I can't speak for the diltiazem, but the dose of Metoprolol is on the low end of the scale. I'm currently on 75mg, and had been as high as 200mg per day. The high end dosages are 400mg. This is not to that you could be adversely affected by your dose level.
The best way to take care of your SVT is to eliminate rather than try to supress it. An electrophysiologist can eliminate for good with a cardiac ablation procedure. Have you considered this?
Like you, I'm in my early 60's. My cardiologist was concerned with the extremely high rates I was exhibiting during my near lifelong history of SVT and my otherwise healthy but aging heart. I agreed to the procedure and was in and out in one day. It's been three years since my last event. I highly recommend this procedure, and many of us here on the forum have had it done.
I am taking 25mg metoprolol and 60mg diltiazem daily. I was up to 50mg metoprolol and 120mg diltiazem daily at one point but have cut back recently. Since cutting back I have been less fatigued and seem to have less difficulty breathing although I still have the annoying headaches. I am trying to find the best way to deal with the SVTs and palpitations but maintain a decent quality of life. I don't believe these particular meds are going to do it for me.
After reading many of the posts on this forum I have a better perspective. My SVTs are usually less then 5 minutes and the maximum heart rate that was measured was between 160 and 180. So not very severe when compared to many posters. I also have hiccups and irregular rhythms that are a more recent development and have not been recorded by a monitor yet. The hiccups, etc. are many times daily now, while the SVTs are several a week, usually daily. Unfortunately I believe the increase in frequency is related to the reduction in meds. Another thing to mention is my diastolic blood pressure is normally low (50-65) while my systolic is often in the 130s. Makes blood pressure medications more of a challenge.
My response is not to say this is what you have, but I went through a period similar to this while trying a large dose of Metoprolol for my SVT. Headache, chest heaviness when attempting to anything that would raise the heart rate; climbing steps, walking any distance, etc. What dosages are you taking of each drug?
Also if you could ask, how long are your SVT events?