I recently took a CPR class and they taught us that there are situations when mouth-to-mouth CPR should not be given. I found the following information that explains this theory:

“Bottom Line: Push Hard, Push Fast
The body’s transportation system is the most important. It’s not complicated – not nearly as complicated as the brain is, anyway – but it is vital to keep the other parts working. It takes a few pumps to get the blood moving. Stopping chest compressions to do mouth-to-mouth interrupts that flow.
Two studies have clearly showed a benefit to chest compressions without mouth-to-mouth. It’s time to consider changing the way CPR is taught. Focusing on pumping blood during CPR, rather than on moving air, makes a lot of sense.” Ewy, GA, et al.”Cardiocerebral resuscitation for cardiac arrest.” Am J Med. Jan 2006.