If Jahi's heart stops while she is on the ventilator, it depends on "how closely" she is being monitored, how quickly this is detected, and whether or not they have decided to try to do anything about it.
It is "possible" that her heart and other functions (like pulse oximetry) are not being monitored continuously, if she is in a long term care facility. We have no idea what they are monitoring, and the capabilities of the staff and the facility. Particularly if Mrs. Winkfield had decided not to try to do anything medically if her heart enters an abnormal rhythm-- for instance, if she is on palliative care, she may not be on typical ICU monitoring, but just have the ventilator cycling.
If she is receiving heart monitoring and pulse oximetry, and if appropriate alarm limits are enabled and audible, then if her heart enters an abnormal rate or rhythm, the alarms will sound as the limits are violated. Then the staff would have to decide if they are going to attempt to treat whatever is going on (with drugs, and intravenous fluids, and/ or electricity), or "let her go", or call 911, etc. I have no idea what they have decided to do.
If, for example, they have decided to "let her go", hopefully they will silence alarms, allow family to sit quietly with her body, hold her hands, etc, and at some point an appropriate staff member will quietly turn off the ventilator.
Very seldom does a heart go from a normal rhythm to complete stand still (asystole) abruptly-- it's more likely her heart would experience rate and rhythm abnormalities as a prelude to the final pings of electricity that precede asystole (standstill). That could be minutes, or hours. Often, the final bits of electricity can kick out for 15-20 min or more, once a rhythm has ceased. The other situation, as MMJ explained above, is that the heart continues to put out electricity, but there is no "squeeze" to produce a pulse that can be felt. That is a common ending, too, with that rhythm deteriorating to remnants of electricity that eventually stop.
There is no suffering for someone with brain death. Their color will become pale (most noticeable in the lips and inner mouth, and the palms, in someone with darker complexion), and the skin takes on an ashy, pale color as circulation ceases.
She wouldn't gasp or convulse, if that is what you're asking.
In fact, if she is not on a heart monitor or a pulse oximeter, the staff might not notice that her heart isn't beating well for some time. The vent will continue to cycle, even if her heart isn't beating well enough to circulate blood effectively, or if her blood pressure is very, very low.
I want to add that if she is in a home care, or long term care facility, that the monitors themselves are not as sophisticated as an ICU setting. They may not have any central telemetry monitoring-, but may have a type of transport bedside monitor- meaning, only when someone is in the room looking at her monitor would they be evaluating what is going on. We cannot assume what kind of monitoring her body is receiving, or not receiving, since there is really no "standard of care" for the level of care her body is receiving. Basically, they are free to do whatever monitoring they want, or are capable of doing. I doubt she has continuous arterial blood pressure monitoring, for example-- so may be on intermittent blood pressure cycling using an external cuff (such as a dynamap). They might set that to go off every one minute, or set it to go off every 15 min, or longer, etc. Or just manually cycle the automatic BP when they are in the room.I doubt they have noninvasive cardiac output capability (which uses the ventilator circuit to calculate cardiac output). All of this monitoring is dependent on having staff who understand how to do it, and how to interpret it. We just have no idea what the capabilities are where she is.
