Bradycardia Induced by Intravascular Versus Direct Stimulation of the Vagus Nerve

Background. Electrical stimulation of the parasympathetic nervous system results in slowing of the heart. We sought to determine whether cardiac vagal efferent axons can be stimulated adequately to induce bradycardia without disturbing the integrity of the thorax. Methods. Cardiodepressor effects elicited by direct stimulation of a vagus nerve in anesthetized dogs and pigs were compared with those generated when the same nerve was stimulated indirectly through bipolar electrodes placed in the adjacent superior vena cava. Results. The heart rate of dogs decreased by about 80% when electrical stimuli were delivered to the right thoracic vagus at the level of the thoracic outlet through bipolar electrodes placed either in the adjacent superior vena cava (intravascular method) or directly on the nerve (direct method). Maximal responses were achieved with 10-V, 5-ms, and 20-Hz stimuli. In anesthetized pigs, similar bradycardia occurred when the right cervical vagus or the right cranial thoracic vagus was stimulated either directly or indirectly through the intravascular method. Atrial dysrhythmias occurred when the stimulating electrodes were placed by either method within 1 cm of the right atrium in both animal models. Conclusions. Controlled bradycardia can be induced during operation without the risk of generating cardiac dysrhythmias using electrical stimuli (10 V, 5 ms, and 10 to 20 Hz) delivered to the right cervical vagus nerve or the right cranial thoracic vagus nerve through adjacent intravascular electrodes.