Arrows point to the P wave, which is inscribed before the QRS complex. The reentrant circuit involves anterograde conduction over the atrioventricular node, followed by retrograde conduction over a slowly conducting (or decremental) accessory pathway, usually located in the postero- septal region, to provide a retrograde P wave with long RP interval. This ECG does not exclude atypical AVNRT or a low septal atrial tachycardia, which can appear very similar to this ECG. AVNRT indicates atrioventricular nodal reentrant tachycardia; ECG, electrocardio- gram; and PJRT, permanent form of junctional reciprocating tachycardia. Page et al 2015 ACC/AHA/HRS SVT Guideline e521 3.1. Physiological Sinus Tachycardia Sinus tachycardia is regarded as physiological when it is the result of appropriate autonomic influences, such as in the set- ting of physical activity or emotional responses. Physiological sinus tachycardia may result from pathological causes, includ- ing infection with fever, dehydration, anemia, heart failure, and hyperthyroidism, in addition to exogenous substances, including caffeine, drugs with a beta-agonist effect (eg, alb- uterol, salmeterol), and illicit stimulant drugs (eg, amphet- amines, cocaine). In these cases, tachycardia is expected to resolve with correction of the underlying cause. 3.2. Inappropriate Sinus Tachycardia Inappropriate sinus tachycardia (IST) is defined as sinus tachycardia that is unexplained by physiological demands at rest, with minimal exertion, or during recovery from exercise. Crucial to this definition is the presence of associated, some- times debilitating, symptoms that include weakness, fatigue, lightheadedness, and uncomfortable sensations, such as heart racing. Patients with IST commonly show resting heart rates >100 bpm and average rates that are >90 bpm in a 24-hour The cause of IST is unclear, and mechanisms related to dysautonomia, neurohormonal dysregulation, and intrinsic sinus node hyperactivity have been proposed. 