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Transcript
BME Class (Lecture 6 – LV pressure-volume relationship) September 14, 2004 Lecture #6 - Left Ventricular Pressure-Volume Relationship - (September 14, 2004) A. P-V Loop (a) Phase I (D – Diastolic Filling) - blood passively fills from atrium into ventricle, followed by additional volume due to atrial contraction. Characteristics: mitral/tricuspid valve open and aortic/pulmonic valve closed, low pressure changes, high volume changes. (b) Phase II (IC - Isovolumic Contraction) - Ventricle contracts generating high pressure level within the ventricle. Characteristics: mitral/tricuspid valve and aortic/pulmonic valve closed, high pressure changes, no volume changes. (c) Phase III (EJ - Ejection) - Pressure in ventricle now higher than aorta/pulmonary artery, opening aortic/pulmonic valve and blood flow to arterial system occurs. Characteristics: mitral/tricuspid valve closed and aortic/pulmonic valve open, low pressure changes, high volume changes. (d) Phase IV (IR - Isovolumic Relaxation) - Ventricle relaxes. Characteristics: mitral/tricuspid and aortic/pulmonic valves closed, high pressure changes (decreases), and no volume changes. 1 BME Class (Lecture 6 – LV pressure-volume relationship) September 14, 2004 End - Systolic Elastance Left Ventricular Pressure, mmHg Pes Increasing Elastance External Work Diastolic Elastance Vo Stroke Volume Ved Ventricular Volume, cc Figure 6-1. Illustration of key landmarks for the left ventricular pressure-volume loop, where Pes = left ventricular end-systolic pressure; Vo = left ventricular volume at 0 mmHg; and Ved = left ventricular enddiastolic volume. The area inside the P-V loop represents the external work of the ventricle. Figure 6-2. Illustration of key landmarks for the left ventricular pressure-volume relationship. The PV loops are generated by performing a vena cava occlusion to reduce pre-load to the heart. The slope of the line tangent to the PV loop at end-systole is called the End-Systolic Pressure Volume Relationship (ESPVR or Ees) that has been used as a load independent index of ventricular contractility. 2 BME Class (Lecture 6 – LV pressure-volume relationship) September 14, 2004 Ea Pes Ees EW Vo SV Ved Figure 6-3. Illustration of key landmarks for the left ventricular pressure-volume loop, where Pes = left ventricular end-systolic pressure; Vo = left ventricular volume at 0 mmHg; and Ved = left ventricular enddiastolic volume. The area inside the P-V loop represents the external work of the ventricle. Cardiovascular Coupling Theory: (1) To maximize power transfer Ees = Ea (2) Does the heart function to ‘maximize’ or ‘optimize’ power transfer? REFERENCES: Koenig SC, DL Ewert, DA Ludwig, JW Fanton, and VA Convertino. Effects of fluid shifts during bed rest confinement on left ventricular and systemic arterial elastances. Aviat. Space Environ. Med., 75:7-15, 2004. Ewert DL, B Wheeler, C Doetkott, C Ionan, GM Pantalos, and SC Koenig. The effect of heart rate, preload, and afterload on the visco-elastic properties of the swine myocardium. Ann. Biomed. Eng. (in press). 3 BME Class (Lecture 6 – LV pressure-volume relationship) September 14, 2004 2. Applications A. Heart Failure Figure 6-4. Illustration of difference in P-V loops for normal and failing heart. In top figure, heart failure is induced by (1) betablocker and (2) Monensin. 4 BME Class (Lecture 6 – LV pressure-volume relationship) September 14, 2004 B. Ventricular Assist Devices (VAD) Figure 6-5. Comparison of pressure-volume loops between a normal and failing left ventricle (LV) without assistance and a failing LV assisted by an (a) artificial vasculature device (AVD), and (b) continuous (CVAD) and (c) asynchronous pulsatile (PVAD) flow ventricular assist devices. Unlike VAD, the AVD produces counterpulsation to the circulation as can be seen in left ventricular (LVP) and aortic (AoP) pressure waveforms (d). 5 BME Class (Lecture 6 – LV pressure-volume relationship) September 14, 2004 100 90 LV pressure (mm Hg) 80 70 60 baseline cVAD 50% cVAD 100% 50 40 30 20 10 0 0 50 100 150 200 250 300 350 400 450 LV volume (ml) 80 70 LV pressure (mm Hg) 60 50 40 baseline failure 75% bypass 100+% bypass 30 20 10 0 -10 -20 0 50 100 150 200 250 LV volume (ml) Figure 6-6. Comparison of pressure-volume loops in calves with normal (top) and failing ventricles (below) during continuous flow (CF) ventricular assist device (VAD) support at 75% and 100% bypass. Notice that in normal ventricle model, the only effect is a leftward shift of PV loop. 6 BME Class (Lecture 6 – LV pressure-volume relationship) September 14, 2004 Assignment #6 – For each of the 5 data sets from lecture 4 assignment, plot the left ventricular pressure-volume relationship and calculate the stroke volume and external work of the ventricle. Again, indicate whether the heart is normal/failure, supported by an assist device, and/or whether the therapy is effective. NOTE: LVV = left ventricular volume and LVP = left ventricular pressure. *Assignment 6 due in class on Tuesday, September 21, 2004 7