What might cause abnormally low End-Systolic Left Ventricular Pressures in a Working Heart experiment?

If abnormally low End-Systolic Left Ventricular Pressure (ESLVP) is being measured during a Working Heart experiment, the first thing that should be investigated is the preload and afterload pressures that are being applied to the heart.
  • Preload is set by the height of the atrial bubble trap, and should be approximately 5mmHg in most animal models.
  • Afterload is generally set by the height of the compliance loop, shown in the image below. Regarding Afterload, 50-80mmHg is normal, depending again on animal model.
Both of these pressure values should be checked against previously published literature to find the proper settings for the chosen animal model.
Another troubleshooting step would be to ensure the proper use of the Compliance Loop. The compliance loop can greatly affect the developed pressure of the ventricle at the end of Systole. The bubble trap portion of the compliance loop should have an ~2mm bubble at the top of the bubble trap (or attached syringe) to mimic the elastic compliance of the arterial vasculature. If the bubble that the heart is beating against is too small, this may model a very stiff vasculature causing an increase in resistance and an increase in ESLVP.  A large bubble would model a very compliant vasculature, and may result in low ESLVP.
Finally, ensure proper atrial filling by applying the appropriate preload to the heart. In some circumstances, high resistance in the filling line may cause enough restriction as to limit the Atrial Filling Volume. If this is the case, low ESLVP will result.
If your preload (atrial bubble trap height) is set properly, a test can be performed to ensure proper atrial filling. First, without a heart attached, measure the amount of fluid draining from the atrial cannula while the fluid is free to empty from the bubble trap. This should be well in excess (2x recommended) of the cardiac output expected. 
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