The class I recall is due to a malfunction in the device’s outflow graft assembly that may cause the outflow graft to twist and close up (occlusion) over time. Occlusion of the outflow graft can reduce or stop pump flow and set off a persistent low flow alarm in the system. A reduction in pump can lead to serious adverse events such as blood clots and death.

https://www.fda.gov/Safety/MedWatch/SafetyInformation/SafetyAlertsforHumanMedicalProducts/ucm608444.htm