Context: a boxing champion is arm-wrestling against another boxer. The champion is on the verge of losing, but he still doesn't give up, so the other boxer thinks:
In Kenkyusha dictionary I found these two examples:
生命線を守る guard one's lifeline
生命線を脅かす threaten [menace] sb's lifeline
So it seems that it is a common collocation, but what is the meaning in the sentence above and in general? Other than referring to the lifeline on the palm of one's hand or to something of vital importance, can 生命線 simply be a synonym of "life"? Can it refer to a person's survival istinct?
I would also be grateful if you could explain the value of that で after ちぎれそう. Thank you for your help!