What puts one person out for six hours might only last two on another. In traumatic injuries to the heart and great vessels, this sac can fill with blood or fluid and put pressure on the heart, resulting in an arrest. This is perhaps the most correctable cause of traumatic arrest and one of the few justifiable reasons to perform a thoracotomy. Due to the location of the knife wounds on Lucy, they elected to go with a midline incision, and I can't really blame them for that. The trick to treating this is to relieve the pressure on the heart by opening the pericardium, tamponading the hole that's causing the leak, and getting the heart back into a perfusing rhythm.
Younger people survive injuries better.
Once up in the OR, Romano and Corday removed the spleen and set about patching the holes in the body, which is really what trauma surgery is all about. Greenfield filters are small umbrella-shaped nets that are threaded into the pulmonary arteries to catch clots. You place the filter and then open the net up. This is usually a job for interventional radiology, but this hospital seems to feature people trained and capable of doing everyone's job, so I won't complain about Romano having done a hundred of them. No, that's where the clots go.