`Coronory steal phenomenon' is caused by ?

Ans. is 'a' i.e., Dipyridamole o There are two types of vessels in coronary circulaton ? 1. Larger conducting aeries which run epicardially and send perforating branches. 2. Smaller resistance aerioles - These are perforating branches of conducting aeries. These aerioles supply blood to endocardium. o During ischemia, adenosine (a local mediator) causes dilatation of resistance aerioles in ischemic area only, tone of resistance vessels of nonischemic area does not change --> increased flow to ischemic area. o Dipyridamole is a powerful coronary dilator; increases total coronary flow by preventing uptake and degradation of adenosine. o It dilates resistance vessels in nonischemic zone as well ----> Dives the already reduced blood flow away from ischemic zone - Coronary steal phenomenon. o It has no effect on larger conducting vessels (contrast nitrates which dilate larger conducting vessels and cause redistribution of blood towards ischemic zone). o Dipyridamole inhibits platelet aggregation by increasing cAMP (by inhibiting phosphodiesterase) which potentiates PGI, and interferes with aggregation. o It is not useful as an antianginal drug (due to coronary steal phenomenon), but can be used for prophylaxis of coronary and cerebral thrombosis in post MI and post stroke patients as well as to prevent thrombosis in patients with prosthetic hea valve.