Describe the regulation of the coronary circulation.
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.
- 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
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