Penetrating injury to chest, back or upper abdomen? Think tension, think massive haemothorax, think tamponade.
Exclude or confirm tamponade with eFAST.
Cardiac tamponade is not an on/off phenomenon (yet another continuum), though the progression to PEA cardiac arrest may be rapid. 150 to 200 ml of blood in the pericardial sac may be enough to cause this.
Cardiac tamponade as a result of blunt injury is exceptionally rare in those patients reaching hospital alive.