A little-known device is shaking conventional wisdom for reviving people who suffer sudden cardiac arrest: People may be able to go much longer without a pulse than the 20 minutes previously believed. The capnograph, which measures carbon dioxide being expelled from the mouth of the patient, can tell rescuers when further efforts at cardiopulmonary resuscitation, or CPR, are futile or whether they should be continued. It is the latest effort that cardiology experts and emergency teams are devising that aim to improve a patient's odds. The American Heart Association recently revised its guidelines for first responders, with particular emphasis on initiating hard, rapid chest compressions to keep the stricken victim's blood circulating. Rescue squads increasingly are chilling victims of cardiac arrest with ice packs and other cooling approaches, a technique known as hypothermia, in order to protect the brain from injury when blood flow is restored.