Having injected this stuff into people on quite a few occasions, it makes you feel weird and a bit faint for a second or two, often with a feeling of chest tightness and flushing, and then you feel OK again. It slows down electrical transmission between the atria and the ventricles for a few seconds and can stop the heart transiently. It is generally a very safe drug.
I am presuming that in your case you are due to have electrophysiological studies on your heart, including the injection of adenosine, because otherwise it is used only in the acute treatment of supraventricular tachycardia.
QDM