Cardiac Preload: Definition, Determinants, and Role in the Frank-Starling Mechanism

Cardiac preload is the ventricular wall stress (or stretch) at end-diastole, just before contraction. It reflects the volume of blood in the ventricle at the end of filling (end-diastolic volume, EDV).

Determinants

1. Venous return: ↑ VR → ↑ EDV → ↑ preload (Guyton's venous return curves)
2. Ventricular compliance: Stiff ventricle (hypertrophy, fibrosis, infiltrative) → ↓ compliance → ↑ pressure for same volume
3. Heart rate: Tachycardia ↓ diastolic filling time → ↓ EDV → ↓ preload
4. Blood volume: Hemorrhage ↓, fluid overload ↑ preload
5. AV valve function: Mitral/tricuspid regurgitation → volume overload → ↑ preload; stenosis → ↓ filling → ↓ preload
6. Pericardial constraint: Tamponade, constriction limits filling

Frank-Starling Mechanism

Within physiological limits, ↑ preload → ↑ sarcomere length → ↑ actin-myosin overlap → ↑ force of contraction → ↑ stroke volume. This autoregulates cardiac output to match venous return.

Clinical Assessment

• Surrogates: CVP, PAOP (wedge pressure), EDV on echo
• Dynamic measures (stroke volume variation, pulse pressure variation) better for fluid responsiveness

Preload optimization is central to hemodynamic management in critical care and surgery.