Caffeine (1,3,7- trimethylxanthine) blocks adenosine receptors via its properties as a competitive antagnoist of adenosine. This has flow on diffuse physiological effects, given the abundance of four distinct adenosine receptor subtypes throughout the body. The normal physiological effects of adenosine are on the cental nervous system, where adenosine would normally have a sedative effect, and on cardiac function where adenosine would normally slow down the heart rate - with caffeine antagonising this activity.
Caffeine is therefore contraindicated prior to particular cardiac imaging such as coronary CT angiography and myocardial perfusion scintigraphy.
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