My heart rate won't come down after exercise. What do I do?

This is actually a question on my mom's behalf. About two years ago she had an episode where her heart rate shot up to over 200 bpm, supposedly brought on by a strong cup of coffee, even though she had drank coffee for years. She had tests and her heart was fine - they said she had supraventricular tachycardia. She's been taking a beta blocker, atenolol I think, and that helps but she still has problems with her heart rate escalating and not being able to get it down. Recently, she's become very determined to lose weight and has joined a gym and started walking slowly. But her heart rate still gets up high and takes a long time to get back to a normal rate. Has anyone else dealt with this or knows of someone who has? Any ideas as to natural remedies she can try?

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