What does preload mean in cardiac physiology?

Preload in Cardiac Physiology

Preload refers to the initial stretching of cardiac myocytes before contraction, which directly affects sarcomere length and subsequent contractile force through the Frank-Starling mechanism. 1

Definition and Physiological Basis

• Preload represents the degree of myocardial fiber stretch at the end of diastole, just before ventricular contraction, determining the initial length of myocardial fibers 1
• At the sarcomere level, preload influences the force of contraction through the Frank-Starling mechanism, which states that increased stretch leads to increased contractile force 1
• Preload is a key determinant of stroke volume, as it establishes the starting volume from which the ventricle ejects blood during systole 1

Clinical Measurement and Assessment

• Left ventricular end-diastolic volume (LVEDV) is the most direct clinical representation of preload, reflecting the volume that stretches the ventricle at the end of diastole 2
• There is a curvilinear relationship between left ventricular end-diastolic pressure (LVEDP) and LVEDV, with higher pressures at near-maximal volumes 2
• While central venous pressure (CVP) is often used clinically as a surrogate for preload, it has significant limitations as it only indirectly reflects ventricular filling 3
• Echocardiography provides direct visualization of ventricular dimensions, allowing assessment of end-diastolic volume as a measure of preload 1

Preload Reserve

• Preload reserve represents the heart's ability to increase stroke volume in response to increased venous return 1
• It is defined as the difference between the current and the maximal possible LVEDV 2
• In heart failure, the ventricle may develop eccentric hypertrophy with addition of new sarcomeres to maintain preload reserve despite chamber dilation 4, 1
• As heart failure progresses, preload reserve may become exhausted, limiting the heart's ability to increase stroke volume in response to increased filling 4

Clinical Implications

• In aortic regurgitation, the left ventricle responds with compensatory mechanisms including increased end-diastolic volume and chamber compliance to accommodate increased volume without raising filling pressures 4
• The greater diastolic volume permits the ventricle to eject a larger total stroke volume to maintain forward stroke volume in the normal range 4
• In heart failure, preload augmentation may precipitate decompensation, necessitating preload reduction therapies 5
• During exercise in healthy individuals, stroke volume increases primarily through increased end-diastolic volume (preload), whereas patients with heart failure have limited preload reserve and rely more on heart rate increases 4

Comprehensive Understanding

• From a mechanical perspective, preload can be defined as all factors that contribute to passive ventricular wall stress at the end of diastole 6
• Preload is differently recruited in response to various degrees of hemodynamic overload and typically parallels afterload, in accordance with the concept of preload-afterload mismatch 7
• Minor or absent LV preload reserve indicates that there will be minimal or no increase in stroke volume following intravenous fluid administration 2
• Optimal preload management is crucial in perioperative cardiac surgery and critical care settings 1