Patients with junctional tachycardia may mimic the pat- tern of slow-fast AVNRT and may show AV dissociation and/ or marked irregularity in the junctional rate. *RP refers to the interval from the onset of surface QRS to the onset of visible P wave (note that the 90-ms interval is defined from the surface as opposed to the 70-ms ventriculoatrial interval that is used for intracardiac diagnosis 80 ). AV indicates atrioventricu- lar; AVNRT, atrioventricular nodal reentrant tachycardia; AVRT, atrioventricular reentrant tachycardia; ECG, electrocardiogram; MAT, multifocal atrial tachycardia; and PJRT, permanent form of junctional reentrant tachycardia. Modified with permission from Blomstr m-Lundqvist et al.11 Figure 8. Acute treatment of regular SVT of unknown mechanism. Colors correspond to Class of Recommendation in Table 1; drugs listed alphabetically. *For rhythms that break or recur spontane- ously, synchronized cardioversion is not appropriate. IV indicates intravenous; and SVT, supraventricular tachycardia. of IST is difficult, and it should be recognized that lowering the heart rate may not alleviate symptoms. Therapy with beta blockers or calcium channel blockers is often ineffective or not well tolerated because of cardiovascular side effects, such as hypotension. Exercise training may be of benefit, but the benefit is unproven. Ivabradine is an inhibitor of the I-funny or If chan- nel, which is responsible for normal automaticity of the sinus node; therefore, ivabradine reduces the sinus node pacemaker activity, which results in slowing of the heart rate. On the basis of the results of 2 large, randomized, pla- cebo-controlled trials, this drug was recently approved by the FDA for use in patients with systolic heart failure. In the BEAUTIFUL (Morbidity-Mortality Evaluation of the If Inhibitor Ivabradine in Patients With Coronary Disease and Left-Ventricular Dysfunction) study,166 with coronary disease and a left ventricular ejection frac- tion <40% were randomized to ivabradine or placebo. 