What Causes Inappropriate Sinus Tachycardia?

Question: Hello, I've summarised my XXXXXXX condition across all specialities below for your information. What I'm specifically interested in from your point of view is what could the causes of inappropriate sinus tachycardia be? ANS function is normal, EP study was normal and there are no hormone secreting tumours. The 'reaction' is easily replicable and is divided into an active and passive element. What other mechanisms could be causing this? If you have any other thoughts on some other underlying cause that would be most appreciated. Outlandish ideas are fine, my medical team and I need ideas as we're struggling. Feel free to share with colleagues. Male, 21, health was perfect until ~13, was an elite athlete. First noticed a decrease in exercise tolerance around 13, if I hit a certain threshold of exercise my heart rate would just sit at 180/150 at not come down for ages. Accompanied with breathlessness. Cardio was the only symptom until 19 when I picked up pneumonia for a month. I was extremely ill and living on my own was too ill to seek treatment. Lasted about a month. Afterwards, all other symptoms but cardio started within 0-9 months. Most crippling thing is overbearing, extreme exhaustion. Varies, appears to by cyclic but unknown aggrovating factors. Cardio: * Threshold decreased from ages 13-20 until *anything* would cause a rapid heart rate and breathlessness. * Holtier monitors show no arythmia, ECG clear * Exercise tests confirm racing heart and decreased ventilation * Treated with ivabradine 5mg 2xdaily, is a tradeoff with exhaustion. * EPS study confirms no arrythmia, 24hr urine for VMA, catecholamines and 5 HIAA normal * Bracycardia (40bpm = NORMAL) only while sleeping * Inappropriate sinus tachycardia, unknown cause Neurology: * Almost permanent headache/migrane, rear left. No obvious trigger * Ineloquence/forgetting words. “Mental cloudiness” * ANS functions fine, ice water, tilt table and 3 min grip okay. One boarderline fail. * Brain MRI"Few non-specific white dots in the brain substance" * Somatic disorder ruled out due to this * Awaiting repeated MRI with contrast Sleep: * Diagnosed with UARS, although CPAP suggests central SA * Undergoing CPAP * AHI varies periodically in weeks between 0-1 and 2-3. Short periods between sleep cycles removed. * Breathing stops for 16-30 seconds, no restriction of flow or leakage * Two sleeping patterns identified: 1) Sleep at 11pm for 7 hours, refreshing (RARE) 2) Sleep at 11pm for 12 hours if not disturbed, unrefreshing. 3) Sleep 8 hours, awake 6 hours, sleep 4 hours, awake 6 hours, repeated. * Awaiting second sleep study for confirmation of central events Eyes: * Strong photophobia * Constant 'white noise', like interference on an old tv or camera in low light. 'Visual snow'. * Physiology of eyes fine, all tests passed - therefore neurologically based * Light sensitivity variable, white noise fixed * Maximum focal distance shifts inwards at random. Sometimes can't focus after 5m, sometimes 20m, 50m, other days fine * "Don't know". Passed to Neurology. Digestive: * Developed long term IBS from 18-21. Transient. * Stomach emptying/transit would just STOP, no particular trigger. * Keep developing antibodies to milk, yeast and wheat even after 6-12 month exclusion diets (IgE). * Permanent anal fissures. Diet is excellent, topical solutions have no effect. Botox injections little effect, nothing can overcome the internal sphincter spasm. But then randomly it relaxes itself for a few days. * Mebeverine MR 200mg when required * "Don't know" Bladder: * Detrusor over activity (overactive bladder) * Repeated protein present in urine * VERY overactive during some periods, no apparent trigger. * Up to 20x a day inc 4x at night. Other weeks almost normal. * No longer responsive to detrusol XL 4mg mr, terminated. * Currently mirabegron MR 50mg daily, helps a bit * "Don't know" Rhumatology: * Calcium deposit in wrist, other deposits in soft tissue * Other lumps on trunk, armpits and neck. Some hard, some soft. One soft excised and described as a lipoma * Small joins very stiff, require significant mechanical force * Constant ache/pain in fingers, getting worse. Now spread to ankles and wrists * No inflammation in blood tests * Checking for connective tissue disorders Medication list: Ivabradine 5mg 2xdaily Mirabegron MR 50mg daily Lymecycline 400mg 2xdaily Vitamin D 5000IU daily Mebeverine MR 200mg as required Ibuprofen 200mg as required Kind Regards, Adam

Thank you for your detailed analysis, however I do not think it fits. Echo was unremarkable and I do not have a big heart. There are no filling problems and there is no arrhythmia. There is no coronary artery stenosis, and the heart is fine running at high bpm. The only thing that is wrong is inappropriate sinus tachycardia. Any normal influence that would raise the heart rate is magnified.

Several holter monitors have been performed, no arythmians (other than sinus tach when expected and bracycardia at night). Autonomous nervous function was normal, as was everything else. It just seems very, very strange that IST could develop along side so many other things and them not be related.