Define cardiac preload and describe its determinants.
College Answer
This question required synthesis and application of knowledge derived from multiple sources
rather than regurgitation of a published list in a text. Many candidates failed to recognise that
venous return is not the only determinant of preload. Most candidates failed to discuss
determinants of venous return. Factors such as contractility, afterload or chamber filling and
emptying can all impact preload. In addition to listing determinants the question required an
explanation of their relationship with preload (e.g. the direction of change).
A discussion about the determinants of cardiac output was not asked for as did not score marks.
Discussion
It is just as well that "regurgitation of a published list in a text" was not required here, because one would be hard-pressed to find a good text to regurgitate. Indeed, it seems unreasonable for the examiners to ask for a definition of a concept which does not have a widely accepted definition.
- Preload can be defined as:
- Myocardial sarcomere length just prior to contraction, for which the best approximation is end-diastolic volume
- Tension on the myocardial sarcomeres just prior to contraction, for which the best approximation is end-diastolic pressure
- The determinants of preload, if we choose to define it as a volume, are:
- Pressure filling the ventricle:
- Intrathoracic pressure (high pressure decreases preload)
- Atrial pressure
- Atrial contractility and rhythm (SR increases preload, AF decreases preload)
- Atrioventricular valve competence (mitral and tricuspid stenosis decrease preload)
- Ventricular end-systolic volume (increased end-systolic volume increases preload by adding to venous return)
- Ventricular compliance (decreased compliance decreases preload)
- Right atrial pressure (high right atrial pressure increases preload)
- Mean systemic filling pressure
- Total venous blood volume (increased volume increases preload)
- Venous vascular compliance (decreased compliance, i.e. venoconstriction, increases preload)
- Cardiac output, insofar as it supplies the total blood volume
- Compliance of the ventricle:
- Pericardial compliance:
- Compliance of the pericardial walls (decreased compliance decreases preload)
- Compliance of the pericardial contents (increased contents volume decreases preload)
- Ventricular wall compliance:
- Duration of ventricular diastole (a longer diastole increases preload)
- Wall thickness (a thicker wall decreases preload)
- Relaxation (lusitropic) properties of the muscle (poor relaxation decreases preload)
- End-systolic volume of the ventricle (i.e. afterload) - increased end-systolic volume decreases preload