Expansion
Dilating already maximally dilated vessels diverts flow away from ischaemic territory
Expansion
Distal to a significant stenosis, arterioles are already maximally dilated by local metabolites in order to maintain resting flow; their coronary reserve is exhausted.
Giving a systemic vasodilator therefore dilates only the healthy vessels, which still have reserve. Resistance falls preferentially in normal territory, and because flow follows the path of least resistance, blood is diverted away from the ischaemic region.
Consequences:
- Pharmacological stress testing with adenosine, dipyridamole or regadenoson exploits this deliberately, creating a perfusion defect that reveals the stenosed territory on imaging without the patient exercising
- Vasodilators may occasionally provoke angina
- A similar mechanism operates in subclavian steal, where a proximal subclavian stenosis causes retrograde vertebral flow during arm exercise, producing brainstem symptoms
The concept underlines a general principle: in a bed with exhausted autoregulatory reserve, flow becomes pressure dependent and vulnerable to redistribution.