If you are validly served PERSONALLY and not by mail, you have to appear or a warrant may be issued for your arrest. I suggest retaining counsel to deal with the subpoena. Going to a local judge won't do any good because a local judge has nothing to do with the subpoena. The issuing state has to provide you with travel expenses, including plane or bus fare, motel expenses and food.
The information and legal suggestions made herein do not in any way create an attorney-client relationship. The responses provided herein discuss general principles of law and should not be relied upon by the asker in making legal decisions. Only an attorney who has met with the asker and fully reviewed the facts and circumstances of the asker's individual case should be relied upon for legal advice. If you find my suggestions helpful, please mark the appropriate box as helpful.
A subpoena must be served personally. DAs often serve subpoenas by mail and ask you to call to acknowledge receipt. If you do that a Court may consider you served.
Beyond that there is a procedure that must be followed when another state wants to command your presence in their courts. This usually involves getting a CA court to issue a subpoena commanding your attendance and personal service.
If your health is truly a concern you need an attorney to help you deal with this.
Hire an attorney. Dont speak to DAs or law enforcement who may wish to trap you into acknowledging that you received a mailed subpoena (which isn't sufficient). An attorney will advise you further on how to legally avoid having to testify in this matter. Don't try to handle this yourself, by going to court and attempting to prove "undue hardship". This won't work and may end up acting as a waiver of any right you have to legally avoid testifying. Since this is a state court trial, you most likely will not be provided any witness protection that you might need in a serious death penalty case. Beat of luck
A subpoena mailed or personally delivered from an out of state court has no legal effect in the State of California. A lawful order requiring out of state attendance at a trail requires the party seeking attendance of the witness to obtain or order from a court of lawful jurisdiction in the state where the witness resides compelling the witness to attend trail in the other state.