I am a certified CPR trainer.
The purpose of CPR is to keep the circulation running such that the vital organs receive enough oxygen to survive the period until the medical professionals take over. We literally buy time for the victim.
If there is a serious injury such that a significant part of the available blood is pushed out of the body, we have to treat the wound first.
- The simplest way is to find a second helper that can push something (preferably a sterile bandage, but we have other priorities right now) on the wound such that it is closed. So you can focus on the CPR.
- If you have proper training to apply a tourniquet, you can use this if the wound is on an arm or leg.
- Or just improvise to close up the wound.
But there is also the psychological effect. Sometimes it is better to do something (even in vain) than to do nothing. Miracles do happen and it can be important for the next of kin that you can tell them that you have done all that was possible.