Coronary Autoregulation: Adenosine, Diastole & Steal

Coronary autoregulation is the heart's ability to keep its own blood flow steady across a wide range of perfusion pressures (about 60-200 mm Hg in CV Physiology's figures) by changing the tone of small resistance arterioles, while matching flow to oxygen demand. Because the myocardium already extracts most of the oxygen delivered to it at rest (about 60-70% in StatPearls' figures), extra demand can only be met by more flow, which local metabolic vasodilators provide: adenosine (the classic exam answer), NO, CO2, H+, K+ and hypoxia. Systole squeezes the intramyocardial vessels, so left ventricular flow happens mostly in diastole; tachycardia therefore cuts supply just as it raises demand, and the subendocardium is hit first. A stenosis is compensated by dilating the arterioles beyond it, which spends coronary flow reserve: exertional angina appears long before resting flow falls, and vasodilator stress agents (adenosine, regadenoson, dipyridamole) unmask it by raising flow mainly in normal beds (coronary steal).

Supply vs demand: why flow, not extraction

The heart has a very high basal oxygen consumption (8-10 mL O2/min/100 g) and the highest arteriovenous O2 difference of any major organ (10-13 mL/100 mL). With extraction already near its ceiling, coronary flow rises almost in proportion to oxygen consumption (active hyperemia). Merck lists the three main determinants of demand: heart rate, systolic wall tension and contractility.

What sets coronary tone

Memory hook: a HAPPI heart dilates. H+, Adenosine, low PO2, high PCO2, Increased K+, plus NO from the endothelium.

Lore vs lab: exam answers name adenosine, but StatPearls notes that its role in physiologic coronary dilation has not been conclusively demonstrated, and that NO is not necessary for it either: the signals are redundant. On test day, still pick adenosine.

The autoregulatory plateau and its floor

Plot flow against perfusion pressure and you get a flat plateau: as pressure falls, arterioles dilate and resistance drops, so flow holds. At the floor of the range (about 60 mm Hg for the coronaries in CV Physiology's figures), the vessels are maximally dilated and flow then falls passively with pressure.

• Hypotension and shock make coronary flow pressure-dependent; the endocardium suffers first.

Diastole: why heart rate counts twice

During systole the contracting LV compresses its intramural vessels, most strongly in the endocardial layer and least in the epicardium, so most left coronary flow occurs in diastole. Tachycardia shortens the diastolic time available for perfusion while raising demand, which matters most in coronary disease. A beta-blocker slows the heart, lengthening diastole while cutting demand. Map the phases on a Wiggers diagram.

Coronary flow reserve: the dilation left in the tank

Coronary flow reserve is the bed's maximal flow capacity relative to resting flow: the vasodilation still available. A stenosis lowers pressure beyond it; the distal arterioles dilate to keep resting flow normal, which spends reserve. Reserve is lost first, so symptoms start with exertion. Sources draw these lines differently: read the table as approximate landmarks from three references, not one agreed cut-off.

In the cath lab, fractional flow reserve (FFR), measured with a wire during maximal hyperemia (most often with adenosine), is in Merck's words the ratio of maximal flow through the stenotic area to normal maximal flow (1.0 = no limitation). Merck considers FFR <0.75-0.8 abnormal; lesions with FFR >0.8 do not seem to benefit from a stent.

Why the subendocardium goes first

• Greatest systolic compression, so it is perfused almost entirely in diastole.
• Most susceptible at low perfusion pressure (shock, a tight stenosis).
• High LV diastolic pressure squeezes it further; lowering preload, as nitrates do, improves subendocardial flow.
• Result: demand ischemia is typically subendocardial and shows as ST depression (ST depression and T-wave inversion); an occluded artery or spasm causes transmural ischemia with ST elevation.

Coronary steal and pharmacologic stress tests

Beyond a significant stenosis the arterioles have already dilated to hold resting flow (maximally, if the narrowing is severe). A drug that dilates arterioles everywhere therefore raises flow mainly in the normal beds. If both beds share an upstream narrowing (CV Physiology's example: left main plus LAD lesions), the extra flow through it lowers pressure downstream, and LAD flow can actually fall while circumflex flow rises. Either way, the diseased territory gets relatively less flow: a perfusion defect. In Merck's words, dipyridamole "increases myocardial blood flow in normal coronary arteries but not in arteries distal to a stenosis."

• Methylxanthines (caffeine, theophylline, aminophylline) block adenosine receptors: Merck says to avoid caffeine and theophylline for 24 hours before a vasodilator test (the regadenoson label: at least 12 hours) to prevent false negatives; IV aminophylline reverses severe or persistent side effects.
• Exercise is preferred; drugs are for patients who cannot reach the target heart rate (deconditioning, musculoskeletal problems, obesity, peripheral arterial disease).
• Nitrates rarely steal: nitroglycerin dilates mainly veins and, in the coronaries, the large epicardial arteries rather than resistance arterioles, so steal is unlikely; dipyridamole provokes it.

Clinical links

• Stable angina: a fixed stenosis with reduced reserve; exertion raises demand that flow cannot follow, and rest or nitroglycerin relieves it (stable vs unstable angina).
• Nitrates: venodilation cuts preload and wall stress (less demand); epicardial dilation and reversal of spasm add supply. Never combine with PDE-5 inhibitors (sildenafil and others): together they potentiate cGMP and can cause severe hypotension. Constant levels cause tolerance, so schedule a nitrate-free interval (Merck: 8-10 hours for nitroglycerin, 12 hours for isosorbide dinitrate).
• Beta-blockers: Merck advises against beta-blockers other than labetalol and carvedilol in vasospastic angina because unopposed alpha activity may provoke spasm.
• Angina without CAD: hypertension, aortic stenosis or regurgitation and hypertrophic cardiomyopathy raise workload; severe anemia or hypoxia lowers supply.
• Demand ischemia and secondary MI (formerly type 2): tachyarrhythmia, hypotension, hypoxia, anemia, sepsis or bleeding can outstrip a limited supply. Under the 2026 Fifth Universal Definition, secondary MI is considered when such a trigger causes a troponin rise with ischemic symptoms or ECG changes, and confirmed by obstructive CAD without an acute coronary lesion or by imaging showing a new regional wall motion abnormality or loss of viable myocardium in an ischemic pattern. Most such patients have neither, and secondary MI can then be excluded: the troponin rise is acute myocardial injury. Treat the trigger (STEMI vs NSTEMI management).
• Vasospasm: transient ST elevation at rest, treated with calcium channel blockers and nitrates; an MI caused by spasm now counts as primary MI (Prinzmetal angina).

How USMLE tests it

Practice question

A 66-year-old man with diabetes has 3 months of chest pressure when he climbs stairs, relieved by rest. Severe knee osteoarthritis keeps him off the treadmill, so he has regadenoson myocardial perfusion imaging. Tracer uptake in the anterior wall is reduced at stress and normal at rest. Angiography later shows a 75% proximal left anterior descending (LAD) stenosis. Which best explains the stress perfusion defect?

A. Arterioles beyond the stenosis had already dilated to keep resting flow normal, so regadenoson increased flow mainly in normal territories B. Regadenoson stimulated β1 receptors, raising heart rate and contractility beyond what the LAD could supply C. Regadenoson constricted the LAD beyond the stenosis D. Oxygen extraction in the anterior wall fell during stress E. Coffee that morning blocked regadenoson's effect only in the LAD territory

Answer: A. This is the coronary steal principle. To keep resting flow normal, the LAD territory has already spent much of its dilator reserve; regadenoson, an A2A agonist, dilates the normal beds, whose flow rises far more than flow beyond the stenosis, so the LAD bed takes up relatively less tracer at stress. At rest, autoregulation keeps LAD flow normal, so the defect fills in: reversible ischemia, not scar.

• B: that describes dobutamine, a demand stressor used when vasodilators are contraindicated.
• C: regadenoson dilates coronary resistance vessels; it doesn't constrict them.
• D: reduced flow makes the myocardium extract more oxygen, not less, and perfusion tracers map flow.
• E: methylxanthines block adenosine receptors in every territory, blunting hyperemia everywhere and risking a false-negative scan; they don't create a regional defect.

Key takeaways

• Myocardial O2 extraction is near-maximal at rest (highest A-VO2 difference of any major organ), so more demand = more flow.
• Metabolic control dominates: adenosine (exam answer), NO, CO2, H+, K+, hypoxia; sympathetic drive raises flow through metabolites despite α1 constriction.
• Autoregulation holds flow steady across a wide pressure range (about 60-200 mm Hg, CV Physiology); below its floor, flow follows pressure.
• LV flow is mostly diastolic: tachycardia cuts supply and raises demand; beta-blockers do the reverse.
• A stenosis spends flow reserve first (exertional angina); resting flow falls only with severe narrowing (about 80% in StatPearls).
• The subendocardium is most vulnerable: demand ischemia means ST depression.
• Coronary steal explains vasodilator stress defects; nitrates dilate epicardial vessels and rarely steal.
• A troponin rise with ischemia during tachycardia, hypotension, hypoxia or anemia raises secondary MI (formerly type 2): confirmed by obstructive CAD without an acute lesion or by imaging; treat the trigger.

Sources

• Coronary Anatomy and Blood Flow — CV Physiology, Richard E. Klabunde (2023)
• Metabolic Mechanisms of Vasodilation — CV Physiology (2022)
• Myogenic Mechanisms Regulating Blood Flow — CV Physiology (2023)
• Vascular Steal — CV Physiology (2022)
• Critical Stenosis — CV Physiology (2023)
• Physiology, Coronary Circulation — StatPearls, NCBI Bookshelf (2023)
• Angina Pectoris — Merck Manual Professional Edition (2026)
• Cardiac Stress Testing — Merck Manual Professional Edition (2026)
• Cardiac Catheterization — Merck Manual Professional Edition (2026)
• Lexiscan (regadenoson) injection: prescribing information — DailyMed, NLM (2018)
• Nitrodilators and Soluble Guanylyl Cyclase Activators — CV Pharmacology (2024)
• Fifth Universal Definition of Myocardial Infarction (2026) — Global Heart (2026)