Evaluation for and treatment of reversible causes are recommended in patients with suspected IST.160,185 See Online Data and 5. It is important to distinguish IST from physiological sinus tachycardia or focal AT from the high right atrium, which can have P-wave morphology similar to the sinus P wave. A careful history and physical examination, with further laboratory and imaging studies, are necessary to determine reversible causes of tachycardia, such as exogenous substances and drugs, infection, anemia, and hyperthyroidism. A focal AT would have sudden onset and termination, which would not be the case for IST. IIa B-R 1. Ivabradine is reasonable for ongoing management in patients with symptomatic IST.168 176 See Online Data and 5. In 1 small randomized crossover trial,168 ivabradine given at a dosage of 2.5 to 7.5 mg twice daily significantly reduced daytime heart rate from 98.4 11.2 at baseline to 84.7 9.0, compared with 98.6 11.1 on placebo (P<0.001), and improved exercise tolerance and symptoms in patients with IST. Similar findings have been observed in several additional nonrandomized observational studies.169 176 Furthermore, a significant number of patients in these studies reported complete resolution of symptoms, with persistent clinical benefit observed in some even after discontinuing the drug. In 1 observational study, ivabradine was more effective than metoprolol in reduction of the heart rate and amelioration of symptoms.170 The drug is well tolerated, with an excellent safety profile demonstrated in 2 large RCTs in patients with heart Ivabradine can cause phosphenes, an enhanced brightness in a portion of the visual field; this side effect, which is usually transient, was reported in 3% of patients taking the drug in the SHIFT trial.167 IIb C-LD 1. Beta blockers may be considered for ongoing management in patients with symptomatic IST.170,172 See Online Data and 5. 