Cardiac preload refers to the normal stretching of the muscle fibers of the heart as it expands from filling with blood. It cannot be directly measured, so indirect measurements are used, such as the volume or pressure of the heart when it is completely full of blood prior to contracting. Preload can be thought of as similar to the stretch in a rubber band. If cardiac preload is insufficient or, in other words, if the initial stretch of the heart is inadequate, the heart will not contract as forcefully.
Volume Deprivation
According to "Harrison's Principles of Internal Medicine," by Anthony Fauci, M.D., one of the most common causes of preload insufficiency is volume deprivation, as with blood loss. If there is a low amount of blood in the vessels, less will be delivered to the heart. The heart will not fill completely, not stretch and have a decreased preload.
Valve Problems
The heart has four chambers, two atria and two ventricles. One atria and one ventricle are located on the right and left sides of the heart. The ventricles are located below the atria and provide the pumping pressure that delivers blood to the body. Valves separate the atria from the ventricles. If the valves are narrowed, the atria cannot effectively deliver blood to the ventricles, resulting in decreased preload.
Atrial Problems
The atria contract to deliver blood to the ventricles, which then contract to deliver blood to the body. If the atria are not functioning properly, they will not deliver an optimal amount of blood, and hence less preload, to the ventricles. Atrial fibrillation is a heart rhythm problem in which the atria quiver and contract erratically instead of in a coordinated fashion and do not deliver blood adequately to the ventricles.
Increased Heart Rate
If the heart is contracting at a normal rate, it has time to fill completely. At faster rates, the filling time is decreased, which results in a lower preload, according to "Hurst's The Heart,' by Valentin Fuster, M.D.
Stiff Ventricles
Problems with the ventricle can also cause decreased preload. Since preload involves the stretch of the ventricles, if they become stiff or less distensible, the preload will decrease. In diastolic heart failure, the heart has the ability to adequately squeeze blood out to the body but does not relax well. This impairs blood filling the heart. According to "Harrison's Principles of Internal Medicine," in amyloidosis, a protein called amyloid is deposited throughout the tissues of the body, including the heart. The amyloid makes the heart thick and less stretchy, so it cannot develop adequate preload.
Decreased Back-Pressure on the Heart
When the ventricle contracts, it ejects blood into the vessels. However, it does not normally eject all the blood. It normally ejects about 55 percent of the blood in the chamber, according to "Hurst's The Heart." The heart has to push against pressure in the blood vessels in order to eject the blood. The pressure the heart has to overcome is called the afterload. If the afterload is too low, all the blood will be ejected from the heart. In that case, it will not fill up all the way. In other words, the forward flow out of the heart will be too high, and the preload will drop.
References
- "Harrison's Principles of Internal Medicine"; Anthony S. Fauci et al.; 2008
- "Hurst's The Heart"; Valentin Fuster, Robert O'Rourke, Richard Walsh and Philip Poole-Wilson; 2007
