One of the main functions of the heart and blood vessels is to supply the body with oxygenated, nutrient-rich blood. The ability of the heart to pump blood out to the body, called cardiac output or stroke volume, is determined by three characteristics, according to the University of Virginia School of Medicine. The first is how hard the heart muscle can pump, the second is the pressure the heart has to pump against in the arteries, called afterload. The third characteristic is preload, defined as the length of the heart muscle just before it starts to pump; it is measured clinically by the volume of blood in the left ventricle, the main pumping chamber, when it is most relaxed. For the most part, increased preload results in increased cardiac output, decreased preload results in decreased cardiac output. Preload can be altered by several factors.
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Cardiovascular Physiology Concepts, a website managed by Richard E. Klabunde, Ph.D., the author of a book by the same name, states changing the volume inside the left ventricle is the most important method of changing preload. Increasing the volume with fluids or blood products increases preload and, therefore, the amount of blood pumped out into the body. This is true only to a certain point; too much volume can cause heart failure. Dehydration or blood loss decreases preload and decreases cardiac output.
The left ventricle of the heart fills when it is most relaxed, called diastole. The slower the heart rate, the more time the heart has to be in a relaxed state and filling increases which results in increased preload, according to "Cardiovascular Physiology Concepts." On the other hand, fast heart rates, both normal and abnormal, decrease the filling time and preload decreases.
Left Ventricular Relaxation
The more the left ventricle is relaxed, the more it can distend, fill with blood and increase the preload. The heart can become stiff with certain diseases such as hypertension and diabetes and cannot fill normally with blood which decreases the preload. Keeping blood pressure and diabetes under good control is important in keeping ventricular relaxation within the normal range. Sometimes medications, such as beta-blockers, can be utilized to relax the heart muscle. Tanabe and colleagues demonstrated in a study published in the January 2005 "International Journal of Cardiology" that measuring preload by echocardiography, a sound wave test of the heart, can actually estimate the amount of left ventricular relaxation.
Alterations in Afterload
Changes in afterload, what the heart has to push against to get the blood out of the heart, can secondarily affect preload. Increased afterload, such as high blood pressure or narrowed heart valves, increases preload because more blood stays in the heart after it pumps. Often there is an excessively high preload in heart failure. Drugs that decrease afterload either by decreasing volume such as diuretics or by decreasing blood pressure including angiotensin converting enzyme inhibitors and nitrates can secondarily decrease preload because less blood is left in the left ventricle after pumping.