What is inappropriate sinus tachycardia (IST)?

What is Inappropriate Sinus Tachycardia (IST)?

Inappropriate sinus tachycardia is a persistent elevation in resting heart rate (>100 bpm) that is unrelated to, or out of proportion with, the level of physical, emotional, pathological, or pharmacologic stress. 1

Definition and Core Characteristics

IST represents a distinct clinical syndrome where the sinus node generates an inappropriately fast heart rate without identifiable secondary causes. 1 The key distinguishing feature is that this tachycardia is nonparoxysmal (continuous rather than episodic), which differentiates it from other arrhythmias like sinus node re-entry tachycardia. 1

The P-wave morphology and endocardial activation pattern remain identical to normal sinus rhythm, confirming the sinus node as the origin. 1

Underlying Mechanisms

The pathophysiology is likely multifactorial, with two primary mechanisms proposed by the ACC/AHA/ESC guidelines: 1

• Enhanced automaticity of the sinus node - an intrinsic abnormality causing the sinus node to fire at inappropriately high rates 1
• Abnormal autonomic regulation - characterized by excess sympathetic tone and reduced parasympathetic (vagal) tone to the sinus node 1

Research supports that IST involves a primary sinus node abnormality with high intrinsic heart rate, depressed cardiovagal reflex, and beta-adrenergic hypersensitivity. 2

Clinical Presentation

IST has a striking demographic pattern: 1

• Approximately 90% are female patients 1
• Mean age of presentation is 38 ± 12 years 1
• High proportion are healthcare professionals 1

Symptoms

The predominant symptom is palpitations, but patients may also experience: 1

• Chest pain 1
• Shortness of breath 1
• Dizziness and lightheadedness 1
• Pre-syncope 1

The degree of disability varies tremendously - from completely asymptomatic patients discovered incidentally during routine examination to individuals who are fully incapacitated by their symptoms. 1

Diagnostic Criteria

Diagnosis requires meeting specific criteria while excluding secondary causes. The ACC/AHA/ESC guidelines establish the following diagnostic framework: 1

Required Features:

1. Persistent sinus tachycardia (heart rate >100 bpm) during the day with excessive rate increase in response to minimal activity 1

2. Nocturnal normalization of heart rate as confirmed by 24-hour Holter monitoring 1

3. Nonparoxysmal nature - the tachycardia and symptoms are continuous, not episodic 1

4. P-wave morphology identical to sinus rhythm on ECG and during electrophysiologic study 1

5. Exclusion of secondary systemic causes including: 1

   • Hyperthyroidism
   • Pheochromocytoma
   • Physical deconditioning
   • Medications (anticholinergics, beta-blocker withdrawal, caffeine, alcohol)
   • Medical conditions (panic attacks, pulmonary embolism, fever, hypovolemia, anemia, pain)

Important Diagnostic Pitfall

Postural Orthostatic Tachycardia Syndrome (POTS) must be excluded before diagnosing IST, as these conditions can overlap but require different management approaches. 1 POTS is not a disorder of the sinus node itself but rather a dysautonomic condition. 1

Clinical Significance

The long-term prognosis of IST appears to be benign. 3 The risk of tachycardia-induced cardiomyopathy in untreated patients is unknown but is likely to be small. 1 This benign natural history influences treatment decisions, which should be predominantly symptom-driven rather than aimed at preventing long-term complications. 1

Clinical examination and routine investigations are generally not helpful in establishing the diagnosis but are essential for eliminating secondary causes of tachycardia. 1