I Have Suffered From Depression Most Of My Life. I

I have suffered from depression most of my life. I have been on tofranil for the past 38 years. 300 milligrams daily. My depression has been severe, Is it possible that my anti-depressive is no longer effective. How would I ever be able to change meds. Very fearful of change at this stage of my life. I am 66 and had a heart attach 5 years ago. My ejection fraction is only 17 at this point. Would it be risky to change meds.
Thanks for your help,