The NEJM publishes a study using 3 simple predictive rules:

The predictive rule recommended termination of automated external defibrillator and basic life support efforts when there was no return of spontaneous circulation, no shocks were administered, and the arrest was not witnessed by emergency medical services personnel.

Using that predictive rule, 776 patients were recommended for termination of which four survived (0.05%), which was significantly lower (P=0.04) than the threshold of 1% that has been suggested as reflective of medical futility, they wrote. Three of the four patients survived with good cerebral performance and one had severe cerebral disability.