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. It is important to distinguish IST from secondary causes of tachycardia, including hyperthyroidism, anemia, dehy- dration, pain, and use of exogenous substances and drugs of abuse. Anxiety is also an important trigger, and patients with IST may have associated anxiety disorders.160 Structural heart disease, such as cardiomyopathies, must also be excluded, though the development of a cardiomyopathy secondary to sinus tachycardia is extremely rare.164,165 IST must also be distinguished from other forms of tachycardia, including AT arising from the superior aspect of the crista terminalis and sinus node reentrant tachycardia (Section 4). It is also important to distinguish IST from postural ortho- static tachycardia syndrome, although overlap may be pres- ent within an individual. Patients with postural orthostatic tachycardia syndrome have predominant symptoms related to a change in posture, and treatment to suppress the sinus rate may lead to severe orthostatic hypotension. Thus, IST is a diagnosis of exclusion. 3.2.1. Acute Treatment There are no specific recommendations for acute treatment of IST. 3.2.2. Ongoing Management: Recommendations Because the prognosis of IST is generally benign, treatment is for symptom reduction and may not be necessary. Treatment Figure 7. Differential diagnosis for adult narrow QRS tachycar- dia. 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. 