Most of the time, if you see somebody ambulating go down and don't get up after you open her airway and give her a few compressions, you will be needing an automatic external defibrillator.

All the ventilating and cardiac massaging in the world won't help a bit, if the conduction system isn't working. You might get a little oxygen to the brainstem, but that only means you're looking at twenty years in a persistent vegetative state instead of dead. Odds of leaving the hospital in a position other than supine zipped up in plastic are less than 5%.

Yeah, OK, it's a little over a thousand dollars, but that's just a couple of PLB's right? Besides, you're EDCing because you want to help, so get the right equipment. If not, just resign yourself to the fact that for every minute that goes by without defibrillation, survival goes down by 10% or more.

Improving Survival From Sudden Cardiac Arrest (external link to JAMA)

PS. I have done mouth-to-mouth resuscitation just once when I was a student and came across a "Found Dead" on rounds with the Critical Care fellow. She smacked me good on the back of the head and started Ambu-bagging. It's a strange feeling to actually see that chest rise as you blow in and hear that rattly exhalation sound. Still remember it all these decades later.

PPS. If it sounds like I'm a firm believer in everyone having an AED, it's only as much as Doug is a firm believer in everyone having a PLB. The technology is here to save lives, so go get it!