Question 11

College Answer

For a good pass candidates were expected to cover at least the following areas -
Anatomy of the coronary arteries and their supply, variations in supply and venous
circulation
Other unique features - coronary sinus saturation <30%, the diastolic aortic pressure,
Tachycardia reduces the coronary blood flow through a reduction in diastolic time, left
ventricle perfused mainly during diastole and right ventricle perfused mainly during systole,
different pattern of left and right ventricular coronary perfusion (drawing a figure of Rt and
Lt coronary blood flow), lack of capacity for the myocardium to increase its extraction ratio
Physiological control: The most important mechanism through which coronary blood flow
can be changed is by autoregulation which changes the coronary vascular resistance to
maintain constant flow in response to different coronary perfusion pressure and changing
metabolic demand. Important mediators are adenosine, nitric oxide, and opening of the ATPsensitive
K+ channels, prostaglandins, carbon dioxide, lactic acid or hydrogen ion.
Sympathetic stimulation to heart increases coronary blood flow.
Overall the greatest deficiency by candidates was lack of detail, use of illustrations and
clarity in their response. It is important that candidates take note of the distribution of marks
given within the question.

Reference Text: Berne and Levy Cardiovascular Physiology Chapter 11 Coronary circulation.

Discussion

Adult 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
    
    The illustration which (I think) they wanted is probably something like this diagram from the old 6th edition of Barash:

Physiological control of coronary circulation:

• 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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