fibrillating

adjective2

1. •
   Of a muscle, especially in the heart: undergoing fibrillation; quivering.

   • A publicity plane flew along the Croisette, its pennant fluttering like the trace of a fibrillating heart, unnoticed by the sunbathers stretched in their loungers in the hotel concessions. — 2000
2. •
   Splitting into fibrils or fibres.