From the Guidelines
Inappropriate Sinus Tachycardia (IST) can be caused by Postural Orthostatic Tachycardia Syndrome (POTS), a type of dysautonomia characterized by an abnormal increase in heart rate upon standing, as noted in the 2017 ACC/AHA/HRS guideline for the evaluation and management of patients with syncope 1.
Types of Dysautonomia
The following types of dysautonomia can cause IST:
- Postural Orthostatic Tachycardia Syndrome (POTS): characterized by an abnormal increase in heart rate upon standing, usually with heart rates >120 bpm 1
- Autonomic neuropathy: involves damage to the autonomic nerves, disrupting normal heart rate regulation leading to IST
- Baroreflex failure: occurs when the body's blood pressure sensing mechanism malfunctions, resulting in inadequate heart rate control
Pathophysiology
These dysautonomias cause IST through disruption of the autonomic nervous system's normal regulation of heart rate, leading to inappropriate sinus node activation and persistent tachycardia even without physiological demand.
Diagnosis and Treatment
Diagnosis of IST involves distinguishing it from secondary causes of tachycardia, such as hyperthyroidism, anemia, dehydration, pain, and use of exogenous substances, as well as other forms of tachycardia, including atrial tachycardia and sinus node reentrant tachycardia 1. Treatment typically involves beta-blockers like metoprolol (25-200 mg daily) or ivabradine (5-7.5 mg twice daily) to control heart rate, as well as lifestyle modifications such as increased fluid intake, salt supplementation, regular exercise, and compression garments to manage symptoms.
Key Considerations
It is essential to note that IST is a diagnosis of exclusion, and patients with postural orthostatic tachycardia syndrome may have predominant symptoms related to a change in posture, and treatment to suppress the sinus rate may lead to severe orthostatic hypotension 1.
From the Research
Types of Dysautonomia Causing Inappropriate Sinus Tachycardia
- Inappropriate sinus tachycardia (IST) can be caused by various types of dysautonomia, including:
- These conditions can lead to an imbalance in the autonomic nervous system, resulting in inappropriate sinus tachycardia 2, 3, 5
- The exact mechanisms behind IST are still unclear, but it is believed to involve a combination of intrinsic abnormality of the sinus node, autonomic dysfunction, and hypersensitivity of the sinus node to catecholamines 3, 5
Overlapping Syndromes
- IST can overlap with other conditions, such as POTS, making diagnosis and treatment challenging 2, 3, 4
- A comprehensive diagnostic approach, including autonomic testing and electrophysiological studies, may be necessary to differentiate IST from other supraventricular tachycardias 2, 5
Autonomic Modulation and Sinus Node Automaticity
- Studies have shown that patients with IST have a higher sympathetic contribution to heart rate compared to those with POTS or healthy controls 3
- The treatment of IST should focus on sympatholysis, reserving sinus node modification for patients with continued debilitating symptoms after beta-blockade and possibly ivabradine 3