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Without the information already requested all we can do is guess.
So my first guess is atrial fibrillation. It causes the rapid heart beat and is often accompanied by a panic attack as described.

Next guess is undiagnosed and untreated centrals.

A wild guess a panic attack in response to a dream.

Since this has only happened on rare occasions, any of the above could make sense and Sleepyhead could easily miss at least 2 out of the 3.

It is interesting that Jeff hasn't been back to respond all day.

Data would be really helpful. Maybe the machine was cutting out.

Mine was, it turned out the plug had come really loose in the outlet and the machine kept shutting itself off.