Question 15

College Answer

For this question a good answer should have encompassed a description of the following components; the arterial supply (epicardial, endocardial supplies), venous supply (major and minor venous drainage), the phasic ventricular blood supply, a description of and the relevance of the high oxygen extraction ratio, an emphasis on metabolic autoregulation whilst including any other mechanisms of physiological regulation. Most candidates provided reasonable detail surrounding the arterial-circulation and the area of the heart these vessels supply. Less well described were the perforating branches, the capillary network and the venous drainage, these components were often incorrectly described or left out of the candidate answers. Most candidates correctly identified the phasic flow and the difference in flow that occurs between the right and left ventricle. Those candidates that used diagrams to explain this were rewarded when these diagrams were labelled and accurate with respect to their systolic and diastolic distinction. The control of regulation was less well answered with generic statements not specifically identifying the local metabolic controls importance over any other mechanism. Those answers that scored well coupled the metabolic control to the flow dependence highlighting the high oxygen
extraction within the coronary circulation.

Discussion

• Coronary circulation:
  •  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

Ramanathan, Tamilselvi, and Henry Skinner. "Coronary blood flow." Continuing Education in Anaesthesia, Critical Care & Pain 5.2 (2005): 61-64.

Goodwill, Adam G., et al. "Regulation of coronary blood flow." Comprehensive Physiology 7.2 (2011): 321-382.

Raphael, M. J., D. R. Hawtin, and S. P. Allwork. "The angiographic anatomy of the coronary arteries." British Journal of Surgery 67.3 (1980): 181-187.