Coronary Steal and its Clinical Relevance

Coronary steal is a brilliant concept to break down visually, as it perfectly illustrates the limits of microvascular autoregulation and explains the seemingly paradoxical effects of certain cardiac medications. Here is a breakdown of the pathophysiology and its direct clinical applications.

The Core Mechanism: The Path of Least Resistance

Normally, the coronary microvasculature constantly adjusts its resistance to maintain stable blood flow over a wide range of perfusion pressures (autoregulation).

However, when a patient has a severe, flow-limiting epicardial stenosis, the arterioles distal to the blockage must fully and permanently dilate just to maintain adequate resting blood flow. In this ischemic territory, the coronary flow reserve (CFR) is exhausted—the vessels cannot dilate any further.

If a potent systemic vasodilator is introduced:

1. Arterioles in healthy territories (which still have their CFR intact) dilate massively.
2. Arteriolar resistance in these healthy zones drops significantly.
3. Because the vessels in the diseased territory are already maxed out, they cannot match this drop in resistance.
4. Blood takes the path of least resistance, diverting away from the ischemic zone and into the healthy tissue.

The ischemic myocardium is physically “robbed” of its blood supply, precipitating angina or ischemic ECG changes without any actual increase in myocardial oxygen demand.

The Clinical Relevance

The steal phenomenon isn’t just a physiological curiosity; it forms the foundation of modern myocardial perfusion imaging and dictates our pharmacotherapy choices for angina.

1. Pharmacological Stress Testing (MPI, PET, CMR)

This is the most common intentional application of coronary steal. When patients cannot exercise, we use potent microvascular vasodilators like adenosine, regadenoson, or dipyridamole.

Unlike dobutamine, which induces true demand ischemia by driving up heart rate and contractility, these agents trigger a coronary steal. By inducing maximal hyperemia in the healthy vessels, they create a stark relative perfusion defect in the territory supplied by the stenotic artery, which is then captured by the imaging tracer.

2. Why We Don’t Use Certain Vasodilators for Angina

Understanding steal explains why not all vasodilators relieve angina—and why some actually worsen it.

Drug Class	Target Vessel	Effect on Ischemia	Mechanism
Dipyridamole / Hydralazine	Small resistance vessels (arterioles)	Worsens	Induces microvascular steal, diverting flow away from post-stenotic zones.
Nitrates	Large epicardial vessels & venous capacitance	Relieves	Dilates collateral channels and large arteries; reduces preload (LVEDP), lowering wall tension without causing microvascular steal.

3. Collateral Circulation Steal

In patients with chronic total occlusions (CTOs), the myocardium often survives entirely on collateral flow from a donor artery. If the donor artery itself develops proximal disease, vasodilation causes a pressure drop distal to the new stenosis. This pressure drop at the origin of the collateral network reduces the driving force across the collaterals, effectively “stealing” blood away from the CTO territory.

4. Coronary-Subclavian Steal Syndrome

A macroscopic, structural variation of the steal phenomenon occurs in patients who have had a CABG using the Left Internal Mammary Artery (LIMA) to the LAD.

If the patient develops a proximal left subclavian artery stenosis, the pressure in the distal subclavian drops. When the patient exercises their left arm, the arm vessels dilate, and blood flow is diverted to the arm reducing flow to the LIMA, ‘stealing’ blood flow away from the LAD territory to the arm. The patient presents with classic angina triggered purely by upper extremity exertion.