Autonomic reflexes in patients with cardioinhibitory carotid sinus syncope

Can neurons in the nucleus ambiguus selectively regulate cardiac rate and atrio-ventricular conduction?

Previous anatomic data have described the distribution of presumptive negative chronotropic and negative dromotropic neurons in the ventro-lateral nucleus ambiguus (NA-VL) following injections of retrograde tracers into physiologically selective parasympathetic intracardiac ganglia. Negative dromotropic neurons were preferentially distributed in the rostral NA-VL (rNA-VL). Negative chronotropic neurons were preferentially distributed in the caudal NA-VL (cNA-VL). Significant numbers of both types of cardio-inhibitory neurons were observed to overlap in an intermediate level of the NA-VL (iNA-VL). In the present report, we have examined the effects of microinjections of the excitatory amino-acid glutamate (GLU) into the cNA-VL and iNA-VL on cardiac rate and AV conduction while recording the electrocardiogram in paced and non-paced cat hearts. The data indicate that: (i) excitation of neurons in the cNA-VL causes a 58 +/- 17% reduction in cardiac rate, without influencing AV conduction; and (ii) excitation of neurons in the iNA-VL causes both a reduction in heart rate (68 +/- 12%) and a decrease in the rate of AV conduction (38 +/- 7%). These physiological results support the anatomical inference that neurons in the cNA-VL that are retrogradely labeled from physiologically selective parasympathetic intracardiac ganglia selectively exhibit negative chronotropic properties. Furthermore, the data indicate that there is a longitudinal cardiotopic organization of both negative chronotropic and negative dromotropic neurons in the NA-VL. This CNS organization mirrors the peripheral organization of functionally selective cardiac components of the vagus nerve. Finally, the data are consistent with the hypothesis that anatomically separated and functionally selective parasympathetic preganglionic vagal motoneurons in the NA independently control cardiac rate and AV conduction.

Observations on the arterial baroreflex in neurally mediated vasodepressor syncope

The arterial baroreflex was studied in subjects who had recently had an episode of vasodepressor syncope. This was determined using 2-3 mcg/kg intravenous boluses of phenylephrine and assessing the bradycardic response. The values were measured in ms/mmHg and expressed as the angular coefficient of the regression line between the increase in R-R interval on the electrocardiograph and the systolic arterial pressure. In subjects examined immediately after the vasodepressor syncope episode the bradycardic response was much more marked than in controls (p < 0.01) and in the subjects themselves 6 months after the episode, provided that they were symptom-free (p < 0.01). It is concluded that in vasodepressor syncope there is a phase in which the baroreflex is highly sensitive and that this is due not to a lowering of the stimulation threshold but to a gain in the efferent arc, which explains a 'vagotonic' response.

Pattern of inhibition of parasympathetic activity in response to incremental bolus doses of atropine in carotid sinus hypersensitivity

Patients with established reproducible cardioinhibitory carotid sinus hypersensitivity were studied to define the dose of atropine required to abolish the heart rate slowing in response to carotid sinus massage, the heart rate response to the Valsalva manoeuvre and salivary gland flow. Bolus doses of intravenous atropine were given to a cumulative dose of 700 mcg. Cardioinhibition was abolished in all patients with a total dose of 700 mcg. The heart rate ratio during the Valsalva manoeuvre did not vary significantly. The decline in salivary gland flow was evident earliest, at a dose of 75 mcg. In clinical studies, 700 mcg of atropine will abolish the diagnostic cardioinhibitory response to carotid sinus message in patients with the syndrome. This may not prevent syncope, as could occur in the key frequent vasodepressor form of carotid sinus hypersensitivity syndrome.