THC has been known to increase your heart rate so I would steer clear of strains very high in THC, if not avoiding them completely. Conversely, CBD has been shown to have little to no impact on your cardiovascular system and has been shown to decrease your heart rate. CBD can counteract the increase in heart rate from the THC as well, though in your situation I would imagine that you would only want to try something that would reduce your heart rate. I am not a doctor so above all you will want to consult your MD about this.
Here are the (possibly outdated) studies that I have obtained this info from:
"Effects and interactions of natural cannabinoids on the isolated heart."
Nahas G, Trouve R. (1985)
"Short-term effects of smoked marihuana on left ventricular function in man."
Tashkin DP, Levisman JA, Abbasi AS, Shapiro BJ, Ellis NM. (1977)
When dealing with a racing heart, or any situation where you are feeling overstimulated, you’re going to want to stay clear of Sativas and go with a hich CBD strain or an indica strain. Indica as both will help relax the body both physically and mentally. Our customers continually give high marks to our CBD strain Harlequin and Indica strains such as Grape Ape, Grandaddy Purple, Abbot Kinney OG, and Key Lime Pie.