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Coronary blood flow
Coronary blood flow is regulated to meet the heart's high oxygen demands. The heart extracts 70-80% of oxygen from coronary blood flow, which can increase five-fold during exercise. Coronary blood flow is determined by coronary perfusion pressure, perfusion time, and vessel wall diameter, which is controlled by vasomotor tone in response to oxygen demand and various humoral and neurological factors. Maintaining adequate coronary perfusion pressure and perfusion time is important for myocardial oxygen balance.
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Coronary blood flow
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- 2. Oxygen consumption • Highestper tissue mass of all human organs • 250ml per min • 0.8 ml per min per gram of heart muscle • 5% of cardiac output • Oxygen extraction is 70-80% (25%)
- 3. Coronary blood flow •Increased oxygen consumption must be met by increasing the blood flow • May increase five fold during exercise • Supply usually matches the demand • Gregg effect: an increase in coronary blood flow can increase myocardial oxygen consumption
- 4. ANATOMY • Two maincoronary arteris • Left and Right • Epicardial arteries • Intramuscular arteries penetrate the myocardium to form subendocardial arterial plexuses • Coronary sinus • Anterior coronary vein, Thebesian veins
- 5. Determinants of coronaryblood flow • Coronary perfusion pressure • Perfusion time • Vessel wall diameter
- 6. • Coronary perfusionpressure: • Systole: blood flow to LV is lowest • Diastole: flow resumes • Aortic diastolic pressure-LVEDP=CPP • Phasic changes in blood flow are less in rt.ventricle
- 7. • Perfusion time: Anyincrease in heart rate impinges on diastolic time more than systolic time and reduces the perfusion time.
- 8. • Vessel WallDiameter: • Vasomotor tone • Deposits • Mechanism/regulation
- 9. Factors-Vasomotor Tone • Myocardialoxygen demand • Hypoxia: adenosine, ATPase K+ • Autoregulation – Intraluminal pressure changes – Metabolic regulation – Myocardial o2 tension, vasoconstrictors, vasodilators
- 10. • Nervous control:weak • Epicardial vessels alpha receptors • Anti-steal effect • Intramuscular, subendocardial blood vessels beta- 2 receptors • Sympathetic stimulation: 02 demand, beta activation – increased flow • Parasympathetic : vasodilatory, intact endothelium, acetyl choline
- 11. • Humoral Control: •Intact endothelium, peptide harmones • ADH causes vasoconstriction in stress • Others vasodialtion – ANP, VIP, calcitonin gene related peptide • AT-2 causes vasoconstriction • ACE inhibits bradykinin - vasodilator
- 12. • Vascular Endothelium: •Final common pathway in regulation blood flow • Modulates the contractile activity of the underlying smooth muscle • vasorelaxants: EDRF, NO, prostacyclin, bradykinin • Vasoconstrictos: endothelin, ThromboxaneA2 • Balance between these determines the flow
- 13. Myocardial oxygen balance •Oxygen delivery is the product of arterial oxygen carrying capacity and myocardial blood flow. • diastolic pressure time index (DPTI): • Useful measure of coronary blood flow • Product of coronary perfusion pressure and diastolic time • Tension time index (TTI): • Oxygen demand • Product of systolic pressure and systolic time
- 14. • EVR (endocardialviability ratio): • DPTI/TTI Ratio • Myocardial oxygen supply-demand balance • Normal is 1 or more • ratio <0.7 is associated with subendocardial ischemia
- 15. diseases • CAD (coronaryartery disease): • Change in artery diameter • Coronary steal • Hypertension: LVH • Heart Failure: impaired ejection, LDV, LVEDP, lowered coronary perfusion pressure • Vasoconstriction: increases o2 demand, work load, improves perfusion
- 16. DRUGS • Antiplatelet drugs,anticoagulants and lipid lowering drugs • Nitrates • Calcium channel blockers • Drugs acting on angiotensin • Potassium channel openers • B-blockers • Vasopressors and inotropes
- 17. Anesthesia and myocardialoxygen balance • Halogenated inhalational agents • Isoflurane • Sevoflurane • Halothane • Desflurane??? • Central neuraxial block • Sympathetic stmulation
- 18. • Key points: •Blood flow to the heart occurs mainly during diastole. • Coronary blood flow is mainly determined by local oxygen demand. • The vascular endothelium is the final common pathway controlling vasomotor tone. • When anaesthetising patients with coronary artery disease, maintain coronary perfusion pressure and avoid tachycardia.