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PHYSIOLOGY OF HEART CONTRACTILITY| Tutorial D-1 CVS

130110110177|Gabriella Chafrina| 09/05/1

Determinants of Contractile Function in the Intact Heart

CO = bodys total metabolic need

CO = SV x HR

3 major determinants of SV are preload, afterload, and myocardial contractility

1. PRELOAD

Preload is the ventricular wall tension at the end of diastole. In

clinical terms, it is the stretch on the ventricular fibers just

before contraction, often approximated by the end-diastolic

volume or end-diastolic pressure

Frank-Starling curve/ventricular function curvethe more a

normal ventricle is distended during diastole, the greater the

volume that is ejected during the next systolic concentration

intravascular volume (dehydration/severe hemorrhage)

ventricular preloadsmaller EDV SV

2. AFTERLOAD

Afterload is the ventricular wall tension during contraction; the resistance that must be overcome for the

ventricle to eject its content. Often approximated by the systolic ventricular (or arterial) pressure

LaPlaces relationship:

=

2

Higher pressure load (ex: hypertension) or an chamber size (ex: a dilated left ventricle)

ventricular wall stress

wall thicknesscompensatory role in wall stress (because the force is distributed over a greater

mass per unit surface area of ventricular muscle)

3. MYOCARDIALCONTRACTILITY / INOTROPIC STATE

Myocardial contractility is property of heart muscle that accounts for changes in the strength of contractio

independent of the preload and afterload. Reflects chemical or hormonal influences (ex: catecholamine) o

the force of contraction

contractility by using positive inotropic agent (ANS sympathetic, hormones, Ca in interstitial fluid,

drugs)

contractility by using negative inotropic agent (sympathetic inhibition, acidosis, K level in interstitial

fluid, Ca channel blocker)

Positive inotropic promote Ca inflowreinforcing force for next contraction

= Wall stress (force per unit area)

= ventricular pressure

= ventricular chamber radius

= ventricular wall thickness

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PHYSIOLOGY OF HEART CONTRACTILITY| Tutorial D-1 CVS

130110110177|Gabriella Chafrina| 09/05/1

PressureVolume Loops

Normal Left Ventricular pressure-volume loops:

Early diastole: mitral valve openfilling of chamber

During diastole: volume small rise in pressure --> in accordance with passive length-tension propertie

or COMPLIANCE

Compliance: intrinsic property of a chamber that describes its pressure-volume relationship during filling.

Reflects the ease or difficulty with which the chamber can be filled. Strict definition: Compliance = volum

: pressure

Early systole: LV pressure exceeds LA pressuremitral valve is forced to close

Pressure continue to increase, the ventricular volume does not immediately change (because the aorticvalve has not yet opened) = ISOVOLUMETRIC CONTRACTION

During systole: ventricle pressure > aortic pressureaortic valve forced to openejection of blood

During ejection: volume in ventricle , but its pressure continue to , until ventricular relaxations begins

Ends of ejection: during relaxation phaseventricular pressure falls below aortaaortic valve closed

Ventricle continue to relaxpressure , volume remains constant (because the mitral valve has not yet

opened) = ISOVOLUMETRIC RELAXATION

Difference between end-diastolic volume (EDV) and end-systolic volume (ESV) = Stroke Volume

Changes in any of the determinants of cardiac function are reflected by alterations in the pressure-volume

loop (by analyzing the effects of a change in preload, afterload, and myocardial contractility)

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PHYSIOLOGY OF HEART CONTRACTILITY| Tutorial D-1 CVS

130110110177|Gabriella Chafrina| 09/05/1

1. Alterations in Preload

Preload (afterload and contractility constant)LV EDV SV (Frank-Starling mechanism) and

constant ESV

Normal LV is able to adjust its SV, as long as contractility and afterload keep constant

Relationship between filling volume and pressure:

Poorly compliant ability of the chamber to fill during diastole EDV SV and ESV unchanged2. Alterations in Afterload

Afterload (preload and contractility constant)pressure during ejection ventricle work

Afterload LV systolic pressure LV ESV SV

3. Alterations in Contractility

contractilityventricle empties more completely / SV ESV

ESV is dependent on the afterload against which the ventricle contracts and the inotropic state, but is

independent of the EDV prior to contraction

4. Important physiologic concepts:

Ventricular stroke volume is a function of preload,afterload and contractility. SV rises when there is an

increase in preload a decrease in afterload or augmented contractility Ventricular end-diastolic volume (or end-diastolic pressure) is used as a representation of preload the end

diastolic volume is influenced by the chambers compliance

Ventricular end-systolic volume depends on the afterload and contractility but not on the preload