| Cardiac Output |
| Cardiac Output (CO) = Stroke Volume (SV) x Heart Rate (HR) Oxygen delivery = CO x arterial oxygen content (CaO2)Cardiac output must be the same in both sides of the heart except patients with regurgitant murmurspatients with structural heart abnormalitiesFick principle VO2 = (Ca-Cv) x COvolume of oxygen consumed = tissue extraction of oxygen times COapplies “conservation of mass” concept to oxygen consumption in bodyCO = O2 consumption / [ (O2 in pulmonary vein) – (O2 in pulmonary artery) ]Key the equation above refers to difference of oxygen content between the pulmonary vein (oxygenated blood) and pulmonary artery (deoxygenated blood)when dealing with systemic circulation, one has to substract O2 in systemic veins (deoxygenated blood) from O2 in systemic arteries (oxygenated blood) to avoid negative valuesChanges in CO↓ SV in ventricular tachycardiaif HR is too high, diastolic filling is incomplete and CO decreasesexercise CO maintained by SV in early stages of exerciseCO maintained by HR in late stages of exercise |
| Stroke Volume |
| Stroke Volume (SV) SV = CO/HR = End-Diastolic Volume (EDV) – End-Systolic Volume (ESV) volume of blood ejected per heart beatSV ~ 70 mLVariables (assessed by echocardiogram)EDVvolume of blood in ventricle before ejectionESV volume of blood in ventricle after ejectionChanges in SV ↑ SV in anxiety, exercise, pregnancy↓ SV in failing heart“SV CAP“Stroke Volume affected by Contractility, Afterload, Preload↑ SV via (1) ↑ contractility, (2) ↓ afterload, (3) ↑ preload↑ Contractility, ↑ SV viacatecholamines↑ activity of Ca2+ pump in SR↑ intracellular Ca2+↓ extracellular Na+↓ activity of Na+/Ca2+ exchangerdigitalis↑ intracellular Na+ → ↑ intracellular Ca2+↓ Contractility, ↓ SV viaβ1 blockade↓ cAMPheart failuresystolic dysfunctionacidosishypoxia/hypercapnea↓ PO2/↑ PCO2non-dihydropyridine Ca2+ channel blockers↑ Myocardial O2 demand via↑ afterload (∝ arterial pressure)↑ contractility↑ HR↑ heart size aka ↑ wall tension |
| Ejection Fraction |
| Ejection Fraction (EF) = SV/EDVfraction of EDV ejected per SVan index of ventricular effectiveness↓ EF via systolic heart failurean index of cardiac contractility↑ EF, ↑ contractilityEF ~ 0.55 or 55% |
| Pulse Pressure |
| Pulse Pressure (PP) = Systolic Pressure – Diastolic PressurePP reflects volume of blood ejected from left ventricle on a single beatPP ∝ SV |
| Mean Arterial Pressure |
| Mean Arterial Pressure (MAP)average pressure in a complete cardiac cycleEquationsMAP = CO x Total Peripheral Resistance (TPR)MAP = 2/3 Diastolic Pressure + 1/3 Systolic PressureMAP = 1/3 PP + Diastolic Pressureas HR increases, diastole decreases and systole increases in the percentage of time spent, thus influencing the MAP |
| Preload and Afterload |
| Preloadaka ventricular EDVpreload “pumps up the heart”↑ preload byexercise (slightly)↑ blood volumee.g., over-transfusionexcitementsympatheticsPathologyright wall myocardial infarction (inferior wall)ST elevation seen in leads II, III, and avFpreload dependent for cardiac function, thus do not diureseAfterloadaka MAP∝ TPRPharmacologyvenodilators (e.g., nitroglycerin) ↓ preload vasodilators (e.g., hydrAlAzine) ↓ Afterload (Arterial) |