In cases of cardiac arrest, rescue breathing is something only certified rescuers should perform.
This is because, by expanding the chest artificially, rescue breathing can inhibit the blood flow to the heart. Professionals certified in CPR are trained to ensure this doesn’t undermine the effectiveness of chest compression or the survival of person being treated.
On the other hand, if a person still has a pulse but is breathing abnormally, rescue breathing may help keep them alive until emergency help arrives.
Be sure you’re following the proper procedure for rescue breathing in an emergency situation.
In cases of cardiac arrest, the American Heart Association (AHA) recommends chest compressions without rescue breathing if you are untrained or unable to competently perform professional CPR.