A Health Care Proxy (HCP) names an agent and a successor agent (if desired) to make medical decisions for your when you are unable to make them yourself.
So long as the HCP is signed while you have capacity, it can still be used after you lose capacity. An HCP, combined with a Power of Attorney, can avoid the necessity of having an Article 81 guardian appointed for you if you become incapacitated later in life.
You should make sure that your agent(s) named in your HCP are willing to act in that capacity and are aware of your wishes in the event you are unable to make medical decisions for yourself. A copy of your HCP should also be provided to all of your medical providers to have in their files.