The Preload is the force which, during the relaxation and filling phase of the heart (diastole), stretches the fibers of the heart chambers, which can contract. As a result, the preload is involved in the basic function of the human heart, which, as a vital organ, pumps blood through the body. Backward failure in the context of a weak heart can result in complications such as edema in the lungs.
What is the preload?
The 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 able to contract and thereby shorten: they are contractile.
The preload ensures that the muscle fibers stretch after they were previously tense and thus shortened. Therefore, the maximum elongation of these heart muscle fibers corresponds to their maximum length at rest; In other words, the fibers do not behave like a rubber band, the length of which is less at rest than when it is under tension, but exactly the other way around.
The heart muscles are part of the smooth muscles that are found in some other organs and in the walls of blood vessels in the human body. In contrast to the striated (skeletal) muscles, the smooth muscles have contractile elements that are not easily recognizable from the outside and thus form the eponymous smooth surface.
The preload acts at the end of the relaxation phase, also known as diastole. The ventricles and atria of the heart do not relax at the same time: while the fibers of the ventricles stretch, 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 & task
A number of factors and biological functions depend on the preload or in turn influence this force. First of all, the preload belongs in the broadest sense to the beating rhythm of the heart and thus helps it to pump blood through the body. The blood reaches the organ through veins and leaves it through arteries. The veins of the pulmonary circulation - also called the small circulation - transport oxygen-rich blood, while the veins of the body circulation or large blood circulation transport oxygen-poor blood. Due to the different physical properties, oxygen-rich blood has a lighter shade of red than oxygen-poor blood.
In connection with the preload, central venous pressure plays an important role, along with other factors. Central venous pressure is the blood pressure in the superior vena cava (vena cava superior) and in the right atrium of the heart. A measurement using 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 have a potential to affect the preload. The return flow of the venous blood also influences the preload.
Together with other factors, the preload in turn influences the stroke volume of the heart, among other things. In medicine, stroke volume is the blood capacity that the heart pumps out of the chamber into the arteries. This process takes place during the tension phase (systole), when the hollow organ drives the blood out of itself.
The heartbeat volume changes as a function of physical exertion, with doctors generally assuming a normal value of 70-100 ml. However, the individual ideal value can deviate from this guide value. When calculating the stroke volume, a formula helps that subtract the left ventricular end-systolic volume (i.e. the filling volume of the left ventricle at the end of a systole) from the left ventricular end-diastolic volume (i.e. the filling volume of the left ventricle at the end of a diastole). The right and left ventricles normally have approximately the same stroke volume, which is due to the Frank Starling mechanism, for which the afterload is important in addition to the preload.
Illnesses & ailments
The preload plays a role in connection with various diseases, the effects and causes of which do not have to be limited to the heart alone. Taking dehydrating agents or diuretics can reduce the preload and thereby affect the function of the heart. The same seems to apply to nitrates. Angiotensin converting enzyme inhibitors (ACE inhibitors) and other drugs can also cause this effect.
If the heart is weak (heart disease), the pressure in the heart chambers can rise or the end-diastolic volume can increase. One possible consequence is a so-called backward failure, which is characterized by increased filling pressure in the heart chamber while at the same time normal ejection performance. The Forrester classification, which divides acute cardiac insufficiency into different classes, assigns backward failure to class II. The increased filling pressure in the heart chamber is accompanied by a backlog of blood. Both chambers of the heart, just the right one, or just the left one can be affected.
In addition to backward failure, there is also forward failure, which can also be the result of cardiac insufficiency and corresponds to class III in the Forrester classification.
Edema in the periphery or the lungs can manifest itself as a result of backward failure - however, numerous other causes can also be considered for such water retention. People who suffer from such symptoms cannot therefore automatically conclude from the presence of these or similar symptoms that they have heart failure.
In any case, signs of illness require individual medical clarification. The same applies to treatment options, which can differ in individual cases depending on the patient and conditions.
