Define preload and describe the determinants of preload.

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College Answer

A definition based on stretch of the isolated myocyte prior to contraction, and 
extrapolation to the human heart, was expected. Surrogate measures of preload 
used in clinical practice needed to be explained and related to the definition (for 
example, end-diastolic volume and central venous pressure). The Frank-Starling 
Law was relevant to discussion of the significance of preload to cardiac performance.
A diagram illustrating the interaction of important factors would have been helpful in 
answering this question. At a minimum, detail should have included atrial 
contractility, diastolic filling time, ventricular compliance, and the determinants of 
venous return.
Better answers included discussion of the effects of afterload, arrhythmias, and 
valvular pathology. A distinction between the factors determining left and right
ventricular preload would have demonstrated a more sophisticated understanding of 
the physiology.
Syllabus: C1c, 2b,c.
Reference: Cardiovascular Physiology, Berne and Levy, p64-65

Discussion

It is not remarkable that only 10% of the candidates passed this question, because preload is something that has always defied 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 decreases preload) 
        • 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

References

Norton, James M. "Toward consistent definitions for preload and afterload."Advances in physiology education 25.1 (2001): 53-61.

ROTHE, CARL. "Toward consistent definitions for preload and afterload—revisited." Advances in physiology education 27.1 (2003): 44-45.