CPR question

In the section re: First Aid for Cardiac Arrest for Baby, the rescuer is

placing two fingers on the middle of the chest to do 30 compressions.

In the section re: Two-Rescuer CPR for a Baby, one rescuer is to

encircle the baby's chest with both hands, spread fingers around baby's

back, and place thumbs on the lower half of the baby's breathbone.

Why is the thumb method being used instead of the method used for

the Cardiac Arrest situation?

the anterior/posterior circle method is prefered but since you have to alternate between compressions and ventilations it is easier to do over hand compressions from the side of the baby. once you have a second rescuer you become the primary compression giver so you dont have to switch as much.. also encircling the babies chest gives you the advantage of a firm surface if one is not available.