Mechanism of 'inappropriate' sinus tachycardia. Role of sympathovagal balance.

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Abstract
BACKGROUND: "Inappropriate" sinus tachycardia (IST) is an uncommon and poorly defined atrial tachycardia characterized by inappropriate tachycardia and exaggerated acceleration of heart rate with "normal" P wave. The mechanism of this tachycardia is unknown. The purpose of the present study was to determine the role of autonomic balance in the genesis of IST. METHODS AND RESULTS: Six female patients aged 23 to 38 years with IST and 10 age- and sex-matched control subjects were assessed with the following autonomic function tests: (1) sympathovagal balance to the sinus node assessed by calculating the LF/HF (low frequency/high frequency) ratio using power spectral analysis both in the supine position and after 10 minutes of head-up tilt to 60 degrees, (2) cardiovagal reflex assessed by cold face test (CFT), (3) beta-adrenergic sensitivity as determined by calculating isoproterenol dose-response curves and isoproterenol chronotropic dose 25 (CD25), and (4) intrinsic heart rate (IHR) assessed after autonomic blockade with atropine 0.04 mg/kg and propranolol 0.2 mg/kg administered as an intravenous bolus. No significant differences in the LF/HF ratio both in the supine position (2.8 +/- 0.3 versus 2.6 +/- 0.4) and during upright tilt (8.7 +/- 1.3 versus 8.5 +/- 0.5) were observed between control subjects and IST patients. Cardiovagal response to CFT was markedly depressed in all patients (6.3% IST patients versus 24.2% control subjects, P < .001). beta-Adrenergic hypersensitivity to isoproterenol was noted in all patients (mean CD25, 0.29 +/- 0.10 microgram IST patients versus 1.27 +/- 0.4 microgram control subjects; P < .001), and high IHR was noted in all cases. The patients were treated with high doses of beta-blockers with adequate short-term control. Radiofrequency catheter ablation of the sinus node area was performed in one drug-refractory patient. CONCLUSIONS: These findings suggest that the mechanism leading to IST is related to a primary sinus node abnormality characterized by a high IHR, depressed efferent cardiovagal reflex, and beta-adrenergic hypersensitivity.