In the wilderness there will be no ambulance arriving, so we have to take extreme measures to keep going. At Far Out First Aid we suggest that you use your knee to continue compressions on the victim's chest when everything else fails. You won't find this in any of the conventional texts on CPR, but using your knees might provide just enough additional time to make a difference, particularly when the victim goes into and out of cardiac arrest.
Even more effective is using your legs. Place the ball of your foot in the same spot you placed your hands, mid-sternum, and continue compressions. Your legs are much stronger than your arms and back, and you can actually do CPR for quite a while using this technique.
These are extreme measures, but when there is nothing else available and you are exhausted from doing conventional CPR, why not add a few more minutes using your knee or foot? It could make a difference.