Preload

Preload represents the stretch on the muscle just prior to its contraction. Preload in the intact heart is  influenced by a number of factors that impact on ventricular filling (and hence end-diastolic stretch). These factors include:

•  blood volume, 
• venous capacitance,
•  ventricular compliance,
•  atrial function,
•  valvular function, 
•   heart rate.

The Frank-Starling relationship demonstrates increasing stroke volume

with increasing preload. Notice that stroke volume is more sensitive to changes in

preload at low values compared to high values:

How do we estimate preload clinically? With the widespread use of

transesophageal echocardiography (TEE), it is possible to get a reasonable

estimate of chamber size and, hence, preload. In the absence of

TEE, we substitute the pressure at end-diastole with the assumption

that the higher the pressure, the greater the size and the greater the

stretch on the contractile elements . EDP can be assessed clinically by

measuring the pulmonary capillary wedge pressure (PCWP) using a

Swan-Ganz catheter that is placed through the right ventricle (RV) into

the pulmonary artery (PA). When the balloon at the tip of the

catheter is inflated, it occludes flow in that small branch of the PA; the

pressure distal to the balloon quickly equilibrates with the pressure in

the pulmonary venous system. As the risk of pulmonary rupture and

hemorrhage is increased with anticoagulation, the pulmonary arterial

diastolic pressure is often used instead of the PCWP. In the absence of

a PA catheter, central venous pressure is used to assess volume status

with the assumption that there is a positive association between changes

in central venous pressure and LV size.

(On Bypass : Advanced Perfusion Techniques, edited by Linda B. Mongero)

Normal:

Pulmonary capillary wedge pressure 2-15 mmHg

Pulmonary artery pressure sys. 15 - 30 mmHg, diast 4-12 mmHg

Central venous pressure 3-8 mmHg