Adenosine is a natural vasodilator. Endogene Adenosine is syntesized (from ATP) and metabolized in myocytes and vascular smooth cells.
Adenosine plays an important role in the natural regulation of the coronary flow (vasodilation) and cardiac demand (lowering BP).

Endogene and exogene Adenosine stimulates the A2 purine receptor DIRECTLY leading to vascular smooth muskle cell relaxation and therefore coronary vasodilation (increased flow) and systemic vasodilation (decrease in BP = demand).
Adenosine also stimulates the A1 purine receptors in the SA and AV node leading to slowing of HR and AV conduction delay.

The Adenosine receptors are block by CAFFEINE and THEOPHILLINE. Adenosine stress testing is unrelible if there is caffeine (coffe, thee, coke) on board (i.e. consumed within the last 12 h).
The effects (or side effects) of Adenosine can be blocked by theophylline.

During IV infusion of Adenosine vasidilation has been noted after 20-30 sec. Max effect has been observed invasively after about 60 sec, and continues as long as the drug infused.
T1/2 in plasma is in the order of 15 sec. The effect of Adenosine therefore disappear quickly after discontinuation of infusion.

The 2 plus 2 Adenosine Protocol is very convenient, effective, and well tolerated. The radioactive tracer is injected after 2-min's infusion, and the infusion continues for 2 more min to clear the tracer from the blood.

Adenosine increased blood flow (up to 2-3 times baseline) in coronaries where the flow CAN increase (normal or non-significant CAD), while the flow increases little or not at all in arteries with significant CAD. Adenosine thus creates unevennes in flow between coronary artery territories -- which is reflected in uneven tracer uptake.
Myocardial perfusion imaging with this protocol thus illustrate regional relative hypoperfusion, not real regional ischemia.

The most common side effects is shortness of breath and mild non-specific chest pressure.
Occasionally (2-5%) significant AV block occurs. Discontinuation of the infusion usually quickly resolves the problem, rarely is it necessery to administer an antidote: Theophylline.
Due to this potentially serious side effect continuous ECG monitoring is obligatory, and one doesn't turn ones back to the patient and the monitor th 4-min infusion.

Chest pain and ECG changes of ischemic nature happens and is usually a sign of "coronary steal". This phenomenon is attributed to situation where an area south of significant stenosis (where the flow can not increse) is supplied by collateral's from a coronary without stenosis. When the flow in the latter increases during Adenosine infusion and the pressure decrease, flow in the collateral'smay reverse, blood is being "sucked" over fro the stenotic vessel leading to real hypoperfusion and ischemia in the stenotic artery.

The Adenosine protocol is particularly well suited for patients with LBBB.
The false positive rate with this protocol is 2-5% compared 30-40% for treadmill testing.

Ongoing treatment with beta -blockers does not effect the efficiency of Adenosine. Pharmacological vasodilation is the protocol of choice for patient on beta-blockers.

Adenosine is the protocol of choice in patient with significant arrhythmia and in patient with psychiatric history.
Adenosine is safe for stress testing shortly after acute MI.