Adenosine

Common indications

• As a first-line diagnostic and therapeutic agent in supraventricular tachycardia (SVT).

 

Mechanisms of action

• Adenosine is an agonist of adenosine receptors on cell surfaces. In the heart, activation of these G protein-coupled receptors induces a number of effects, including reducing the frequency of spontaneous depolarisations (automaticity) and increasing resistance to depolarisation (refractoriness). In turn, this transiently slows the sinus rate, conduction velocity, and increases atrioventricular (AV) node refractoriness. Many forms of SVT arise from a self-perpetuating electrical (re-entry) circuit that takes in the AV node. Increasing refractoriness in the AV node breaks the re-entry circuit, which allows normal depolarisations from the sinoatrial (SA) node to resume control of heart rate (cardioversion). Where the circuit does not involve the AV node (e.g. in atrial flutter), adenosine will not induce cardioversion. 
• However, by blocking conduction to the ventricles, it allows closer inspection of the atrial rhythm on the ECG. This may reveal the diagnosis. 
• The duration of effect of adenosine is very short because it is rapidly taken up by cells (e.g. red cells). Its half-life in plasma is less than 10 seconds.

 

Important adverse effects

• By interfering with the functions of the SA and AV nodes, adenosine can induce bradycardia and even asystole. 
• This is accompanied by an unpleasant sensation for the patient: a sinking feeling in the chest, often accompanied by breathlessness and a sense of ‘impending doom.’ Due to the drug’s short-lived effect, this feeling is only brief.

 

Warnings

• You should not administer adenosine to a patient who will not tolerate its transient bradycardic effects, including those with hypotension, coronary ischaemia, or decompensated heart failure. 
• Adenosine may induce bronchospasm in susceptible individuals, so should be avoided in patients with asthma or COPD. 
• Patients who have had a heart transplant are very sensitive to the effects of adenosine.

 

Important interactions

• Dipyridamole blocks cellular uptake of adenosine, which prolongs and potentiates its effect: the dose of adenosine should be halved. 
• Theophylline, aminophylline and caffeine are competitive antagonists of adenosine receptors and reduce its effect. 
• Patients who have taken these drugs respond poorly and may require higher doses.