immediate saline flush. Do not refrigerate (crystallisation). Record a continuous rhythm strip during the dose.

Baseline. 12 lead ECG, blood pressure, history of asthma and drug history (theophylline, dipyridamole, carbamazepine).

Monitoring. Continuous ECG, blood pressure, oxygen saturation during and 5 minutes after each dose. Have atropine and defibrillation ready.

Stopping. Single bolus doses only; no taper.

Common prescribing errors. Giving slowly (inactive); using a small peripheral vein or no flush; failing to halve the dose in patients on dipyridamole; giving to a patient with active asthma; using it in a patient on theophylline without raising the dose; missing atrial fibrillation with an accessory tract.