The coma mechanic is based on two aspects - your blood level and non-vital organ damage. If your blood level drops below 50%, you enter a coma, and your squad mates need to stop the bleeding and infuse blood into you to revive you.
Similarly, when your liver or lungs are destroyed, you enter a coma, and your squad mates need to fix it with a surgery kit.
You have 120 seconds after entering a coma before your character dies and can no longer be revived.