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Grand Case Presentation: Coronary Circulation & Structure of Cardiac Valves

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PART I: CORONARY CIRCULATION

1. Overview and Clinical Significance

• Guyton and Hall Textbook of Medical Physiology, p. 268

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2. Anatomical Arrangement of the Coronary Blood Supply

• Supplies the anterior and left lateral portions of the left ventricle
• Divides into:
  • Left Anterior Descending (LAD) - supplies anterior LV wall and anterior septum
  • Left Circumflex (LCx) - supplies lateral and posterior LV

• Supplies most of the right ventricle
• Supplies the posterior part of the left ventricle in 80-90% of people (right dominant circulation)

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3. Coronary Venous Drainage

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4. Normal Coronary Blood Flow

• Resting coronary flow: ~70 mL/min/100 g of heart weight (~225 mL/min total)
• This represents 4-5% of total cardiac output
• During strenuous exercise, coronary flow increases 3-4 fold (cardiac work can increase 6-9 fold)

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5. Phasic Flow: The Systole-Diastole Paradox

• During systole: Ventricular wall contraction compresses intramuscular coronary vessels - LV capillary flow falls dramatically (nearly to zero at peak systole)
• During diastole: Muscle relaxes, obstruction removed - blood flows rapidly. The left coronary is predominantly diastolic-dependent
• RV coronary flow is less affected because RV wall tension is lower
• After the systolic compression is relieved, reactive hyperemia occurs to repay the O2 debt

Key teaching point: The left coronary artery fills predominantly in diastole. This is why tachycardia (shorter diastole) and aortic regurgitation (reduced diastolic perfusion pressure) impair coronary flow.

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6. Epicardial vs Subendocardial Blood Flow

• Epicardial arteries: Large surface vessels, act as conduits
• Intramuscular arteries: Penetrate the myocardium; subject to compression during systole
• Subendocardial arterial plexus: Lies immediately beneath the endocardium; most vulnerable during systole and in states of elevated end-diastolic pressure
• The subendocardial plexus has extra vessels to compensate for systolic compression - but this reserve is exhausted in ischemia (subendocardium is first to infarct)
• Guyton and Hall Textbook of Medical Physiology, p. 269

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7. Control of Coronary Blood Flow

A. Local Metabolic Control (Primary Mechanism)

• ~70% of O2 in coronary arterial blood is extracted at rest (nearly maximal extraction)
• When O2 demand increases, there is essentially no reserve to extract more O2 - the only response is to increase blood flow
• The key vasodilator signal is hypoxia itself and adenosine

1. Increased metabolic rate → decreased O2 in cells
2. ATP degrades → AMP → adenosine released into tissue fluid
3. Adenosine causes arteriolar dilation → increased coronary flow → O2 delivery matches demand
4. After vasodilation, adenosine is reabsorbed and used to regenerate ATP

Costanzo Physiology (p. 996): "Blood flow through the coronary circulation is controlled almost entirely by local metabolites, with sympathetic innervation playing only a minor role. The most important local metabolic factors are hypoxia and adenosine."

B. Nervous Control (Secondary, Largely Overridden by Metabolic)

• Epicardial vessels: Predominance of α1 (constricting) receptors
• Intramuscular arteries: More β2 (dilating) receptors

The indirect metabolic effects usually override the direct neural effects.

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8. Cardiac Muscle Metabolism and Ischemia

• At rest, ~70% of cardiac energy comes from fatty acid oxidation (not glucose)
• Under ischemia/anaerobiasis: switches to anaerobic glycolysis - consumes glucose rapidly and produces lactic acid (source of ischemic cardiac pain)
• ATP degrades to ADP → AMP → adenosine during severe ischemia
• If severe ischemia persists >30 minutes, ~half the adenine base is lost from cells
• New adenine is synthesized at only 2%/hour - explaining why reperfusion after 30+ min may be too late to salvage cells

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9. Ischemic Heart Disease - Overview

• Most common cause of death in Western countries
• ~35% of Americans aged 65+ die from ischemic heart disease
• Most common cause: atherosclerosis - cholesterol deposition under endothelium → fibrous invasion → plaque → lumen obstruction

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PART II: STRUCTURE OF CARDIAC VALVES

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1. Atrioventricular Valves - General Structure

• Leaflets (cusps): Flaps of connective tissue covered on both sides by endocardium, devoid of blood vessels
  • Atrial surface: smooth
  • Ventricular surface: rough, with chordae tendineae attachments
• Fibrous annulus: Ring of fibrous tissue to which the base of each cusp is secured; maintains shape of the orifice
• Chordae tendineae: Fibrous tendinous cords; arise from free margins and inferior surface of leaflets, connecting to papillary muscles
• Papillary muscles: Conical projections of myocardium; contract simultaneously with the ventricular walls
• Color Atlas of Human Anatomy Vol. 2, p. 55

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2. Tricuspid Valve (Right AV Valve)

• Anterior papillary muscle - largest and most constant; arises from anterior wall; has a direct connection via the septomarginal trabecula (moderator band) - which also carries the right bundle branch
• Posterior papillary muscle - may be one, two, or three structures
• Septal papillary muscle - most inconsistent; often small or absent; chordae may arise directly from septal wall

• During diastole: valve opens, cusps project into RV, blood fills
• During systole: papillary muscles contract simultaneously with myocardium, tightening chordae tendineae
• This prevents cusp eversion into the right atrium (i.e., prevents regurgitation)
• Chordae from two papillary muscles attach to each cusp, preventing cusp separation

Clinical note: Papillary muscle necrosis after myocardial infarction → valve prolapse → acute mitral (or tricuspid) regurgitation

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3. Mitral Valve / Bicuspid Valve (Left AV Valve)

• Anterior (anteromedial) cusp - larger; continuous at its septal origin with the wall of the aorta (aortomitral continuity)
• Posterior (posterolateral) cusp
• Two small commissural cusps do not extend to the fibrous annulus

• Anterior papillary muscle - large, arises from anterolateral LV wall
• Posterior papillary muscle - arises from posterolateral LV wall
• Each papillary muscle supports adjacent sides of both leaflets
• Short and thick chordae tendineae compared to the RV

1. Leaflets
2. Fibrous annulus
3. Chordae tendineae (marginal/primary from free edges; secondary from undersurface)
4. Papillary muscles
5. Subjacent myocardium

• Guyton and Hall Textbook of Medical Physiology; Sabiston Textbook of Surgery, p. VALVE ANATOMY

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4. Semilunar Valves - General Structure

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5. Pulmonary Valve

• Located between the infundibulum (conus arteriosus) and the pulmonary trunk
• Three cusps: Anterior, Right, and Left semilunar cusps
• After ventricular contraction ends, blood recoils back and fills the pulmonary sinuses - this forces the cusps to close, preventing backflow into the RV

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6. Aortic Valve

• Located between the aortic vestibule and the aorta
• Three cusps: Right, Left, and Posterior (non-coronary) semilunar cusps
• The vessel wall near the cusps bulges outward forming the aortic sinuses (sinuses of Valsalva), which enlarges the luminal diameter (aortic bulb)
• The left coronary artery arises from the aortic sinus of the left semilunar cusp
• The right coronary artery arises from the aortic sinus of the right semilunar cusp
• The posterior sinus has no coronary ostium ("non-coronary sinus")

Key anatomical point: The coronary arteries originate from the sinuses of Valsalva - their position behind the open cusps during systole protects them from being occluded, and the sinus geometry creates eddy currents that initiate cusp closure.

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7. Valve Mechanics - Summary Table

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8. Functional Anatomy During the Cardiac Cycle

• AV valves open - blood flows from atria into ventricles
• Semilunar valves closed - blood in aorta/PA does not reflux back

• AV valves closed - papillary muscles contract, chordae taut, preventing leaflet eversion into atria
• Semilunar valves open - blood ejected into aorta and pulmonary trunk
• After ejection ends, recoil of blood fills sinuses of Valsalva, forces semilunar cusps closed

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Key Clinical Correlations

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• Guyton and Hall Textbook of Medical Physiology (pp. 268-270)
• Costanzo Physiology 7th Edition (p. 996-998)
• Gray's Anatomy for Students (pp. 233-236)
• Color Atlas of Human Anatomy Vol. 2, Internal Organs (pp. 54-55)
• Sabiston Textbook of Surgery, 21st Edition (Valve Anatomy section)