What is the significance of cardiac blood vessels, including coronary arteries and cardiac veins, in supplying blood to the heart muscle?

Cardiac Blood Vessels and Their Significance

The cardiac blood vessels, including coronary arteries and cardiac veins, are critically important for maintaining myocardial oxygen supply and demand balance, with disruption of this system leading to ischemia, potential myocardial injury, and mortality.

Coronary Arterial System

Major Coronary Arteries

• The coronary arterial system consists of three major arteries: the left anterior descending (LAD), the circumflex, and the right coronary artery (RCA), which supply oxygenated blood to the myocardium 1
• The left main coronary artery bifurcates into the LAD and circumflex branches, with the left main ostium including the first 3 mm of the artery 1

Right Coronary Artery (RCA)

• The RCA primarily supplies the right atrium, right ventricle, sinoatrial node, posterior portion of the interventricular septum, and in right-dominant circulation (approximately 90% of people), the inferior wall of the left ventricle 2
• The RCA gives rise to the posterior descending artery (PDA), which runs in the posterior interventricular groove and supplies septal perforator branches 2
• Right ventricular branches arise from the RCA to supply the right ventricular wall 2

Coronary Dominance Patterns

• Coronary dominance is classified into three categories based on which arteries supply the inferior wall of the left ventricle 1:
  • Right dominance (most common): The PDA and posterolateral artery (PLA) arise from the RCA
  • Left dominance: The PDA and PLA arise from the left circumflex artery
  • Co-dominant: The RCA supplies the PDA, and the circumflex artery supplies the PLA

Coronary Venous System

Major Cardiac Veins

• The great cardiac vein is the longest venous vessel of the heart, typically originating at the lower third of the anterior interventricular sulcus 3
• The great cardiac vein and middle cardiac vein can merge at the apex of the heart, forming with the coronary sinus a complete venous ring around the left ventricle 3
• Cardiac veins usually accompany the coronary arteries but may run independently, particularly at myocardial bridges 4

Thebesian Vessels

• Thebesian vessels provide direct connections between coronary arteries and the chambers of the heart, bypassing the capillary system 5
• Under certain conditions, up to 90% of arterial flow may escape via the Thebesian vessels 5
• These vessels can supply the heart muscle with sufficient blood to maintain circulation when coronary artery orifices are gradually closed 5

Physiological Significance of Coronary Circulation

Coronary Blood Flow Regulation

• Myocardial ischemia results from an imbalance between myocardial oxygen supply and demand 1
• Coronary blood flow provides oxygen supply for myocardial demand and can increase from resting level to maximum level in response to exercise and other stimuli 1
• This increase from baseline to maximal flow is termed coronary flow reserve 1

Resistance Components

• Blood flow has three major resistance components 1:
  • R1: Epicardial vessel resistance (normally trivial in non-atherosclerotic patients)
  • R2: Small arteries and arterioles (primary site of resistance adjustment)
  • R3: Intramyocardial capillary system

Exercise and Coronary Blood Flow

• Exercise is the most important physiological stimulus for increased myocardial oxygen demand 6
• During heavy exercise, left ventricular oxygen demand increases approximately sixfold, met primarily by augmenting coronary blood flow (approximately 5-fold) 6
• Across a normal epicardial artery supplying normal myocardium, coronary blood flow can increase 3-fold in adults 1

Pathophysiological Significance

Coronary Artery Disease

• Atherosclerotic narrowings produce epicardial vessel resistance and can abolish coronary reserve and autoregulation, reducing resting coronary blood flow 1
• The physiological significance of coronary stenosis, especially for intermediately severe luminal narrowing (40-70% diameter), cannot be accurately determined by angiography alone 1
• Pressure and flow measurements provide complementary information to anatomic characterization of coronary disease 1

Demand Ischemia

• Demand ischemia occurs when there is an imbalance between oxygen supply and demand 7
• Factors increasing demand include tachycardia, hypertension, left ventricular hypertrophy, and aortic valve stenosis 7
• Factors decreasing supply include hypoxemia, hypotension, coronary microvascular dysfunction, and coronary vasospasm 7

Myocardial Bridges

• Coronary arteries are frequently covered by cardiac muscles, termed myocardial bridges 4
• Myocardial bridges can cause myocardial ischemic diseases or cardiac arrhythmia 4
• At myocardial bridges, cardiac veins often run independently on the bridge surfaces rather than accompanying the coronary arteries 4

Clinical Implications

Coronary Physiological Assessment

• Coronary pressure and flow measurements help overcome limitations of coronary angiography 1
• These physiological data facilitate timely and objective decision-making about therapy 1
• Pressure loss across a stenosis increases with increasing coronary flow along a quadratic pressure drop-flow relationship 1

Indications for Coronary Angiography

• Coronary angiography is recommended for patients who have survived sudden cardiac death or potentially life-threatening ventricular arrhythmia 1
• It is also indicated for patients with heart failure symptoms to assess cardiac risk 1
• Coronary angiography is recommended when clinical characteristics and quality of life remain unsatisfactory despite evidence-based medical therapy 1

Adaptations to Exercise Training

• Exercise training is associated with adaptations in the coronary microvasculature, including increased arteriolar densities and/or diameters 6
• These adaptations provide a morphometric basis for increased peak coronary blood flow rates in exercise-trained individuals 6
• Training also enhances endothelium-dependent vasodilation throughout the coronary microcirculation 6