From the Guidelines
Inappropriate sinus tachycardia (IST) is caused by unclear mechanisms, but proposed causes include dysautonomia, neurohormonal dysregulation, and intrinsic sinus node hyperactivity. The exact cause of IST is still not well understood, but it is defined as sinus tachycardia that is unexplained by physiological demands, with associated symptoms such as weakness, fatigue, lightheadedness, and uncomfortable sensations like heart racing 1.
Key Characteristics of IST
- Resting heart rates >100 bpm and average rates that are >90 bpm in a 24-hour period
- Associated symptoms like weakness, fatigue, lightheadedness, and uncomfortable sensations
- Proposed mechanisms related to dysautonomia, neurohormonal dysregulation, and intrinsic sinus node hyperactivity
Differential Diagnosis
It is essential to distinguish IST from secondary causes of tachycardia, including:
- Hyperthyroidism
- Anemia
- Dehydration
- Pain
- Use of exogenous substances
- Anxiety disorders, as anxiety can be an important trigger for IST 1
Diagnosis and Management
IST is a diagnosis of exclusion, and it is crucial to rule out other forms of tachycardia, such as atrial tachycardia and sinus node reentrant tachycardia, as well as postural orthostatic tachycardia syndrome (POTS) 1. Management of IST typically involves lifestyle modifications and, if necessary, medications like beta-blockers or ivabradine to control symptoms and heart rate.
From the Research
Causes of Inappropriate Sinus Tachycardia
The causes of inappropriate sinus tachycardia (IST) are not well understood and are considered multifactorial 2, 3, 4.
- Autonomic dysfunction is thought to be the central abnormality in IST 2.
- The etiology of IST remains ill-defined and controversial 3.
- Various explanations for IST have been considered, but a definitive common mechanism is not yet known 4.
- IST is a diagnosis of exclusion, and a thorough evaluation of secondary causes of tachycardia is required in the work-up of all cases 2, 3, 4.
Pathogenesis of IST
- The pathogenesis of IST is not well understood and is considered multifactorial, with autonomic dysfunction being the central abnormality 2.
- Evaluation of cardiac autonomic reflex function is essential to support the diagnosis of IST 3.
- IST is usually a diagnosis of exclusion, and it is important to rule out other causes of sinus tachycardia before reaching a final diagnosis 3, 4.
Mechanisms Involved in IST
- The treatment of IST aims to target the multiple mechanisms involved in this disease 3.
- Multidisciplinary management, including cardiac rehabilitation, pharmacotherapy, and occasionally radiofrequency modification of the sinus node, should be considered 3.
- Effective treatments vary but can include ivabradine, beta-blockers, or calcium channel antagonists; ablation is seldom advised 4.