Differential effect of stimulation of nucleus ambiguus on atrial and ventricular rates.

Acute experiments were performed on open-chest male mongrel dogs that were anesthetized with thiopental sodium and respired. An average of 10 penetrations per side was used to explore the rostral-caudal organization of the nucleus ambiguus (NA) and the area ventrolateral to it. Atrial pacing was performed on some trials to clarify the occurrence of atrioventricular (AV) block. In addition, the vagi were sectioned in some animals to confirm the peripheral pathway of the observed effects. Finally, the twin-pulse technique was used to classify the neurons underlying the observed effects according to their refractory periods. Stimulation in the area of the NA and lateral to it produced bradycardia and bradyarrhythmias without altering blood pressure. Stimulation of either the left or right NA produced slowing of both atrial and ventricular rates. However, slowing of the atrial rate was greatest with right NA stimulation, whereas slowing of the ventricular rate was predominant with left-side stimulation. The presence of AV block was evident from dropped QRS complexes with and without pacing. Vagotomy demonstrated that these effects were conducted by ipsilateral but not contralateral nerves. Measurement of the refractory periods showed that the neurons underlying both atrial and ventricular slowing were characteristic of B-fibers.