[Changes in heart rate after ventricular stimulation; correlations with vagal tone]

The authors report a new method of studying the autonomic nervous system, especially vagal tone, during endocavitary electrophysiological studies. After termination of fixed ventricular pacing at incremental rates to 200/min, an initial acceleration of the heart rate is observed followed about 5 seconds later by a sudden slowing. This phenomenon was studied in 278 patients. Ninety seven patients had no cardiac disease: the variation in heart rate was 33 +/- 18%. In the 181 other patients with cardiac disease (ejection fraction 35 +/- 16%) the variation was only 21 +/- 16% (p < 0.01). When the ejection fraction was less than 30%, the variation was only 13 +/- 4%. When the heart rate variation was less than 10%, the prognosis was poor because, of the 48 patients with this sign, 13 died, whereas there were only 2 deaths in the 133 other patients with cardiac disease and preserved adaptation. In 14 patients without cardiac disease the injection of 2 mg of atropine suppressed all adaptation of the heart rate, whilst in 18 other patients, oral betablockers reduced the variation but it persisted to a significant degree. The authors conclude that the adaptation of the heart rate after rapid ventricular stimulation is probably a reflection of vagal tone and may be used to assess the prognosis of subjects undergoing electrophysiological investigations.