Effect of combined sympathetic and vagal stimulation on heart rate in the dog.

A set of equations is described which permit prediction of the time course of heart rate in a dog anesthetized with pentobarbital from the time course of frequency of stimuli applied to the distal cut ends of vagus and sympathetic efferent nerves to the heart. These equations, although based on assumptions as to the physiological mechanisms involved, were only tested for their ability to describe the kinetic relationship involved. The time course of heart rate resulting from combined vagus and sympathetic nerve stimulation (HR SV) is given by the equation: where the subscripted terms are heart rate values due to vagus stimulation (v), sympathetic stimulation (s), no stimulation (0) and the minimum (min) heart rate achievable by vagus stimulation alone. Clearly the vagus influence is capable of dominating the sympathetic in the control of heart rate. ADDITIONAL KEY WORDS computer nervous control heart rate cardiovascular control • The time course of heart rate resulting from stimulation of the sympathetic or of the vagus nerves to the heart is described by a mathematical model developed by Warner and Cox (1) in 1962. This model predicts the heart rate response to stimulation of either set of nerves alone but will not predict the response to simultaneous stimulation of sympathetic and vagus nerves. It is the purpose of this paper to describe an addition to the model which predicts the response to combined stimulation (2) and to describe experiments designed to test the performance of the model under a variety of stimulus parameters. Methods and Procedure The experiments were performed on 14 mongrel dogs anesthetized with pentobarbital, 30 mg/kg, iv. A cuffed endotracheal tube was inserted, and the animals were maintained on positive-pressure ventilation with 100% oxygen. A single-lead electrocardiogram was monitored on an oscilloscope throughout the procedure to assure that all measurements were made during a time when the animal had a normal sinus rhythm. The isolation and preparation of the sympathetic and vagus nerves was performed as previously described (1). The distal cut ends of the vagus and cardiac sympathetic nerves (usually on the right side) were stimulated supramaximally with a 6-v, 1-msec square wave generated through a digital-to-analog converter by a Control Data 3200 digital computer and passed through an isolation transformer. Timing of each stimulus for either nerve was controlled by the program and the repetition rate varied from zero to 20/sec. In these experiments no attempt was made …