Lactic acid fermentation occurs in muscle cells as a way to produce ATP when there's no oxygen around. After glycolysis, the pyruvic acid is converted into lactic acid.
Depending on the amount of time spent on an exercise, the energy used by the muscles can differ: for quick bursts of energy, the muscles cells use the ATP from lactic acid fermentation; to exercise for a longer period of time, the ATP comes from cellular respiration.
The ATP generated by lactic acid fermentation can last up to 90 seconds.
The provider must continue CPR until either a valid DNR order can be produced, or directed by medical control to discontinue. This is from a first responder point of view.