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| [[Image:Heart_diastole.png|thumb|Heart during ventricular diastole.]]
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| In [[cardiology|cardiac]] [[physiology]], '''preload''' is the end volumetric pressure that stretches the right or left [[Ventricle (heart)|ventricle]] of the [[heart]] to its greatest geometric dimensions under variable physiologic demand. <ref>{{cite journal |last1=Leucke |first1=thomas |last2=Roth |first2=Harry |year=1993 |title=Assessment of cardiac proload and left ventrical function under increasing levels of positive end-expiratory pressure |journal=Intensive Care Medicine |volume=30 |issue=1 |pages=119–126 |url=http://www.springerlink.com/content/2qwhwh6vvq698m70/ |pmid= |accessdate=July 2011 }}</ref> In other words, it is the initial stretching of the [[cardiomyocytes]] prior to contraction; therefore, it is related to the sarcomere length at the end of diastole. Parameters such as ventricular end diastolic volume or pressure are used to measure preload since the ideal length of the cardiac sarcomere cannot be measured. Passive filling of the (heart) [[Ventricle (heart)|ventricle]] and subsequent atrial contraction thus allows an echocardiographically volumetric measurement.
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| Preload is theoretically most accurately described as the initial stretching of a single [[cardiomyocyte]] prior to contraction. This cannot be measured ''[[in vivo]]'' and therefore other measurements are used as estimates. Estimation may be inaccurate, for example in a chronically dilated ventricle new [[sarcomere]]s may have formed in the heart muscle allowing the relaxed ventricle to appear enlarged. The term [[end-diastolic volume]] is better suited to the clinic, although not exactly equivalent to the strict definition of ''preload''. Atrial pressure is a surrogate for preload.
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| ==Calculation==
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| Quantitatively, preload can be calculated as
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| :<math>\frac{LVEDP \cdot LVEDR}{2h}</math>
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| where LVEDP = [[Left ventricular end diastolic pressure]], LVEDR = Left ventricular end diastolic radius (at the ventricle's midpoint), and h = thickness of the ventricle. This calculation is based on the [[Young-Laplace_equation#Application_in_medicine|Law of Laplace]] which states that (wall stress) = <math>\frac{(pressure) \times (radius)}{2 \times (wall \ thickness)}</math>. Hence, ''preload'' is the ''wall stress''. Preload is measured in pressure units (mm Hg).
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| ==Factors affecting preload==
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| Preload is affected by venous blood pressure and the rate of venous return. These are affected by venous tone and volume of circulating blood.
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| Preload is related to the ventricular [[end-diastolic volume]]; a higher end-diastolic volume implies a higher preload. However, the relationship is not simple because of the restriction of the term '''preload''' to single myocytes. Preload can still be approximated by the inexpensive echocardiographic measurement [[end diastolic volume]] or EDV.
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| Preload increases with exercise (slightly), increasing blood volume (overtransfusion, [[polycythemia]]) and neuroendocrine excitement (sympathetic tone).
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| An [[arteriovenous fistula]] can increase preload. <ref name="urlPulmnary: Heart Failure">{{cite web |url=http://www.indyrad.iupui.edu/rtf/teaching/medstudents/stf/pul1/pul1.htm |title=Pulmnary: Heart Failure |work= |accessdate=2008-12-21}}</ref>
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| Preload is also affected by two main body "pumps."
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| '''The Respiratory pump''' - Intrathoracic pressure decreases during inspiration and abdominal pressure increase, squeezing local abdominal veins, allowing thoracic veins to expand and increase blood flow towards the right atrium.
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| '''Skeletal muscle pump''' - In the deep veins of the legs, surrounding muscles squeeze veins and pump blood back towards the heart. This occurs most notably in the legs. Once blood flows past valves it cannot flow backwards and therefore blood is “milked” towards the heart.
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| == See also ==
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| * [[Afterload]]
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| * [[Cardiac output]]
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| * [[Frank–Starling law of the heart]]
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| * [[Passive leg raising test]]
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| ==References==
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| {{reflist}}
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| ==External links==
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| * [http://www.cvphysiology.com/Cardiac%20Function/CF007.htm Overview at cvphysiology.com]
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| [[Category:Cardiovascular physiology]]
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| {{Cardiovascular physiology}}
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| <!-- simply, the load or volume exerted on the ventricleat end diastole;determines the force of contraction;Frank–Starling Law(as with valvular regurtation and/or septal defects
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