Endocrine Abstracts

A 70 year old female was referred with a putative diagnosis of phaeochromocytoma. She was awaiting hiatus hernia surgery; the diagnosis was based on a history of hypertension and a persistent sinus tachycardia of 110 bpm. Investigations had shown modest elevation of 24 hour normetadrenaline at 4.71 to 4.98 micromol/24 hr (<4.4) on 3 occasions and plasma normetadrenaline of 2398 and 2504 pmol/l (<1180). Review of her medical records confirmed that the onset of tachycardia coincided with commencing treatment with nortriptyline 6 years earlier for symptoms of irritable bowel syndrome. She was also taking bisoprolol 7.5 mg OD, ramipril 5 mg OD and ivabradine 7.5 mg BD at time of investigation. Further in hospital investigation off nortriptyline, bisoprolol and ramipril (known to elevate noradrenaline levels) revealed plasma normetadrenaline 1261.3 and 1658.7 pmol/l (supine) rising to 1735.7 and 2503.7 pmol/l (sitting). The tachycardia resolved off these drugs. Suspicion that ivabradine was causing spurious results lead to repeat investigation off treatment. Results were plasma normetadrenaline 1170 pmol/l supine and 1470 pmol/l sitting (a physiological response). A clonidine test resulted in normal suppression of plasma normetadrenaline to 618.3 pmol/l. The ivabradine effect was confirmed by restarting 7.5 mg BD with repeat investigations showing plama normetadrenaline 1730.9 pmol/l (sitting) and 1498.8, 1600.2 and 1621.4 pmol/l (supine) at 30, 60 and 120 minutes respectively. We concluded that the clinical suspicion of phaeochromocytoma based on tachycardia was due to a drug effect (nortriptyline) as were abnormal biochemical results (nortriptyline, bisoprolol, ramipril and ivabradine). The effect of the latter has not been described previously and our report will alert investigators to the problem. She underwent uncomplicated surgery.