ADENOSINE

This endogenous nucleoside is safe and effective in ending > 90% of re-entrant paroxysmal SVT. However, this is not the most common type of SVT in the critically ill patient.After an IV bolus effects are immediate (10–30 seconds), dose-related and transient (half-life < 10s; entirely eliminated from plasma in < 1 minute, being degraded by vascular endothelium and erythrocytes). Its elimination is not affected by renal/hepatic disease. Adenosine works faster and is superior to verapamil. It may be used in cardiac failure, in hypotension and with beta -blockers, in all of which verapamil is contraindicated.

USES

It has both therapeutic and diagnostic uses:

• Alternative to DC cardioversion in terminating paroxysmal SVT, including those associated with WPW syndrome

• Determining the origin of broad complex tachycardia; SVT responds, VT does not (predictive accuracy 92%; partly because VT may occasionally respond).Though adenosine does no harm in VT, verapamil may produce hypotension or cardiac arrest

CONTRAINDICATIONS

• Second- or third-degree heart block (unless pacemaker fitted)

• Sick sinus syndrome (unless pacemaker fitted)

• Asthmatic – may cause bronchospasm

• Patients on dipyridamole (drastically prolongs the half-life and enhances the effects of adenosine – may lead to dangerously prolonged highdegree AV block)

ADMINISTRATION

• Rapid IV bolus: 3mg over 1–2 seconds into a large vein, followed by rapid flushing with sodium chloride 0.9%

If no effect within 2 min, give 6mg

If no effect within 2 min, give 12mg

If no effect, abandon adenosine

Need continuous ECG monitoring

More effective given via a central vein or into right atrium

How not to use adenosine

• Without continuous ECG monitor

ADVERSE EFFECTS

• Flushing (18%), dyspnoea (12%) and chest discomfort are the commonest side-effects but are well tolerated and invariably last < 1min.

• If given to an asthmatic and bronchospasm occurs, this may last up to 30 min (use aminophylline to reverse).

CAUTIONS

• AF or atrial flutter with accessory pathway (conduction down anomalous pathway may increase)

• Early relapse of paroxysmal SVT is more common than with verapamil but usually responds to further doses

• Adenosine’s effect is enhanced and extended by dipyridamole – if essential to give with dipyridamole, reduce initial dose to 0.5–1mg