It's been a while since I was AED-certified, but positioning the pads on the front of the chest and the back was recommended for infants, who may not have enough space to position both the pads on the chest.
But in a different class, I learned that it can be hard to roll someone over in order to position the pad on their back, especially if they are larger than you. If they have a suspected neck injury there can be significant risks.
In cases where someone is undergoing an elective medical procedure with significant risk of arrhythmia, front-and-back placement in adults is standard.
I would be curious if AED's can analyze the patterns equally well regardless of position.
I'm an anesthesiologist, so in those kind of procedures, they're generally covered in electrodes to watch for arrhythmia, and there's a cardiologist or cardiac surgeon in the room with me who can double-check before we shock. Plus, we can distinguish pulsatile from pulseless VT because we have other monitors (pulse oximetry, and in these procedures usually an intra-arterial catheter that can clearly distinguish whether or not a pulse is present).
> If they have a suspected neck injury there can be significant risks.
Not entirely unreasonable to consider, but if they need defibrillation and are unconscious / unable to roll themselves over, they might have bigger problems. Just the movement of properly executed CPR is going to be considerable.