Preload

The heart is known to be one of the most important muscles in the body. It’s made up of fibres which can easily expand and contract to fill and empty the blood in the heart chambers. There are four chambers present in the heart, two atria and two ventricles. The atria are commonly known to be the pathway for the blood which has to be received by the body and reaches to ventricles and causes its contraction, during which time the ventricles help in pumping the blood out into the body.

The term preload is defined as the initial stretching of the muscle which is present in the heart before its contraction. This term preload is related to the filling up of ventricles of the heart. 

As soon as the relaxed ventricle, fills during the process of diastole, the walls of the heart stretch and the length of the sarcomeres increases. The length of the sarcomere can be considered as approx by the volume of the ventricle since each shape has a conserved surface area/volume ratio. This is clinically useful because measuring sarcomere length can destroy the tissue of the heart to some extent. To be able to view the sarcomeres under the microscope, a piece of heart muscle must be severed. Direct measurement of the bias voltage in the beating heart of a living animal is not currently possible. Preload can be easily calculated from ventricular end-diastolic pressure and measured in millimetres of mercury (mmHg).

Factors affecting preload

Preload can vary with different conditions. For example, it increases with continuous exercise for a long time, with increasing blood volume i.e with over transfusion,  and also with neuroendocrine excitement. It can be seen that the preload is also affected by two of the body pumps- 

• Respiratory pump – Intrapleural pressure is found to be decreased when there will be an inspiration and also abdominal pressure increases, squeezing local abdominal veins which allows the thoracic veins to expand in shape and increase blood flow towards the right atrium.

• Skeletal muscle pump – It is seen mostly in the leg of the body where deep veins are present that surround the muscles that squeeze the veins and pump blood to the heart. This happens mainly in the legs. Once blood flows through the valves, it cannot flow back and so blood is pumped back to the heart.

Preload Formula

By monitoring the mean blood pressure of the atrium in the correspondence, the preload value can be calculated. The measurement of preload is done by taking into consideration central venous pressure (CVP), which is for the right ventricle. When we consider the left ventricle, the preload can be easily measured by taking the consideration of PAOP which is commonly known as pulmonary artery occlusion pressure, which is also referred to as pulmonary capillary wedge pressure (PCWP) or pulmonary artery wedge pressure (PAWP).

The end-diastolic volume of the heart is measured to estimate the preload value.

We can easily relate the value of end-diastolic volume with the  value of end-diastolic pressure as we know that the pressure and volume are related to one another according to Boyle’s law, which can be simplified to;

P∝1/v

Terms

Preload- It is defined as the initial stretching of the muscle present in the heart, which occurs before contraction. Blood fills in the ventricle have a direct connection with preload.

Afterload- It is that type of force or load against which the heart will be contracting which is very necessary to eject the blood.

Contractility- It is defined as the intrinsic strength of the cardiac muscle independent of preload, but a change in preload will affect the force of contraction.

Conclusion 

The preload is known to be an important function of the heart that makes up the amount of blood that is to be ejected from the heart per minute. The concept of preload can be applied to any of the chambers whether it is the ventricles or atria. In any type of chamber, the preload is related to the volume of the chamber, and therefore the length of sarcomere also, just before contraction. Cardiac preload is of two types i.e left ventricular preload and right ventricular preload. 

The measurement of left ventricular preload is denoted as pulmonary arterial wedge pressure (PAWP). Normal PAWP will vary between 6 and 12 millimetres of mercury (mmHg). Measurement of right ventricular preload is denoted as right arterial pressure (RAP). Normal levels of RAP vary from 2 to 7 mmHg. Some Important terms to keep in mind: