What is preload?

Understanding Preload in Cardiac Function

Preload is defined as all factors that contribute to passive ventricular wall stress (or tension) at the end of diastole, primarily determined by the end-diastolic volume of the ventricular chamber. 1

Physiological Basis of Preload

• Preload represents the initial stretching of cardiac myocytes before contraction, which directly affects sarcomere length and subsequent contractile force 2
• At the sarcomere level, preload determines the initial length of myocardial fibers, influencing the force of contraction through the Frank-Starling mechanism 2
• Preload is a key determinant of stroke volume, as it establishes the starting volume from which the ventricle ejects blood during systole 2

Clinical Assessment of Preload

• Echocardiography provides direct visualization of ventricular dimensions, allowing assessment of end-diastolic volume as a measure of preload 2
• Inferior vena cava (IVC) diameter and collapsibility during respiration can provide qualitative estimates of preload status - collapse of 50-99% is normal, complete collapse may indicate volume depletion, and <50% collapse may suggest volume overload 2
• Static pressure measurements like central venous pressure and pulmonary capillary wedge pressure are generally insensitive indicators of preload status, as low values may reflect hypovolemia but high values don't necessarily indicate volume overload 2

Preload in Heart Failure

• In heart failure, the ventricle may develop eccentric hypertrophy with addition of new sarcomeres to maintain preload reserve despite chamber dilation 2
• Preload reserve represents the heart's ability to increase stroke volume in response to increased venous return, which can become exhausted in advanced heart failure 2
• In aortic regurgitation, the greater diastolic volume permits the ventricle to eject a large total stroke volume to maintain forward stroke volume, accomplished through rearrangement of myocardial fibers and eccentric hypertrophy 2

Dynamic Assessment of Preload

• Dynamic indicators of preload responsiveness (like stroke volume variation) are more reliable than static parameters for predicting fluid responsiveness in critically ill patients 2
• Volumetric estimates of preload, such as intrathoracic blood volume or global end-diastolic volume measured by transpulmonary thermal-dye indicator dilution, are more predictive of volume status than pressure-based measurements 3
• Passive leg raising or respiratory systolic variation tests can be used as alternative methods to assess preload responsiveness when other dynamic parameters have limitations 4

Clinical Implications of Preload Management

• In hypovolemic shock, echocardiography can rapidly document a small hyperdynamic unloaded ventricle with reduced left ventricular end-diastolic area, indicating decreased preload 2
• Optimal preload management is crucial in perioperative cardiac surgery, as heart failure cannot be properly assessed unless volume loading is optimal 2
• In right ventricular failure, patients may require higher right ventricular end-diastolic pressure (preload), though reduced left heart filling is more likely caused by right ventricular dilation and ventricular interdependence than reduced right ventricular forward output 2

Preload and Ventricular Interdependence

• Ventricular interdependence refers to forces directly transmitted from one ventricle to the other through the myocardium and pericardium 2
• In right ventricular volume overload, right ventricular dilation can compress the left ventricular cavity, impeding left ventricular filling and equalizing biventricular diastolic pressures 2
• Increased transmural pressure caused by right ventricular dilation with pericardial constraint impairs left ventricular filling (preload), affecting overall cardiac output 2