Describe the regulation of the coronary circulation.

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College Answer

Some answers suffered from listing things rather than describing things as the question required. Better answers included a description of metabolic, physical and neuro-humoral factors and the relative importance of each. Many described detailed anatomy which was not necessary.

Discussion

  • 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
    • Relative importance:
      • Adenosine and nitric oxide are most important during conditions of hypoxia or ischaemia
      • ATP-sensitive potassium channels are most important under normal loading conditions, and regulate coronary flow in response to sustained changes in systemic arterial pressure
      • Potassium ions have greatest importance in regulating brief changes in coronary flow, rather than sustained changes
  • 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

References

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Hoffman, J. I. "Determinants and prediction of transmural myocardial perfusion." Circulation 58.3 (1978): 381-391.

Gregg, Donald E. "Phasic blood flow and its determinants in the right coronary artery." American Journal of Physiology-Legacy Content 119.3 (1937): 580-588.

Downey, H. Fred. "Coronary—Ventricular Interaction: The Gregg Phenomenon." Cardiac-Vascular Remodeling and Functional Interaction. Springer, Tokyo, 1997. 321-332.

Messer, Joseph V., and William A. Neill. "The oxygen supply of the human heart∗." The American Journal of Cardiology 9.3 (1962): 384-394.

Mymin, D., and G. P. Sharma. "Total and effective coronary blood flow in coronary and noncoronary heart disease." The Journal of clinical investigation 53.2 (1974): 363-373.

Suga, Hiroyuki., et al. "Oxygen consumption and pressure-volume area of abnormal contractions in canine heart." American Journal of Physiology-Heart and Circulatory Physiology 246.2 (1984): H154-H160.

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.

Thangavel, Periyasamy, et al. "Anaesthetic challenges in cardiac interventional procedures." (2014). World Journal of Cardiovascular Surgery, 2014, 4, 206-216

Muller-Delp, Judy M. "The coronary microcirculation in health and disease." Isrn Physiology 2013 (2013).