A 4-year-old child is pulled from a swimming pool and emergency medical services are called. They report the submersion time was estimated to be 6 minutes. Cardiopulmonary resuscitation was initiated by bystanders. In the emergency department, the child is comatose with fixed, dilated pupils and has received 25 minutes of continuous CPR without return of spontaneous circulation. Which factor is the strongest predictor of a poor outcome?