生死关头: an important juncture of life and death; a moment when one's fate hangs in the balance; at the crucial moment of life and death; be a moment [a turning point] of life and death; be a matte
生死未卜: one's life is uncertain.; hard to predict one's chances of life or death; hard to tell whether the person is alive or not; uncertain of one's life; whether one will survive is uncertain
