Question 4

College Answer

Contractility represents the performance of the heart at a given preload and afterload. It is 
the change in peak isometric force (isovolumic pressure) at a given initial fibre length (end 
diastolic volume). All indices of myocardial contractility are dependent on preload or 
afterload to a varying degree. The dP/dT is the maximum rate of change in left ventricular 
pressure during isovolumetric contraction, after mitral valve closes and before the aortic 
valve opens. It is preload dependant and afterload independent. A diagram of a pressurevolume loop is very helpful when describing the ESPV. Absence of a diagram (correctly 
labelled and scaled) was a weakness in many answers. Candidates were then expected to at 
least explain that, as preload is increased a new pressure volume loop is generated. Each 
new PV loop has a new end systolic point that is at a slightly higher pressure and volume 
than the previous end systolic point. The line connecting the end-systolic points is called the 
linear ESPVR. The slope of the ESPVR or Emax is used as an index of myocardial contractility. 
Ejection fraction is the percentage of the ventricular end diastolic volume (EDV) which is 
ejected with each stroke volume (SV). Ejection fraction = stroke volume/end diastolic 
volume X 100 (Normal range 55 to 70%). Only a minority of candidates achieved the depth 
of knowledge required for a Level 1 topic

Discussion

• Contractility is the change in peak isometric force (isovolumic pressure) at a given initial fibre length (end diastolic volume) - from Pappano & Weir (p.78 of the 10th edition) 
• Physiological determinants of contractility include:
  • Preload:
    • Increasing preload increases the force of contraction 
    • The rate of increase in force of contraction per any given change in preload increases with higher contractility
    • This is expressed as a change in the slope of the end-systolic pressure volume relationship (ESPVR)
  • Afterload (the Anrep effect):
    • The increased afterload causes an increased end-systolic volume
    • This increases the sarcomere stretch 
    • That leads to an increase in the force of contraction
  • Heart rate  (the Bowditch effect):
    • With higher hear rates, the myocardium does not have time to expel intracellular calcium, so it accumulates, increasing the force of contraction.
• Contractility is also dependent on:
  • Myocyte intracellular calcium concentration 
    • Catecholamines: increase the intracellular calcium concentration by a cAMP-mediated mechanism, acting on slow voltage-gated calcium channels 
    • ATP availability (eg. ischaemia):  as calcium sequestration in the sarcolemma is an ATP-dependent process
    • Extracellular calcium- availability of which is necessary for contraction
  • Temperature: hypothermia decreases contractility, which is linked to the temperature dependence of myosin ATPase and the decreased affinity of catecholamine receptors for their ligands.
• Measures of contractility include:
  • ESPVR, which describes the maximal pressure that can be developed by the ventricle at any given LV volume.  The ESPVR slope increases with increased contractility.
  • dP/dT (or ΔP/ΔT), change in pressure per unit time. Specifically, in this setting, it is the maximum rate of change in left ventricular pressure during the period of isovolumetric contraction. This parameter is dependent on preload, but is minimally affected by normal afterload.

As the "absence of a diagram (correctly labelled and scaled) was a weakness in many answers", a suggested diagram is produced here, which the author attempted to reconstruct as best he could from what the examiners wrote. 

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