I know CPR is usually commenced before defibrillation in a medical emergency, however we learned that defibrillation should be done before CPR in the case of ventricular fibrillation. Is there a reason for this?
The timing of the first defibrillation attempts depends mostly on when defibrillation is detected. If it is witnessed - that is, the patient is being monitored and the alarm goes off, you're right there at the bedside, and the patient is in V Fib, it's fine to shock first.
However, whereas this used to be the norm - shock first - it no longer is, partly because the goals of CPR and ACLS (advanced cardiac life support) are changing. A bit over a decade ago, the goal was restoration of a perfusing rhythm.
The new Advanced Cardiac Life Support guidelines call for CPR to be initiated immediately, assuming the paddles aren't right there and ready to go the moment someone goes into VFib. It needs to be noted, though, that this recommendation does not rest on defibrillation with the aim of return of spontaneous circulation (ROSC), but on improved neurological outcomes.
If the heart is in V Fib, there is no effective blood circulation; the heart and all the other organs are starving for oxygen. While setting up to defibrillate, the hypoxia continues, and the damage becomes more severe, especially to sensitive tissue like the brain. Compressions circulate blood; even blood with lower oxygen saturation is better than none. The brain is less hypoxic, then, and neurological outcomes improve if defibrillation is successful.
This, not ROSC, is what the new ACLS recommendations are based on, because ACLS is not considered highly successful if the patient leaves the hospital only to be hospitalized elsewhere in a persistent vegetative state. And CPR first has been shown to provide better neurological outcomes.