What is Preload?
Preload represents the initial stretching of cardiac muscle fibers (myocytes) at end-diastole, just before contraction begins, which directly determines the force of ventricular contraction through the Frank-Starling mechanism. 1
Physiological Definition
At the sarcomere level, preload reflects the length of myocardial fibers immediately before systolic contraction, establishing the starting point from which the ventricle generates force. 1 This initial fiber length is determined primarily by the left ventricular end-diastolic volume (LVEDV)—the amount of blood filling the chamber at the end of diastole. 2
The Frank-Starling mechanism explains why preload matters clinically: greater myocardial stretch at end-diastole produces stronger contraction and increased stroke volume, up to physiological limits. 1 This relationship between initial fiber length and contractile force is fundamental to cardiac performance.
Clinical Measurement Challenges
Why Pressure Measurements Are Unreliable
A critical pitfall in clinical practice is equating filling pressures with preload. Central venous pressure (CVP) and pulmonary capillary wedge pressure are generally insensitive indicators of actual preload status. 1
The relationship between left ventricular end-diastolic pressure (LVEDP) and LVEDV is curvilinear, not linear—meaning the same pressure can correspond to vastly different volumes depending on ventricular compliance. 2, 3 Low CVP may suggest hypovolemia, but elevated CVP does not reliably indicate adequate preload or predict fluid responsiveness. 1, 3
Superior Assessment Methods
Echocardiography provides direct visualization of ventricular dimensions and end-diastolic volume, making it the preferred method for assessing preload. 1 Volumetric measures like intrathoracic blood volume or global end-diastolic volume offer higher validity than pressure-based estimates. 3, 4
For predicting fluid responsiveness in mechanically ventilated patients, dynamic indicators (stroke volume variation, pulse pressure variation) are more reliable than static parameters like CVP. 1, 3
The Concept of Preload Reserve
Preload reserve represents the heart's capacity to increase stroke volume in response to increased venous return—essentially the difference between current LVEDV and maximal possible LVEDV. 1, 2
This concept has major clinical implications:
- When preload reserve is exhausted (the ventricle is already operating near maximal volume), further fluid administration will not increase stroke volume. 2
- In chronic heart failure, the dilated ventricle operates near its maximal volume, having exhausted most preload reserve. 5
- Minor or absent preload reserve indicates minimal or no benefit from intravenous fluid administration. 2
Preload in Disease States
Heart Failure with Reduced Ejection Fraction
In heart failure, the ability to augment LVEDV during exercise is blunted because the already dilated ventricle operates near maximal volume. 5 The reduced ability to increase preload during stress contributes significantly to exercise intolerance. 5
Chronic Volume Overload (Aortic Regurgitation)
The left ventricle compensates for chronic volume overload by increasing end-diastolic volume and developing eccentric hypertrophy with addition of new sarcomeres. 5, 1 This adaptation maintains preload at the sarcomere level near normal despite marked chamber enlargement, allowing normal ejection performance for years or decades. 5, 1
Fontan Circulation
Despite central venous hypertension, Fontan patients exist in a ventricular preload-deficient state because systemic venous return must traverse the pulmonary circuit before filling the systemic ventricle, resulting in chronically low end-diastolic volume. 5
Practical Clinical Applications
Perioperative Management
In mitral stenosis, preload must be maintained high enough to allow adequate forward flow across the stenotic valve but low enough to avoid pulmonary edema—requiring invasive hemodynamic monitoring with cardiac output and wedge pressure measurement. 1
For aortic stenosis, maintaining adequate preload is crucial because decreased coronary perfusion pressure combined with inadequate preload can precipitate myocardial ischemia or circulatory collapse. 1
Exercise Physiology
During high-intensity exercise in healthy individuals, stroke volume increases primarily through elevation of end-diastolic volume (increased preload) via the Frank-Starling mechanism. 1 This makes preload augmentation the most important factor for cardiac output during exercise. 1