Preload (cardiology)

In cardiac physiology, preload is the pressure stretching the ventricle of the heart, [eMedicineDictionary|ventricular+preload] after atrial contraction and subsequent passive filling of the ventricle. 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.

Calculation

Quantitatively, preload can be calculated as

:frac{LVEDP cdot LVEDR}{2h}

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).

See also

* Afterload
* Cardiac output
* Frank-Starling law of the heart

References

External links

* [http://www.cvphysiology.com/Cardiac%20Function/CF007.htm Overview at cvphysiology.com]


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