I researched for side effects with Doxycycl and don't find anything that would be related to any heart issues. However, there may be an interaction with other medciations, etc., concomitant health issues, etc. so the medication may be secondary to another condition or medication. For a definitive answer, you should contact your doctor as irregular heartbeats could be serious.
Thanks for sharing and if you have any further questions or comments you are invited to respond. Take care.
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