How does mouth-to-mouth work since you're exhaling mostly carbon dioxide?

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You may be interested in this study about CPR and breathing. You’ll find it on the page under " Subject: CPR Givers May Breathe Easier".

When it comes to cardiopulmonary resuscitation (CPR), half a loaf may often be just as good as, and maybe a bit better than, a whole loaf. A study published today shows the surprising result that giving adults in cardiac arrest chest compressions alone was just as likely to save their lives as giving full CPR, which includes mouth-to-mouth breathing.

The results of the experiment, performed over nine years in Seattle, raise the question whether more people might be saved if the CPR guidelines were simplified to eliminate the mouth-to-mouth part of resuscitation, at least in certain cases…

Blood then stops circulating, although much of it is still loaded with oxygen. Until that oxygen is used up, rescue breathing won’t add any more to it, but immediate chest compressions will distribute it to vital organs.

The trend toward better survival with chest compressions alone has also been seen in experiments where pigs are given CPR after cardiac arrest.