Chapter_16_Mechanical_Aspects_of_Cardiac_Performance__1_slide_per_page

Chapter_16_Mechanical_Aspects_of_Cardiac_Performance__1_slide_per_page

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Chapter 16: Mechanical Aspects of Cardiac Performance The parts of the heart normally beat in an orderly sequence: 1) contraction of the atria ( atrial systole ) 2) contraction of the ventricle ( ventricular systole ) 3) during diastole, the ventricles are relaxed BE501 T. K. Hsiai, MD, PhD, FACC Objectives
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Cardiac Performance: preload and afterload From molecular to cellular and macrostructures Stretch the muscle to desired length by hanging appropriate weight ( preload at the end of ventricular filling ) from the relaxed muscle. The desired preload is determined for the passive length:force relationship . a) Place a support beneath the muscle to allow additional weights ( afterload in the intact heart just before opening of the aortic valve ) to be added without stretching the muscle. b) Stimulate the muscle electrically. c) Record change in muscle length once muscle generates sufficient force to lift total load ( preload + afterload). d) Repeat series for different afterloads.
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Preload is a p assive stretching force exerted on the ventricular muscle at the end of diastole. Preload is caused by the volume of blood in the ventricle at the end of diastole. EDV=end diastolic volume Afterload is the f orce resisting the contraction of the cardiac muscle fibers. Afterload can also be considered as the end-systolic wall stress in the ventricle. ESV=end systolic volume
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Stroke volume is directly related to the force of ventricular contraction. The demands placed upon the body will directly affect cardiac output in two ways. Cardiac output will increase if the blood volume within the body is increased or if the heart beats faster . Cardiac output will decrease if either of these factors decrease. A normal, healthy heart ejects about 60% of the blood volume present in the ventricle. Stroke volume represents the difference between end diastolic volume (EDV), the amount of blood collected in a ventricle during diastole, and end systolic volume (ESV), the amoun of blood remaining in the ventricle after contraction. CO (cardiac output)= SV (stroke volume) x HR (heart rate)
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Myocardial Contractility There are 2 types of regulation: Intrinsic Regulation of the myocardium, and Extrinsic Regulation of the myocardium. 1. Intrinsic Regulation: Starling’s Law of Heart (Otto Frank-Ernst Starling Principle) Energy for contraction is provided by the initial fiber length (EDV ) remember that EDV determines ED pressure (stretch on ventricle) referred to as the preload The load or work the heart muscle must contract against is referred to as the afterload this is usually the peripheral resistance or mean arterial pressure (MAP) - hypertensive individuals have a higher (greater) afterload that the heart must work against to pump blood less efficient overtime - As EDV is increased, myofibers are stretched and the heart generates more force of contraction within limits; important intrinsic mechanism to regulate contractility under different conditions. Important regulators of Cardiac Contractile Performance all are related
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This note was uploaded on 02/27/2008 for the course BME 501 taught by Professor Yamashiro,hsiai during the Fall '07 term at USC.

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