Adenosine

Some of the views expressed in the following notes on newly approved products should be regarded as preliminary, as there may have been limited published data at the time of publication, and little experience in Australia of their safety or efficacy. However, the Editorial Executive Committee believes that comments made in good faith at an early stage may still be of value. Before new drugs are prescribed, the Committee believes it is important that more detailed information is obtained from the manufacturer's approved product information, a drug information centre or some other appropriate source.

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Adenocor (Sanofi Winthrop)
3 mg/mL in 2 mL vials
Indication: supraventricular tachycardia

Adenosine is a nucleoside with an important role in metabolism. It has several effects on the heart including depression of conduction at the atrioventricular node, reduced automaticity of the sinoatrial node and decreased atrial contractility.

Adenosine has been approved for both diagnostic and therapeutic indications, which overlap somewhat. In patients with classical paroxysmal supraventricular tachycardia (SVT), the ECG usually shows normal narrow QRS complexes. Most of these cases are due to a reentry circuit which involves antegrade conduction through the AV node and retrograde conduction through an accessory pathway (e.g. classical Wolff Parkinson White syndrome). Measures which slow conduction through the AV node, e.g. carotid sinus pressure or Valsalva manoeuvre, often terminate the tachycardia. Intravenous verapamil works in this way and is often effective. Adenosine is equally effective for paroxysmal SVT¹ but has some advantages over verapamil. The advantages are that adenosine is much shorter acting (minutes) and can be safely given to patients with heart failure or to patients taking beta blockers.

There are two situations where adenosine may be a useful diagnostic test. Firstly, some patients with narrow complex tachycardia around 150 beats per minute have atrial flutter with 2:1 block rather than SVT. In these patients, adenosine will transiently slow the ventricular rate, revealing flutter waves on the ECG, thus confirming the diagnosis.

Secondly, some patients will have wide QRS complexes, so called wide complex tachycardia, for which the cause may not be clear. If this is due to SVT with aberrant conduction, adenosine will usually terminate the tachycardia. By contrast, if the problem is ventricular tachycardia, the patient will usually have serious underlying heart disease, and will often be hypotensive and very unwell. These patients should never be given intravenous verapamil. Intravenous adenosine will have no effect on the arrhythmia, but will generally not harm the patient. As a diagnostic test in wide complex tachycardias, adenosine has a sensitivity, specificity and predictive value of approximately 90%.²

The most common adverse effects include chest tightness, dyspnoea, bronchospasm and facial flushing. Bradycardia is a serious adverse event and the effects of adenosine are not blocked by atropine. Adenosine should only be used where cardiac monitoring and resuscitation equipment are immediately available.