flatline

/ (ˈflætˌlaɪn) /

verb (intr) informal

to die or be so near death that the display of one's vital signs on medical monitoring equipment shows a flat line rather than peaks and troughs
to remain at a continuous low level
Collins English Dictionary - Complete & Unabridged 2012 Digital Edition © William Collins Sons & Co. Ltd. 1979, 1986 © HarperCollins Publishers 1998, 2000, 2003, 2005, 2006, 2007, 2009, 2012