Inappropriate Sinus Tachycardia (IST)

Also known as primary sinus tachycardia, inappropriate sinus tachycardia (IST) is a generally benign condition that has a prevalence of ~1% in the general population.

Diagnostic criteria
  • Average sinus HR > 90 bpm over 24 hours, or HR > 100 bpm while awake and at rest; and
  • Associated with symptoms such as palpitations, dyspnoea, dizziness; and
  • Other secondary causes of sinus tachycardia have been excluded

Causes

Exact aetiology is uncertain, but both intrinsic and extrinsic mechanisms have been proposed:

Intrinsic:

  • Sinus node dysfunction causing increased automaticity

Extrinsic:

  • IgG beta-receptor stimulating antibodies have been detected in some patient groups with IST
  • Beta-receptor supersensitivity
  • Impaired baroreflex control
  • Vagal denervation

Differential diagnosis
  • Postural Orthostatic Tachycardia Syndrome (POTS)
  • Deconditioning

Clinical significance
  • IST is generally a benign condition that rarely causes any life-threatening cardiovascular issues
  • There is a higher prevalence of psychological disorders, in particular anxiety, in patients with IST. It is unclear as to whether this is a cause or effect of IST
  • Isolated cases of IST-induced cardiomyopathy have been reported that respond to treatment with ivabradine

Treatment
  • Once diagnosis has been established, first line treatment is avoidance of stimulants such as caffeine, tobacco, and illicit drugs
  • Ivabradine is the only medical therapy that has demonstrated efficacy in literature. It is a blocker of cardiac If current, and has been shown to reduce HR and symptoms in patients with IST, even after cessation of therapy
  • Beta-blockers are a second line medical option
  • Ablation is not recommended


References

Advanced Reading

Online

Textbooks


LITFL Further Reading

ECG LIBRARY 700

ECG LIBRARY

Electrocardiogram

MBBS (UWA) CCPU Emergency Medicine Advanced Trainee based in Melbourne, Australia. Co-author of the LITFL ECG Library. Likes Ultrasound, Echo, ECGs, and anything and everything with caffeine. Part of the 2021 ANZCEN Clinician Educator Incubator Programme | @rob_buttner | ECG Library |

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