What are some ways that I can slow my heart rate down?

I 28 and I had been in the hospital for ITP blood disorder and anemia and when I lay down my heart rate is like 98 or 99 but when I get up its raise to like 132 and go down when I lay back down. The doctors said they wasn t concern about my heart and they did an ultrasound on my heart and couldnt find anything
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