Pathophysiology of coronary steel phenomenon

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"coronary steal" pathophysiology

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PMID: 37264699

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Coronary Steal Phenomenon - Pathophysiology

Definition and Historical Context

The term "coronary steal" was introduced in 1967, modeled after the earlier "subclavian steal" concept (coined in 1961). Originally it described literal blood flow reversal in an abnormal vascular network (e.g., a coronary fistula), but the term has since expanded to encompass any scenario where regional coronary perfusion is reduced by diversion of flow to another territory - even without true reversal.

Achim et al., Eur Heart J 2023 identify six distinct steal scenarios, framing it as a broader pathophysiological category.

Core Pathophysiological Mechanism

The Pressure-Flow Substrate

Normal coronary physiology relies on autoregulation: resistance vessels (arterioles) in well-perfused myocardium can actively dilate or constrict to maintain constant flow across a range of perfusion pressures. In contrast, arterioles distal to a significant coronary stenosis operate under ischemic conditions and are already maximally dilated due to local metabolic factors (adenosine, hypoxia, CO₂ accumulation, low pH). They have no further vasodilatory reserve.

"In the absence of drugs, arterioles in ischemic areas of the myocardium are usually maximally dilated as a result of local control factors, whereas the resistance vessels in well-perfused regions are capable of further dilation in response to exercise."

Katzung's Basic and Clinical Pharmacology, 16th Ed.

The Steal Event

When a potent arteriolar vasodilator is administered:

Normal zones - Resistance vessels can dilate further, dramatically dropping arteriolar resistance and increasing flow.
Ischemic zones - Resistance vessels are already maximally dilated; no additional dilation occurs.
Hemodynamic consequence - Blood preferentially flows along the path of least resistance (normal zones), diverting flow away from the ischemic territory.

This creates a paradox: a vasodilator worsens ischemia in the most vulnerable myocardium.

The driving factor is the pressure differential. Because flow through any vascular bed is proportional to the pressure gradient divided by resistance (Q = ΔP/R), reducing resistance in normal zones amplifies their flow at the expense of fixed-resistance ischemic zones. Systemic hypotension caused by the vasodilator can additionally lower the pressure head driving collateral flow.

Anatomical Prerequisites

Two structural configurations make steal possible:

1. Collateral-Dependent Ischemic Zone

A coronary territory supplied only via collateral channels from an adjacent normal territory. When the donor territory vasodilates:

Pressure in the collateral feeding vessels drops.
Flow through collaterals to the ischemic zone falls or even reverses.

This was the classic model debated with isoflurane anesthesia in the 1980s - the drug dilates normal coronary arteries, potentially starving collateral-dependent zones.

2. Coronary Fistula or Anomalous Connection

A direct shunt that preferentially accepts vasodilated flow, bypassing the myocardial capillary bed entirely.

Example - Anomalous Left Coronary Artery from the Pulmonary Artery (ALCAPA): The left coronary artery originates from the low-pressure pulmonary artery. Once pulmonary vascular resistance falls postnatally, blood in the left coronary system reverses direction - flowing away from the myocardium and into the pulmonary artery. This is true steal with anatomical flow reversal. It is among the most common causes of myocardial infarction in infancy, with 90% mortality if untreated within the first year.

Tintinalli's Emergency Medicine, p. 3456

Example - Coronary artery fistula: The fistulous communication creates a low-resistance pathway that diverts blood away from the capillary bed. Angina results from relative ischemia despite normal epicardial arteries.

Goldman-Cecil Medicine, p. 1191

Pharmacological Triggers

Dipyridamole

Inhibits adenosine uptake (and phosphodiesterase), causing profound arteriolar dilation via elevated extracellular adenosine. Because it acts non-selectively, it exposes the steal mechanism. It is contraindicated in unstable angina for this reason - transient steal can precipitate MI. This effect is exploited diagnostically in myocardial perfusion stress testing (steal unmasks areas of fixed stenosis as perfusion defects).

Adenosine / Regadenoson

Act on A2A receptors on coronary resistance vessels, causing brief but intense vasodilation. Both carry FDA-label warnings for coronary steal. Regadenoson (selective A2A agonist) is similarly used for pharmacologic stress imaging.

"In patients with unstable angina, transient coronary steal may precipitate a myocardial infarction."

Katzung's Basic and Clinical Pharmacology, 16th Ed.

Lippincott Illustrated Reviews Pharmacology, 16th Ed. (dipyridamole section)

Sodium Nitroprusside

Releases nitric oxide, causing significant intra-myocardial vasodilation. This can produce a steal phenomenon, which is why nitroprusside is not recommended in active myocardial ischemia.

Braunwald's Heart Disease, p. 143

Isoflurane (Volatile Anesthetic)

A brief controversy arose in the late 1980s regarding isoflurane's potent coronary vasodilatory effects potentially inducing steal in patients with coronary disease and collaterals. In clinical practice, this has proved to be, at most, a rare occurrence.

Barash's Clinical Anesthesia, 9th Ed.; Miller's Anesthesia, 10th Ed.

Mechanism Summary: Subendocardial Vulnerability

The subendocardium is the most vulnerable region to steal because:

It has the highest oxygen demand (greatest wall stress).
It receives coronary flow only during diastole (systolic compression obliterates flow).
Collateral supply to ischemic subendocardium is particularly pressure-sensitive.

Any drop in perfusion pressure in a collateral-feeding vessel immediately reduces subendocardial flow more than epicardial flow.

Key Points

Feature	Explanation
Pre-condition	Pre-existing coronary stenosis with collateral-dependent territory, OR coronary fistula
Trigger	Non-selective arteriolar vasodilation
Mechanism	Vasodilation of non-stenotic vessels → preferential flow to normal zones → reduced perfusion pressure in collaterals → ischemia worsened
Classic drugs	Dipyridamole, adenosine, regadenoson, nitroprusside, isoflurane
Diagnostic use	Dipyridamole/adenosine stress testing exploits this mechanism
Structural examples	ALCAPA (true flow reversal), coronary artery fistula
Clinical risk	Unstable angina: steal can precipitate MI

Recent literature note: A 2023 review in the European Heart Journal (Achim et al., PMID 37264699) provides the most current framework, distinguishing six pathophysiologically distinct "steal" scenarios - ranging from true flow reversal (fistulae, ALCAPA) to relative steal without reversal (pharmacologic, collateral-dependent ischemia) - reinforcing that "coronary steal" is a spectrum rather than a single entity.

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A detailed medical flowchart titled "Pathophysiology of Coronary Steal Phenomenon" with clean white background, professional medical style, using boxes and arrows. The flowchart should show: Top box: "Pre-existing Coronary Stenosis + Collateral-Dependent Ischemic Zone OR Coronary Fistula / Anomalous Connection" Arrow down to: "Trigger: Non-Selective Arteriolar Vasodilator" with a side note box listing: "Dipyridamole | Adenosine / Regadenoson | Nitroprusside | Isoflurane" Then splits into TWO parallel branches with arrows: LEFT branch (green/normal zone): Box 1: "Normal Myocardium - Resistance vessels have FULL vasodilatory reserve" Arrow down: "Arterioles DILATE further" Arrow down: "Vascular resistance DROPS markedly" Arrow down: "Blood flow INCREASES in normal zone" RIGHT branch (red/ischemic zone): Box 1: "Ischemic Myocardium - Arterioles ALREADY maximally dilated (local adenosine, hypoxia, acidosis)" Arrow down: "NO further dilation possible" Arrow down: "Perfusion pressure in collaterals FALLS" Arrow down: "Blood flow DIVERTED away from ischemic zone" Both branches converge with arrow to bottom center box: "CORONARY STEAL: Blood preferentially flows to normal zone (path of least resistance) — ischemic zone is robbed" Arrow down to: "Worsening Subendocardial Ischemia" (subendocardium most vulnerable: highest O2 demand, systolic compression, pressure-sensitive collaterals) Arrow down to final red box: "Angina / ST changes / Myocardial Infarction (in unstable angina)" Add a small separate box on the side showing Structural Examples: "ALCAPA: True flow reversal (LCA from pulmonary artery) | Coronary Artery Fistula: Low-resistance shunt bypasses capillary bed" Use color coding: blue for normal pathway, red/orange for ischemic pathway, yellow for triggers, red for outcome. Clean sans-serif font, medical textbook quality.

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