Cardiopulmonary resuscitation or CPR is a way of immediately trying to rescue a patient who has suffered a cardiac arrest. While performing the CPR the person performing it has to blow oxygen in to the patient's mouth. This is called rescue breathing. During rescue breathing one forces air into the patient's respiratory system, with the patients head tilted back and jaws apart. If the stomach inflates during a CPR, it could mean that the air has entered the stomach. This is not a good sign, call for professional medical help or an ambulance immediately. The person performing the CPR should always blow into the mouth slowly and gently. If you blow hard the lungs and stomach can inflate like a balloon which could lead to death.