CORONARY CIRCULATION. Dr. Amel Eassawi. Objectives. The student should be able to: Know the basic anatomy of coronary arterial and venous circulations. Describe the patterns of coronary blood flow throughout the cardiac cycle.
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Factors Affecting Blood Flow to Coronary Arteries
- Chemical factors
- Nervous stimuli
Effect of Tachycardia on Coronary Blood Flow:
During increased heart rate, period of diastole is shorter therefore coronary blood flow is reduced to heart during tachycardia.
Regulation of Coronary Blood Flow by Nervous Stimuli
• Sympathetic and parasympathetic stimulation control coronary flow directly (action on coronary circulation) and indirectly (through the action on cardiac activity.
• Small vagalinnervation of the coronary circulation: acetylcholine dilates the coronary arteries.
• More extensive sympathetic innervation; more beta adrenergic receptors on the epicardial arteries and more alpha receptors on the intramuscular arteries. Overall effect is vasoconstrictive (alpha)
• Metabolic control (oxygen) overrides nervous control
Oxygen is supplied by coronary blood to meet the demand of the myocardium. A mismatch between supply and demand causes ischemia.
Ischemic Heart Disease (IHD):
IHD is decreased coronary blood flow (Transient Myocardial Ischemia)
Patient complains of tightness or pain in the middle of the chest (retrosternal) for few minutes. Pain often radiates to inner side of left arm. Pain is precipeted by effort and relieved by rest.
Metabolic and Functional Consequences of Ischemia
Reduced or discontinued coronary flow leads to:
1. Depletion of oxygen
2. Anaerobic metabolism
3. Depletion of ATP
4. Cell death
Functional Consequences of Reversible Ischemia
• Preconditioning and post-conditioning
• Ischemia-reperfusion injury
Preconditioning and Post-conditioning
• Preconditioning: episodes of reversible ischemia preceding infarction reduce necrosis. The effect of preconditioning persists for up to 4 days
• Post-conditioning: episodes of intermittent reversible ischemia after reperfusion reduce injury
Collateral circulation compensates coronary obstruction
• Opening of existing interarterialanastomoses
• Formation of new vessels
• Myocardial function is reduced during ischemia
• Myocardial function recovers completely if ischemia lasts less than 2 minutes
• With more severe and more prolonged ischemia, myocardial function recovery is delayed after blood flow is restored. Stunning is this delayed recovery.
Chronic Hibernating Myocardium
• Chronic ischemia
• Chronic reduction of function
• No infarction (viable myocardium)
• Reduced mitochondrial activity but normal ATP reserve
• Increased arrhythmias, increased sympathetic innervation disparity