💊 Adenosine

Adenosine is an endogenous nucleoside that acts as an antiarrhythmic agent, primarily used for acute termination of supraventricular tachycardias (SVTs). It is classified as an unclassified antiarrhythmic.

Adenosine activates A1 adenosine receptors in the atrioventricular (AV) node, leading to increased potassium efflux and inhibition of calcium influx. This hyperpolarises the AV nodal cells, slowing AV nodal conduction and increasing the AV nodal refractory period. This transiently blocks conduction through the AV node, interrupting re-entrant pathways involving the AV node.

The primary indication for adenosine is the rapid termination of paroxysmal supraventricular tachycardia (PSVT), including AV nodal re-entrant tachycardia (AVNRT) and AV re-entrant tachycardia (AVRT). It is also used diagnostically to differentiate between broad and narrow complex tachycardias, particularly to unmask atrial flutter or fibrillation in cases of SVT with aberrancy.

Absolute contraindications include second- or third-degree AV block (unless a pacemaker is in situ), sick sinus syndrome (unless a pacemaker is in situ), and known hypersensitivity to adenosine. Relative contraindications include severe asthma or COPD due to the risk of bronchospasm, and Wolff-Parkinson-White (WPW) syndrome with atrial fibrillation, where it can precipitate ventricular fibrillation.

Common adverse effects are short-lived due to its very brief half-life and include flushing, chest discomfort/pressure, dyspnoea, and headache. Transient bradycardia, AV block, and asystole may occur. Less commonly, bronchospasm can be precipitated, particularly in asthmatic patients. Nausea and dizziness are also reported.

Patients receiving adenosine require continuous ECG monitoring to observe for rhythm changes and AV block. Blood pressure and oxygen saturation should also be monitored. Due to its extremely short half-life, adverse effects are usually transient and resolve quickly without specific intervention.

Before prescribing, always confirm patient allergies, especially to adenosine itself. Be aware of potential interactions with dipyridamole (potentiates adenosine) and methylxanthines (e.g., aminophylline, theophylline, which antagonise adenosine). Counsel the patient about the very brief but intense sensation of flushing, chest discomfort, and breathlessness they may experience, reassuring them it is transient.

Adenosine is the first-line drug for stable, narrow complex SVT. Its ultra-short half-life (seconds) means adverse effects are usually transient. It is ineffective for atrial fibrillation or flutter, and ventricular tachycardias. It can be used diagnostically to unmask underlying atrial activity in wide complex tachycardias.

Classic SBA questions often test the primary indication (SVT termination) and its very short half-life. They might also focus on contraindications like severe asthma or pre-existing AV block. Differentiating its use from other antiarrhythmics for different arrhythmias is a common theme.