Inappropriate Sinus Tachycardia Following Viral Illness

Targeted Treatment of Inappropriate Sinoatrial Node Tachycardia Based on Electrophysiological and Structural Mechanisms

The purpose of this review is to determine the causal mechanisms and treatment of inappropriate sinoatrial tachycardia (IST), defined as a non-physiological elevation in resting heart rate. IST is defined as a resting daytime sinus rate >100 beats/minute and an average 24-hour heart rate >90 beats/minute. Potential causal mechanisms include sympathetic receptor hypersensitivity, blunted parasympathetic tone, or enhanced intrinsic automaticity within the sinoatrial node (SAN) pacemaker-conduction complex. These anomalies may coexist in the same patient. Recent ex-vivo near-infrared transmural optical imaging of the SAN in human and animal hearts provides important insights into the functional and molecular features of this complex structure. In particular, it reveals the existence of preferential sinoatrial conduction pathways that ensure robust SAN activation with electrical conduction. The mechanism of IST is debated because even high-resolution electroanatomical mapping approaches cannot reveal intramural conduction in the 3-dimensional SAN complex. It may be secondary to enhanced automaticity, intranodal re-entry, or sinoatrial conduction pathway re-entry. Different pharmacological approaches can target these mechanisms. Long-acting β blockers in IST can act on both primarily increased automaticity and dysregulated autonomic system. Ivabradine targets sources of increased SAN automaticity. Conventional or hybrid ablation may target all the described abnormalities. This review provides a state-of-the-art overview of putative IST mechanisms. In conclusion, based on current knowledge, pharmacological and ablation approaches for IST, including the novel hybrid SAN sparing ablation, are discussed.

Inappropriate Sinus Tachycardia: Etiology, Pathophysiology, and Management: JACC Review Topic of the Week

Inappropriate sinus tachycardia (IST) is a clinical syndrome that generally affects young patients and is associated with distressing symptoms. Although the most common symptom is palpitations, it can be accompanied by a myriad of symptoms, including anxiety, dizziness, presyncope, and syncope. The pathogenesis of IST is not well understood and considered multifactorial, with autonomic dysfunction being the central abnormality. IST is a diagnosis of exclusion. Management presents a clinical challenge. The overall efficacy of lifestyle modifications and medical therapy may be limited. Recent advances in catheter and surgical sinus node sparing ablation techniques have led to improvement in outcomes. In addition, increased focus has led to development of multimodality team-based interventions to improve outcomes in this group of patients. In this review, we discuss the mechanistic basis of IST, review current approaches to diagnosis, and outline contemporary therapeutic approaches.