Preload is the force that stretches the fibers of the heart‘s ventricles during the relaxation and filling phases of the heart (diastole), which can contract. Thus, preload is involved in the basic function of the human heart, which is a vital organ that pumps blood throughout the body. In reverse failure in heart failure, complications such as edema in the lungs can occur.
What is preload?
Preload is the force that stretches the fibers of the heart’s chambers, which can contract, during the heart’s relaxation and filling phase (diastole). Preload is a force that acts on certain fibers of the heart muscle. These fibers are located in the walls of the heart chambers and are capable of contracting and thus shortening: they are contractile. The preload causes the muscle fibers to stretch after they were previously tense and thus shortened. Therefore, the maximum stretch of these cardiac muscle fibers corresponds to their maximum length at rest; that is, the fibers do not behave like a rubber band whose length at rest is less than under tension, but exactly the opposite. The heart muscle belongs to the smooth muscle, which in the human body is also found in some other organs and in the walls of blood vessels. In contrast to striated (skeletal) muscle, smooth muscle has contractile elements that are not readily visible from the outside and thus form the smooth surface that gives it its name. Preload acts at the end of the relaxation phase, also known as diastole. However, the ventricles and atria of the heart do not relax at the same time: while the fibers of the ventricles are dilating, the atria are in a tense state (systole). During diastole, the relaxed cavities of the heart fill with blood, which flows through veins to the vital organ.
Function and purpose
A number of factors and biological functions depend on preload or in turn influence this force. First of all, preload, in its broadest sense, is part of the beating rhythm of the heart and thus contributes to its ability to pump blood throughout the body. Blood enters the organ through veins and leaves it through arteries. In this process, the veins of the pulmonary circulation – also called the small circulation – transport oxygen-rich blood, while the veins of the systemic circulation or large circulation transport oxygen-poor blood. Due to the difference in physical properties, oxygenated blood has a brighter red hue than deoxygenated blood. In connection with preload, central venous pressure plays an important role, along with other factors. Central venous pressure is the blood pressure in the superior vena cava and in the right atrium of the heart. Measurement by means of a central venous catheter can determine the blood pressure in the vein. Central venous pressure is considered an indicator of preload, and changes in superior vena cava blood pressure potentially affect preload. Venous blood return also affects preload. Together with other factors, preload in turn influences, among other things, the stroke volume of the heart. In medicine, stroke volume refers to the blood capacity that the heart pumps out of the ventricle and into the arteries. This process takes place during the tension phase (systole), when the hollow organ expels blood from itself. The volume of the heartbeat changes depending on physical exertion, whereby physicians generally assume a normal value of 70-100 ml. However, the individual ideal value may deviate from this standard value. A formula that subtracts the left ventricular end-systolic volume (i.e. the filling volume of the left ventricle at the end of systole) from the left ventricular end-diastolic volume (i.e. the filling volume of the left ventricle at the end of diastole) helps to calculate the stroke volume. Right and left ventricles normally have approximately the same stroke volume, which is due to the Frank-Starling mechanism, for which afterload is important in addition to preload.
Diseases and complaints
Preload plays a role in the context of various diseases, the effects and causes of which need not be limited to the heart alone. The use of dehydrating agents or diuretics may reduce preload and thereby affect cardiac function. The same appears to be true for nitrates.Angiotensin-converting enzyme inhibitors (ACE inhibitors) and other drugs can also cause this effect. In the context of heart failure (heart disease), the pressure in the heart chambers may increase or the end-diastolic volume may increase. A possible consequence is a so-called reverse failure, which is characterized by increased filling pressure in the ventricle while the ejection fraction remains normal. The Forrester classification, which divides acute heart failure into different classes, assigns reverse failure to class II. The increased filling pressure in the ventricle is accompanied by a backlog of blood. This can affect both chambers of the heart as well as only the right or only the left. In addition to backward failure, there is also forward failure, which can also be the result of heart failure and corresponds to class III in the Forrester classification. Edema in the periphery or lungs can manifest as a result of backward failure – although many other causes can be considered for such water retention. People who suffer from such symptoms cannot therefore automatically conclude that they have heart failure from the presence of these or similar symptoms. Signs of disease always require individual medical clarification. The same also applies to treatment options, which may differ in individual cases depending on the patient and conditions.
