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Week_4_Cardiology

Week_4_Cardiology - Lec 20 PHYSIOLOGY OF THE CORONARY...

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Lec. 20 PHYSIOLOGY OF THE CORONARY CIRCULATION 11/1/2010 Oxygen usage of the heart .4% of body weight but accounts for 10% of O 2 consumption Myocardial demand MVO 2 must meet supply for every beat o VO 2 is O 2 demand for rest of body Most of heart energy (70%) is from free fatty acids (glucose 12-30%) o Lactate production indicates ischemia Humans have basically no anastomoses in the heart Determinants of MVO 2 Heart rate o Linear relationship with MVO 2 Inotropic state o Close to linear relationship with MVO 2 Afterload (developed pressure) o Linear relationship with MVO 2 Wall shortening (external work) o Relatively low O 2 cost o Work to eject large volume is relatively small compared to work against a large afterload Basal oxygen consumption (“housekeeping” O 2 consumption in the absence of contraction) o Relatively fixed Cardiac Efficiency (Work and Energy) Work = Pressure*Volume W=PV o Commonly multiplied by HR to get work per minute W=P*V*HR Energy is O 2 consumption = MVO 2 Efficiency = work / MVO 2 o Units are (P x V/min)/(ml O 2 /min) Efficiency does not always increase with ↑ Work o depends→ volume work cheaper than pressure work
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Main determinants of MVO 2 and equation Heart rate, afterload, inotropic state Heart can not go into O 2 "debt" MVO 2 = CBF*AVO 2 D o Heart O 2 consumption = coronary blood flow*Δ[O 2 ] o Similar to Fick eq for cardiac output Regulation of O 2 to the heart Not controlled by ↑ O 2 extraction o ΔO 2 in Hg is already very high at about 65% Controlled by changing coronary blood flow (CBF) CPP=CBF*CVR (coronary perfusion pressure=coronary blood flow*coronary vascular resistance) aka P=QR o CBF= (1/CVR)*CPP CVR is 1/slope of the line at a given resistance o Coronary Blood flow determined by pressure and resistance o Can't ↑ or ↓ pressure by 400% to meet changing needs of heart o Coronary blood flow determined by resistance Regulation of CBF by resistance Tailored CBF to MVO 2 at given blood pressures
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Avoidance of swings in CBF due to changes in blood pressure Auto regulation allows heart to maintain constant CBF in different CPP if MVO 2 stays the same Horizontal lines represent maintained CBF at differing CPP under two different levels of MVO2 Straight line = vessels maximally dilated and CBF is linear to CPP o Vessels can't dilate anymore to ↑ CBF Difference between horizontal line and vertical line is "vasodilator reserve" o Ability to ↑ CBF if demand increases simply by dilating vessels Usually occurs at about 60mm Hg when resting Line D pressure flow relationship at maximal vasodilation Lines A, B, C auto-regulatory curves at different MVO 2 levels When horizontal lines meet line D, max vasodilation and auto-regulation can no longer occur
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Can give vasodilator to ↓ resistance and ↑ flow which will use coronary flow reserve Physiological mechanisms of controlling CBF (coronary blood flow)
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