Preload, afterload, heart rate: the dark side of the force. When a stress echo is scheduled, interest is focused on wall motion segmental contraction abnormality to diagnose ischemic response to stress and on LVEF to assess contractile reserve. However, ejection fraction is a very gross index of left ventricular performance. It is affected by pre-load and after-load changes and heart rate. Left panel. The graph shows how two additional pressure-volume loops appear with an acute increase in afterload or preload. Contractility is quantified by the end-systolic pressure volume relation slope: the E es (end systolic elastance). Right panel. Increased contractility is reflected in higher myocardial fiber shortening velocity, with a more highly developed tension peak and a steeper pressure rise, when preload, after load, and heart rate are constant: the E es (end systolic elastance) moves upward and to the left. Lower panel. Force-frequency relation or Bowditch treppe. In the healthy heart, a frequency increase up to 180 beats per minute provides for faster systolic calcium SR release (increased contractility or developed force) and for faster diastolic SR calcium reuptake (positive lusitropic effect).