MECHANISM OF INAPPROPRIATE SINUS TACHYCARDIA - ROLE OF SYMPATHOVAGAL BALANCE

Background ''Inappropriate'' sinus tachycardia (IST) is an uncommon an d poorly defined atrial tachycardia characterized by inappropriate tac hycardia 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 g enesis of IST. Methods and Results Six female patients aged 23 to 38 y ears with IST and 10 age- and sex-matched control subjects were assess ed with the following autonomic function tests: (1) sympathovagal bala nce to the sinus node assessed by calculating the LF/HF (low frequency /high frequency) ratio using power spectral analysis both in the supin e position and after 10 minutes of head-up tilt to 60 degrees, (2) car diovagal reflex assessed by cold face test (CFT), (3) beta-adrenergic sensitivity as determined by calculating isoproterenol dose-response c urves 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 posi tion (2.8+/-0.3 versus 2.6+/-0.4) and during upright tilt (8.7+/-1.3 v ersus 8.5+/-0.5) were observed between control subjects and IST patien ts. Cardiovagal response to CFT was markedly depressed in all patients (6.3% IST patients versus 24.2% control subjects, P<.001). beta-Adren ergic hypersensitivity to isoproterenol was noted in all patients (mea n CD25 0.29+/-0.10 mu g IST patients versus 1.27+/-0.4 mu g control su bjects; P<.001), and high IHR was noted in all cases. The patients wer e treated with high doses of beta-blockers with adequate short-term co ntrol. Radiofrequency catheter ablation of the sinus node area was per formed in one drug-refractory patient. Conclusions These findings sugg est that the mechanism leading to IST is related to a primary sinus no de abnormality characterized by a high IHR, depressed efferent cardiov agal reflex, and beta-adrenergic hypersensitivity.