On its own, it has a very low percent chance of recovery. Though it does change based on the mechanism of injury. However, it is extremely useful in prolonging the period in which other medical interventions can be successful. It very much gives time for EMTs to arrive and use defibrillators or chemical intervention.

I’m confident in your training they made it extremely clearly to call 911 and start emergency responders before starting CPR if you are the only person there.