I am a 58 yo male, i have diastolic congestive heart failure. is it safe for me to masturbate?

Probably; but treat.: It's probably best to treat heart failure before engaging in strenuous activities. While there's no direct evidence that masturbation is harmful, history has examples of cardiac events occurring during arousal. Heart failure, while incurable (ie, heart won't go back to way it was before without medications), is also treatable (medications DO restore some function). So get it treated 1st, then TTYD