749-2 Autonomic Imbalance in Patients with Inappropriate Sinus Tachycardia

Abstract

Mechanisms responsible for inappropriate sinus tachycardia (IST) may include an intrinsic defect in the sinus node, autonomic imbalances, or both. To disclose the existence of spontaneous autonomic abnormalities, we analyzed heart rate variability (HRV) in 24-hour ECG recordings in 6 ambulatory adult pts with IST (defined as 24-hr average heart rate ≥ 95 bpm) and in 6 ageand sex-matched normal controls. All subjects were free of heart disease, diabetes, and autonomic disfunction. Mean average HR was 98 ± 2.6 bpm in pts with IST and 69.6 ± 7 bpm in controls. Percent of difference between adjacent RR intervals greater than 50 msec (pNN50) was 2.63 in pts and 23.9 in controls (p < 0.001). Standard deviation of RR intervals (SDNN), root mean square of successive differences (RMSSD), and standard deviation of 5-minute RR intervals (SDANN)forthe 24 hour period were significantly lower in pts than in controls for both day- and night-times (table). SDNN RMSSD SDANN Day Night Day Night Day Night Patients 47.59 51.22 19.04 23.4 62.3 30.7 Controls 98.1 97.78 41.9 58.55 50.55 29.9 P value < 0.001 < 0.001 < 0.001 < 0.001 < 0.001 0.01 Hourly variation for RMSDDs is plotted in the figure. Low and high frequency (freq) power were estimated at 10-min intervals throughout the 24 hour-period. Average Ln of low freq power was 10.26 in pts with IST and 11.27 in controls (p = 0.09). Average Ln of high freq power was 9.81 in pts and 11.31 in controls (p = 0.002). 1) HRV is markedly decreased in pts with IST; 2) This seems to be related to an abnormally low parasympathetic tone; 3) The long-term prognosis of pts with IST deserves further investigation.