Fixed Coupling and Short P-R Interval Induced in the Dog by Stimulation of the Sympathetic Nervous System

¹ From the Department of Physiology, University of Louisville School of Medicine, Louisville, Kentucky

Injection of 0.06–0.12 ml/kg of m/6 mixture of monobasic and dibasic potassium phosphate (ph 7.6) into the cisterna magna of barbitalized dogs increases the blood pressure 50–100%, increases the heart rate 20–80%, and induces cardiac arrhythmia. The effects of strong stimulation of the sympathetic nervous system, thus demonstrated, were studied in dogs with and without the vagi intact. Stimulation of the sympathetic nervous system in vagotomized dogs induces scattered ectopic ventricular beats, ventricular tachycardia and bigeminy. The bigeminy usually appears as an alternation of a normal sinus beat with an abnormal ventricular beat showing a short P-R interval and a prolonged QRS complex. This abnormal beat resembles the aberrant ventricular beat observed in the Wolff-Parkinson-White (WPW) syndrome. However, it is shown, by peripheral vagal stimulation, that the abnormality of the QRS complex is due to an ectopic ventricular impulse which usually fuses with a sinus impulse. Vagal stimulation inhibits the sinus impulse without changing the interval between the ectopic ventricular impulse and the preceding normal beat. In this way fixed coupling of the ectopic ventricular impulse with the preceding beat is established as the mechanism of the bigeminy.