Haemodynamic effects of withdrawal of efferent cervical vagal stimulation on anesthetized dogs—relative importance of chronotropic and non-chronotropic mechanisms

Abstract

The aim of the present work was to study changes in cardiac output (CO) and arterial blood pressure (ABP) following either interruption of artificial efferent vagal stimulations (STOP), or suppression of negative chronotropic effects, during uninterrupted vagal stimulations (PACE). Experiments were performed on 7 anesthetized, open-chest dogs. A computerized data acquisition system was used to record CO (electromagnetic flowmeter), ABP, right atrial pressure and electrocardiogram; 9 parameters were automatically elaborated. The peripheral stumps of both vagus nerves, sectioned at the neck, were stimulated for long control periods (at least 3 min) with brief trains of stimuli triggered by atrial P waves. Records were started during steady-state vagal stimulations, and consisted of paired trials: in the first step the vagal stimulators were turned off (STOP); in the second step the heart was paced at the same rate reached at the end of the preceding step, but vagal stimulation was continued (PACE). Observations lasted two min after each step. Results indicate rapid rise in CO and ABP after STOP, up to 30% and 10%, respectively, in 10 s, followed by slow reduction in CO and further increase in ABP (22% and 15% respectively, at 120 s). Thus STOP caused rapid and sustained improvements in the cardiac performance. After PACE changes in CO and ABP were smaller and followed a slower time-course. The greater effects of STOP with respect to PACE were attributed to non-chronotropic mechanisms, accounting for about 50% of the overall haemodynamic consequences of vagal withdrawal. Since peak aortic flow velocity and acceleration were increased after STOP, stroke volume was reduced much less than after PACE, despite equal rise in heart rate, and similar shortening in the ejection time. Evidence was presented of enhanced atrial and ventricular contractility after STOP. Experiments performed after beta-blockade in 5 dogs substantially confirmed the results. It is concluded that vagal withdrawal, which is an important aspect in many physiological situations, constitutes a rather powerful strategy for rapid enhancement of the cardiovascular performance, through different mechanisms, in addition to cardioacceleration.