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Understanding PV Loops: Technology & Theory

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Understanding PV Loops: Technology & Theory Understanding PV Loops: Technology & Theory www.transonic.com PV Technology Table of Contents Introduction: Why Study PV Loops? ........................................................................................................3 Glossary of Pressure-Volume Terms .......................................................................................................4 Pressure-Volume Conductance Theory of Operation ..............................................................................7 Pressure-Volume Admittance Theory of Operation ................................................................................9 Comparing Conductance vs Admittance ..............................................................................................10 Cardiac Volume Measurement Methods ............................................................................................... 11 Understanding Afterload ...................................................................................................................... 13 Understanding Preload .........................................................................................................................15 Understanding Contractility: Cardiac Inotropy .................................................................................... 17 Understanding Lusitropy ......................................................................................................................20 PV Workbook RPV-2-wb Rev A 2014 2 PV Technology Introduction: Why Study PV Loops? “Physiologists, and in particular physician physiologists, have often fallen into the trap of measuring certain All About Contractility cardiovascular parameters to explain cardiac performance The single greatest advantage of PV loops because they could be measured, rather than because they is the ability to determine the contractility should be measured.” of the heart independent of preload and William J. Mazzei, M.D. 1998 afterload. By an occlusion procedure (typically the inferior vena cava) a series of pressure- volume loops are created which can be Scientists have historically relied on systemic blood pressure, analyzed for a multitude of load independent blood flow, and ventricular pressure to report changes in parameters which are unavailable from other heart performance. These are all important parameters, hemodynamic measurement techniques such but only form part of the picture of heart performance. as echocardiography, MRI and cardiac CT. Pressure-Volume (PV) loops provide a range of hemodynamic parameters which are not readily measurable PV LOOP MEASUREMENTS by other methods; including changes in contractility, elastance, vaRiaBLE DEscRiptioN power, energetics and efficiency. What is even more powerful about PV loops is that they provide quantitative measurements ESP End-Systolic Pressure of parameters, not just qualitative results. This makes PV loops EDP End-Diastolic Pressure the single most comprehensive measurement of hemodynamics ESV End-Systolic Volume and cardiac function available. EDV End-Diastolic Volume There are three main areas of cardiovascular assessment where HR Heart Rate PV loops provide the ideal measurement approach: Max dP/dt Maximum Derivative of Pressure 1. When it is the best method to measure the contractile Min dP/dt Minimum Derivative of Pressure parameter of interest including ESPVR and EDPVR. Max dV/dt Maximum Derivative of Volume 2. When a comprehensive analysis of cardiac function is Min dV/dt Minimum Derivative of Volume needed, such as for phenotyping. CO Cardiac Output 3. When the parameter of greatest interest is unknown EF% Ejection Fraction during drug or genetic studies. SV Stroke Volume EXAMPLES OF CARDIOvaSCULAR patHOLOGY SW Stroke Work THat caN BE EXAMINED BY PV LOOPS Ea Arterial Elastance maxPwr Maximum Power • Myocardial Infarction plPwr Preload Adjusted Power • Dilated (Diabetic) Cardiomyopathy Eff Efficiency • Left Ventricular Hypertrophy PE Potential Energy • Right Ventricular Hypertrophy PVA Pressure-Volume Area • Restrictive Cardiomyopathy ESPVR End-Systolic PV Relationship • Aortic Valve Stenosis EDPVR End-Diastolic PV Relationship • Mitral Valve Stenosis PRSW Preload Recruitable Stroke Work • Aortic Regurgitation (Aortic Insufficiency) E(t) Time-Varying Elastance • Mitral Regurgitation Tau Isovolumic Relaxation Constant • Right Ventricular Function and Pulmonary Hypertension PV Workbook RPV-2-wb Rev A 2014 Brochure: RPV-2-fly 3 PV Technology Glossary of Pressure-Volume Terms AFTERLOAD EJEctION FRactION (EF%) Afterload is the mean tension produced by a chamber Ejection fraction is the ratio of the volume of blood of the heart in order to contract. It can also be ejected from the ventricle per beat (stroke volume) considered as the ‘load’ that the heart must eject to the volume of blood in that ventricle at the end blood against. Afterload is therefore a consequence of diastole. It is widely clinically misunderstood as of aortic large vessel compliance, wave reflection an index of contractility, but it is a load dependent and small vessel resistance (LV afterload) or similar parameter. Healthy ventricles typically have ejection pulmonary artery parameters (RV afterload). fractions greater than 55%. ARTERIAL ELAStaNCE (Ea) E-MaX This is a measure of arterial load and its impact on the Maximum point in the pressure-volume relationship ventricle. Calculated as the simple ratio of ventricular occurring at the end of systole. E-max is directly end-systolic pressure to stroke volume. related to the contractile state of the ventricle chamber. This number is different for each individual CARDIac CONTRactILITY heart beat, representing the maximal systolic elastance (E-max) at that moment in time. The intrinsic ability of the heart to contract independent of preload and afterload. On a cellular level it can be characterized as the change of END-DIASTOLIC PRESSURE (EDP) developed tension at given resting fiber length. Used Pressure in the ventricle at the end of diastole. interchangeably with Cardiac Inotropy. END-DIASTOLIC PRESSURE VOLUME RELatIONSHIP CARDIac INOTROPY (EDPVR) The ability of the heart muscle to generate force The EDPVR describes the passive filling curve for through contraction. Used interchangeably with the ventricle and thus the passive properties of the Cardiac Contractility. myocardium. The slope of the EDPVR at any point along this curve is the reciprocal of ventricular CARDIac OUTPUT (CO) compliance (or ventricular stiffness). Cardiac output is defined as the amount of blood pumped by the ventricle in unit time. END-DIASTOLIC VOLUME (EDV) Volume in the ventricle at the end of diastole. COUPLING RatIO Indication of transfer of power from the ventricle to END SYSTOLIC ELAStaNCE (Ees) the peripheral vasculature. Slope of the end systolic pressure volume relationship. DERIvatIVE OF PRESSURE (dP/dt) END-SYSTOLIC PRESSURE (ESP) Reported as max and min rate of pressure change in Pressure in the ventricle at the end of systole. the ventricle. dP/dt are dependent on load and heart rate. LV dP/dt max occurs before aortic valve closure. DERIvatIVE OF VOLUME (dV/dt) Rate of volume change in the ventricle. Maximum and minimum values of dV/dt are normally reported. PV Workbook RPV-2-wb Rev A 2014 Technical Note: RPV-2-tn 4 PV Technology Glossary of Pressure-Volume Terms Cont. END-SYSTOLIC PRESSURE VOLUME RELatIONSHIP MYOcaRDIAL OXYGEN CONSUMptION (MVO2) (ESPVR) Amount of oxygen consumed by the heart as a The ESPVR describes the maximal pressure that can measure of energy consumption. MVO2 is dependently be developed by the ventricle at any given cardiac correlated with cardiac total mechanical energy (TME). chamber volume. This implies that the PV loop cannot cross over the line defining ESPVR for any given POTENTIAL ENERGY (PE) contractile state. Elastic potential energy of the heart is defined by the area between the ESPVR and EDPVR curves to the left END-SYSTOLIC VOLUME (ESV) of the PV loop. PE = ESP(ESV-V0)/2 -EDP(EDV-V0)/4 Volume in the ventricle at the end of systole. where V0 is the theoretical volume when no pressure is generated. E(T) PRELOAD Time Varying volume elastance provides means to discriminate end systole from the end of ejection as Preload is described as the stretching of a single they might not happen at the same time. Normal LV cardiac myocyte immediately prior to contraction and ejects every shortly after end systole. is, therefore, related to the sarcomere length. Since sarcomere length cannot be determined in the intact EXCItatION-CONTRactION COUPLING heart, other indices of preload such as ventricular end diastolic volume or pressure are used. The cellular relationship between electrical stimulus and contraction which is primarily influenced by PRELOAD RECRUItaBLE StROKE WORK (PRSW) Na+, K+ and Ca2+ ions and the neural, hormonal and exogenous agents which influence their behavior in PRSW is determined by the linear regression of the cell. stroke work with the end diastolic volume. The slope of the PRSW relationship is a highly linear index of FRANKLIN-StaRLING CURVE myocardial contractility that is insensitive to preload and afterload. “The heart will pump what it receives”-Starling’s law of the heart PRESSURE-VOLUME AREA (PVA) SV vs EDP: Afterload dependent measure of inotropy The PVA represents the total mechanical energy (TME) where an increase in inotropy shifts the curve up and generated by ventricular contraction. This is equal to to the left; a decrease in inotropy shifts the curve the sum of the stroke work
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