Mouth to mouth breathing is no longer needed for adults or teens in the first few minutes following a sudden cardiac arrest. The American Heart Association (2019) reports: 

When a teen or adult suddenly collapses with cardiac arrest, his or her lungs and blood contain enough oxygen to keep vital organs healthy for the first few minutes, as long as someone provides high-quality chest compressions with minimal interruption to pump blood to the heart and brain. The cause is usually an abrupt onset of an abnormal heart rhythm, often ventricular fibrillation (VF). VF causes the heart to quiver so it doesn’t pump blood adequately to vital organs. Before a sudden collapse, the teen or adult was probably breathing normally. This means there may be enough oxygen in the person’s blood for the first several minutes after cardiac arrest. Many cardiac arrest victims have gasping, which could bring some oxygen into the lungs. If the victim’s airway is open, allowing the chest to expand back to its normal position after each compression may also bring some oxygen into the lungs. For these reasons, the most important thing someone near the victim can do for a person in sudden cardiac arrest is to pump blood to the brain and to the heart muscle, delivering the oxygen that still remains in the lungs and blood. Do this by giving high-quality chest compressions with minimal interruptions. Interruptions in compressions to give mouth-to-mouth breaths may bring some additional oxygen into the lungs, but the benefit of that oxygen can be offset if you stop the blood flow to the brain and heart muscle for more than a few seconds (especially in the first few minutes after a sudden cardiac arrest when there is still plenty of oxygen in the lungs and blood).

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American Heart Association. (2019). Hands-Only CPR. Retrieved October 23, 2019, from https://international.heart.org/en/hands-only-cpr.