Hello! Thank you for asking on HCM! Regarding your concern, I would explain that sometimes it is difficult to catch a cardiac arrhythmia on a Holter monitoring. Another option would be the implantation of a loop monitoring, which can be held for a longer time and you can switch it on, when you feel the palpitations. Other tests to consider in order to investigate for the possible underlying causes would be: - complete blood count for anemia - thyroid hormone levels for thyroid dysfunction - blood electrolytes for possible imbalance. Hope you will find this answer helpful! Wishing good health, Dr. Iliri