Question 19

College Answer

Good candidates described normal blood flow to the coronary circulation, including differences between the right and left ventricles. Coronary artery anatomy was outlined, including the regions of the heart supplied and the concept of dominance. In addition to epicardial vessels, strong answers also outlined penetrating arteries, subendocardial supply and venous drainage. Regulation of coronary blood flow required an explanation of flow-dependence of the heart given its high oxygen extraction rate. Metabolic autoregulation and its mediators needed to be described, along with the physical factors driving coronary blood flow. Less important mechanisms such as the role of the autonomic nervous system were also described, with an emphasis on indirect effects over direct effects.

Discussion

• Coronary vascular anatomy:
  • Coronary arteries arise from the sinuses of Valsalva at the aortic root
  • Left main
    • Divides into left anterior descending and left circumflex 
    • Supplies most of the septum and LV
  • Right coronary
    • Supplies the RV, the sinoatrial node
  • Coronary sinus
    • Drains into the right atrium; opening is between the IVC and the tricuspid valve
    • Venous blood oxygen saturation here is ~ 30%
• Coronary blood flow
  • 5% of cardiac output, or 50-120ml/100g of myocardial mass
  • 75% of the left main flow and 50% of RCA flow occurs in diastole
  • In systole, LV blood flow is reduced due to the high chamber pressure during contraction
  • For the RV, the systolic chamber pressure is lower, and blood flow is less affected 
  • Thus, diastolic time is more important for LV perfusion, and it can be compromised by tachycardia
• Coronary blood flow is automatically regulated to meet metabolic demand
  • Myocardial oxygen extraction ratio is already very high (60-70%).
  • Thus, the myocardium cannot increase its oxygen extraction efficiency to meet increased metabolic demand
  • Thus, coronary arterial blood flow increases to match myocardial oxygen demand, and the oxygen extraction ratio remains stable.
  • With exercise, coronary blood flow can increase several-fold
• Mechanisms of coronary blood flow autoregulation
  • Metabolic substrates and byproducts are thought to act as vasoactive mediators in the coronary circulation
  • Multiple agents are considered important, including adenosine, O₂, CO₂, lactate, pH, and potassium ions.
  • ATP-sensitive potassium channels also open in response to decreased ATP, resulting in smooth muscle membrane hyperpolarisation and thus relaxation
• Other influences on coronary blood flow
  • Myogenic autoregulation (intrinsic arterial smooth muscle property)
  • Autonomic nervous system
    • α₁-adrenergic receptor activation stimulates vasoconstriction
    • β-adrenergic receptor activation produces vasodilation
    • Muscarinic receptor stimulation produces coronary vasodilation
  • Various pharmacological agents with coronary vasoactive properties include:
    • Vasodilators (adenosine, GTN, dipyridamole)
    • Vasoconstrictors (vasopressin, COX inhibitors

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