Adenosine is a purine analogue. Acts by stimulating outward K+ channel of AV nodal tissue, more specifically in the posteriorly located slow pathway in the vicinity of coronary sinus.
Another action of adenosine is inhibition of cAMP , which is similar to beta blocking action may also help in terminating the tachycardia.
Adenosine : A 10 second cardiac miracle
- 12mg bolus is administered , preferably in a central vein (Not mandatory though)
- Termination is usually abrupt . Transient VPDs are observed during termination.
- Transient flushing may occur.
- If the patient is taking Aminophylline group of drugs (Which are adenosine antagonists) the AV nodal blocking action may be neutralised .
(It may be apt to recall at this juncture , Aminophylline is used in sinus node dysfunction or AV block to increase heart rate )
Reference
A good one from Medscape http://www.medscape.com/viewarticle/585287_2
Dr.S.Venkatesan MD 
A very clear Graph of AVNRT.