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...