In cardiac physiology, preload is the pressure stretching the ventricle of the heart,[1] after passive filling of the ventricle and subsequent atrial contraction. If the chamber is not mentioned, it is usually assumed to be the left ventricle.
Preload is theoretically most accurately described as the initial stretching of a single cardiac myocyte prior to contraction. This cannot be measured in vivo and therefore other measurements are used as estimates. Estimation is inaccurate, for example in a chronically dilated ventricle new sarcomeres 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 laboratory term preload.
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Calculation
Quantitatively, preload can be calculated as
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 Law of Laplace.
Factors affecting preload
Preload is affected by venous blood pressure and the rate of venous return. These are affected by venous tone and volume of circulating blood.
Preload relates to the ventricular end-diastolic volume: a higher end-diastolic volume implies a higher preload, but the relationship is not simple, because of the restriction of the term preload to single myocytes. Single myocytes are not connected simply, but in an interdigitated manner.
Preload increases with exercise (slightly), increasing blood volume (overtransfusion) and excitement (sympathetics).
An arteriovenous fistula can increase preload.[2]
See also
- Afterload
- Cardiac output
- Frank-Starling law of the heart
- Passive leg raising test
- File:PVcurve.bmp
References
- ^ ventricular+preload at eMedicine Dictionary
- ^ "Pulmnary: Heart Failure". http://www.indyrad.iupui.edu/rtf/teaching/medstudents/stf/pul1/pul1.htm. Retrieved 2008-12-21.
External links
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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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