PharmacologyTier 1Medical Sciences concept

Adenosine - mechanism and contraindications

Core concept

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A1 receptor (Gi-coupled)

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A2A receptors -> vasodilation (flushing, coronary steal in stress testing, hypotension)
A2B/A3 on mast cells -> bronchoconstriction

Two distinct uses

Use	Logic
Terminate AVNRT/AVRT	The AV node is part of the re-entrant circuit -> block it and the circuit stops
Diagnose the rhythm	Transient block unmasks atrial activity - flutter waves, atrial tachycardia

*Adenosine does not terminate AF, flutter or VT* - it reveals them
Give it with a continuous ECG rhythm strip running, or you have wasted the dose

Dose and administration

6 mg rapid IV push, then 12 mg, then 12 mg at 1-2 minute intervals
Large proximal vein (antecubital or better), rapid push, immediate 20 mL saline flush, arm elevated
- t1/2 <10 seconds - taken up by erythrocytes and endothelium and deaminated by adenosine deaminase. A slow push simply does not reach the heart.
Halve the dose (3 mg) if: central line, transplanted (denervated) heart - supersensitive, dipyridamole or carbamazepine co-therapy
Double/increase the dose if on theophylline or caffeine (competitive A1 antagonists)

Expected effects - warn the patient

Chest tightness, flushing, dyspnoea, "sense of impending doom" - brief, and universal
Transient asystole of a few seconds, then sinus rhythm
Transient AF in ~1-12% after conversion

Contraindications


Asthma / significant reactive airway disease	Relative - risk of bronchospasm (use verapamil or a beta-blocker instead; verapamil only if the rhythm is certainly supraventricular and the patient is not hypotensive or on a beta-blocker)
Second- or third-degree AV block, sick sinus (without a pacemaker)	Prolonged asystole
Pre-excited AF (WPW with irregular broad-complex tachycardia)	Blocking the AV node accelerates conduction down the accessory pathway -> VF
Haemodynamic instability	Synchronised DC cardioversion, not drugs

Regular narrow-complex tachycardia -> vagal manoeuvres (modified Valsalva - supine leg raise after the strain, ~43% success vs 17%) -> adenosine
Regular broad-complex tachycardia: treat as VT unless proven otherwise. Adenosine is diagnostically useful only if the rhythm is regular and monomorphic, with resuscitation equipment present
Irregular broad-complex tachycardia -> do NOT give adenosine (pre-excited AF)
Adenosine (or dipyridamole/regadenoson) stress perfusion imaging - coronary vasodilation reveals flow-limiting stenoses; aminophylline reverses it
Have defibrillation pads on, IV access secure, and a printed rhythm strip
Theophylline, caffeine and dipyridamole all change the dose you need

Vaughan Williams antiarrhythmic classification (adenosine is unclassified/"class V")
Digoxin mechanism - both slow AV nodal conduction, both dangerous in pre-excited AF
Beta-blocker comparative pharmacology; calcium channel blockers
Anti-anginal drug classes; myocardial perfusion imaging
Asthma pharmacotherapy; theophylline pharmacology
Cardiac transplantation - denervation supersensitivity

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