In order to qualify for a ventricular assist device, one must have serious problems:
There are also situations in which you might anticipate a VAD might be needed, and would have one on standby:
UpToDate has a nice chapter on VADs.
EMCrit brandishes the expertise of somebody who works with these things, and I take that seriously.
Additionally, there is an insanely colourful brochure which has device-specific recommendations.
Grimm, Joshua C., et al. "Duration of left ventricular assist device support does not impact survival after US heart transplantation." The Annals of Thoracic Surgery 102.4 (2016): 1206-1212.