Under California law, you don't. One has to have 'capacity' to give a power of attorney. This is the exact and sad reason we estate planning attorneys urge estate planning while people are young and/or healthy. Accidents happen and now your spouse is not able to contract under the laws of California, and I would assume most states. You may need to look into guardianship if s/he is likely to be in a coma for a long time. Without knowing what you need it for, it is hard to give more advice.
You need to look into guardianship in lieu of power of attorney. Power of attorney is granted when the person granting those powers has capacity to do so. If they are already incapacitated, then someone needs to petition the court for guardianship over the financial affairs of that person.
You will likely need to start a guardianship proceeding through your local county clerk's office.
Our office schedules phone and office consultations to review cases concerning North Carolina legal issues. This answer does not establish an attorney-client relationship but is a response to a hypothetical scenario and should not be construed as legal advice.