Modification and ablation for inappropriate sinus tachycardia: current status.

Inappropriate sinus tachycardia is an ill-defined clinical syndrome with diverse clinical manifestations. Clinical symptoms can range from intermittent palpitations to multisystem complaints. Although there is a general consensus that when the heartbeat exceeds 100 beats per minute at rest or with minimal physiologic challenge, it is considered "inappropriate," this quantitative differentiation is quite arbitrary, while validation of the reproducibility of the heart rate/activity correlation can be challenging. Once the clinical diagnosis of inappropriate sinus tachycardia is expected, other supraventricular tachyarrhythmias and medical conditions causing sinus tachycardia should be excluded. The underlying mechanism of inappropriate sinus tachycardia is not well understood. "Intracardiac" mechanisms such as enhanced intrinsic automaticity, enhanced sympathetic tone, increased sympathetic receptor sensitivity, and blunted parasympathetic tone have been proposed. Evidences for "extracardiac" mechanisms such as length-dependent autonomic neuropathy, excessive venous pooling, beta-receptor hypersensitivity, alpha-receptor hyposensitivity, altered sympathovagal balance, and brainstem dysregulation have also been reported. Currently, our ability to differentiate primary (intracardiac) from secondary (extracardiac) mechanisms of inappropriate sinus tachycardia is limited. It has been reported that ablative therapy of sinus node is effective in treating patients with symptomatic inappropriate sinus tachycardia. Acute success of sinus node modification/ablation can be accomplished in 70%-100% of the various study populations. Although long-term successful outcome may be accomplished in a few patients, symptoms of palpitations and autonomic characteristics frequently persist. Identification and differentiation of patients who are suitable for ablative therapy versus medical therapy should be one of the central clinical research issues in this patient population. This brief review first considers the clinical and electrophysiologic diagnosis of inappropriate tachycardia and then summarizes the mechanisms of inappropriate sinus tachycardia and related syndromes such as postural orthostatic tachycardia syndrome. Techniques of mapping and ablation of sinus node are discussed briefly. A critical review of the acute and long-term clinical outcomes following sinus node ablation and modification is updated. In conclusion, the precise role of sinus node modification in patients with inappropriate sinus tachycardia remains to be determined. Sinus node modification could be considered in patients with inappropriate sinus tachycardia with persistently increased heart rate in the absence of any autonomic abnormalities. Autonomic laboratory testing should be performed to exclude any evidence of autonomic dysregulation. Clinical research on the pathophysiology of inappropriate sinus tachycardia should be pursued vigorously.