Monday, May 18, 2009

Starling's Law of The Heart.

When heart rate is constant, cardiac output is directly proportional to left ventricular end-diastolic volume (preload) unless excessive volumes are reached.

Determinants of ventricular filling
  1. Venous return
  2. Blood volume
  3. Distribution of blood volume- Posture, intrathoracic pressure, pericardial pressure, venous tone
  4. Rhythm of atrial contraction
  5. Heart rate
As most of the other factors affecting venous return are fixed, the major determinant of right ventricular preload is venous tone. In the absence of significant pulmonary or right ventricular dysfunction, venous return (affected by venous tone, etc) is the major determinant of left ventricular preload. Normally, the end-diastolic volume of both ventricles is the same.

Ventricular end-diastolic volume (preload) is difficult to measure clinically. An estimation can be made with the left ventricular end-diastolic pressure only if the relationship between ventricular volume and pressure (compliance) is constant. However, ventricular compliance is usually nonlinear and many conditions may reduce ventricular compliance. However, measurements of LVEDP or pressures approximating it is the most common way of estimating preload. Central venous pressure can be used as a measure of preload in most healthy individuals.

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