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The effect of sympathetic and parasympathetic activity on QT duration. Clinical study in patients with normal and prolonged QT time.

The effect of parasympathetic and sympathetic pharmacologic blockade (atropine and propranolol) on QT duration was studied in 26 patients. Of this number 17 had a normal QT time, while in 9 patients the QT interval was prolonged (greater than 460 ms). QT time was determined before and after the pharmacologic blockade with sinus rhythm (under a non fixed cycle length) and with atrial pacing (with a fixed cycle length). Atropine administration did not change QT duration significantly. On the effect of beta receptor blockade in normal patients only the corrected QT time shortened, while in long QT syndrome a very pronounced shortening in QT time, independent of the changes in the cycle length, could be observed. In QT prolongation an overactivity in the sympathetic tone can be supposed.