Abstract

A new method is presented by which the function and reactivity of a ventricular pacemaker may be studied. This was accomplished by stimulating a vagus nerve so that the sino-atrial pacemaker was depressed to a critical level and in this circumstance superimposing sympathetic activity through injection of a sympathomimetic amine. In this way the automaticity of the ventricular pacemaker is raised and atrio-ventricular rhythms result.

The smallest dose of a sympathomimetic amine to produce atrio-ventricular rhythms during vagal stimulation was considered threshold. Threshold comparisons of certain sympathomimetic amines in the cat and dog revealed isopropylarterenol to be approximately ten times more potent than either l-epinephrine or levarterenol. In the cat, phenylephrine and ephedrine were between fifty and one hundred times less active than isopropylarterenol. Methoxamine was found to be considerably weaker than any of the other amines tested.

Although direct comparison of the sino-atrial and the ventricular pacemakers were not made certain observations suggest that the sino-atrial node is more sensitive to sympathomimetic stimulation.

Isopropylarterenol was found significantly more effective on the sino-atrial pacemaker than either epinephrine or levarterenol, as was the case for the ventricular pacemaker.

The greater effectiveness of isopropylarterenol on the sino-atrial and ventricular pacemaker and its negligible action on myocardial automaticity suggests that this drug may be of value in those instances where an increase in myocardial automaticity may be undesirable.

The results of these investigations reveal that the influence of the vagus on automaticity of the sino-atrial node is great as opposed to a minor influence on ventricular pacemaker automaticity.