Case Presentation: Inappropriate sinus tachycardia (IST) was initially described as a non-paroxysmal increase in the resting sinus heart rate in otherwise healthy individuals (1). It is most often seen in young females and rarely in the elderly (2). Herein we report a case of an elderly female diagnosed with IST. 67-year-old female with past medical history of hyperlipidemia and diverticulosis presented with palpitations associated with generalized weakness and fatigue. On prior visits, she experienced chest pain, flushing, nausea, diaphoresis and anorexia. She noted tachycardia with minimal exertion which would resolve on relaxation. Investigations for multiple endocrine disorders were negative. She was placed on metoprolol which was discontinued due to fatigue and dizziness. An electrophysiology study (EPS) showed no evidence of inducible arrhythmia. She was also evaluated for an accessory cardiac conduction pathway. She was placed on flecainide, but continued to be symptomatic. A transthoracic echocardiogram, stress test and Holter monitor were all unremarkable. She was transferred to a tertiary care center for a cardiac arrhythmia program. It was determined that her symptoms were suggestive of IST. Nadolol 10 mg daily helped improve her symptoms eventually.

Discussion: IST is commonly seen in adolescents and young adults (4, 5). A case series by Gustavo et. al showed only 4 patients in their 6th or 7th decade (5). The challenge of diagnosing IST is due to a wide continuum of symptoms, a poorly understood mechanism, and no universal criteria to establish the diagnosis. IST can go unnoticed, but patients are evaluated for symptoms interfering with their quality of life (5). Our patient’s palpitations were constant; while chest pain and sweating were paroxysmal. In literature, both paroxysmal and non-paroxysmal symptoms have been reported (5). To diagnose IST, explainable causes of sinus tachycardia need to be ruled out and can be categorized into medical conditions, physiological state, drugs, and substance use (5). Our patient was ruled out for hyperthyroidism, Cushing’s disease and pheochromocytoma. Stressors like exercise, emotion, pain, and fever were absent. Use of sympathomimetics, and history of substance use was ruled out.Absence of a universal definition for IST which makes it a challenging diagnosis. It is a diagnosis of exclusion. In most literature the resting heart rate was described above 100 beats per minute (bpm) with a mean HR of ≥ 95 bpm on 24 Holter ECG, exaggerated response to HR with minimal exercise, and P waves during tachycardia which look identical during normal sinus rhythm (3,5). In other literature, distressing symptoms were also part of the definition in patients who had heart rates ≥ 100 bpm while awake and at rest (2). In addition, various heart rate patterns including episodic tachycardia have also been reported (2).

Conclusions: The lack of understanding of the mechanism of IST makes management difficult. Slowing the heart rate with beta blockers and non-dihydropyridine calcium channel blockers has helped improve symptoms. In our patient, beta-blocker therapy ultimately improved symptoms. Ivabradine have shown positive efficacy(6).Catheter sinus node modification (5) can be considered if pharmacological interventions fail. In our opinion, further investigations are required for better understanding of this rare diagnosis and arrive at a diagnosis earlier.