Studies of blood-pressure regulation. IV. The effects of impulse train stimulation of the carotid-sinus nerves

High-resolution measurement of electrically-evoked vagus nerve activity in the anesthetized dog

OBJECTIVE

Not fully understanding the type of axons activated during vagus nerve stimulation (VNS) is one of several factors that limit the clinical efficacy of VNS therapies. The main goal of this study was to characterize the electrical recruitment of both myelinated and unmyelinated fibers within the cervical vagus nerve.

APPROACH

In anesthetized dogs, recording nerve cuff electrodes were implanted on the vagus nerve following surgical excision of the epineurium. Both the vagal electroneurogram (ENG) and laryngeal muscle activity were recorded in response to stimulation of the right vagus nerve.

MAIN RESULTS

Desheathing the nerve significantly increased the signal-to-noise ratio of the ENG by 1.2 to 9.9 dB, depending on the nerve fiber type. Repeated VNS following nerve transection or neuromuscular block (1) enabled the characterization of A-fibers, two sub-types of B-fibers, and unmyelinated C-fibers, (2) confirmed the absence of stimulation-evoked reflex compound nerve action potentials in both the ipsilateral and contralateral vagus nerves, and (3) provided evidence of stimulus spillover into muscle tissue surrounding the stimulating electrode.

SIGNIFICANCE

Given the anatomical similarities between the canine and human vagus nerves, the results of this study provide a template for better understanding the nerve fiber recruitment patterns associated with VNS therapies.

Haemodynamic information encoded in the aortic baroreceptor discharge during haemorrhage

This study concerns the function of aortic baroreceptors during slow haemorrhage, in particular the mechanisms by which baroreceptors respond to hypovolaemia in the absence of hypotension and the manner in which haemodynamic information is encoded in the aortic nerve discharge. Beagle dogs, anaesthetized with morphine and chloralose, were instrumented for the recording of aortic pressure, aortic diameter, aortic flow and aortic nerve activity. The animals were subjected to slow haemorrhage (0.4 ml kg-1 min-1) of 20% of the estimated blood volume. Mean aortic pressure remained unchanged, whereas mean aortic diameter, mean aortic tension and aortic baroreceptor activity decreased during the haemorrhage. Baroreceptor sensitivity to mean aortic pressure, mean aortic diameter and mean aortic tension decreased as the haemorrhage proceeded. Also, the sensitivity of baroreceptors to mean aortic pressure and mean aortic diameter during acute pressure stimulation was attenuated. The results indicate that during non-hypotensive haemorrhage aortic baroreceptor discharge is reduced by two mechanisms: firstly, via constriction and stiffening of aortic smooth muscle and, secondly, via direct effects of the compensatory mechanisms on the baroreceptors. The reduced sensitivity renders the baroreceptors capable of responding to hypovolaemia prior to the onset of hypotension. In the regression analysis, systolic aortic diameter proved to be the best determinant of baroreceptor activity (r = 0.877). Of the neural variables, impulses per cardiac cycle and average discharge rate were the most reliable, but no difference in the relative importance of these two parameters could be observed.

Frequency limitation in the human baroreceptor reflex

In 10 unsedated normotensive subjects, low-level bilateral electrical stimulation of the carotid sinus nerves (0.35 ms, 1-2 V) was applied with frequencies of 20-200 Hz for therapeutic purposes. Both peak and steady-state reflex effects increased with the stimulus rate up to 80 Hz. After 1 min of stimulation, the largest fall of arterial pressure (21.0 +/- 5.8%, mean +/- S.D.) was produced by stimulus frequencies of 80-120 Hz. Right atrial pacing did not modify the results. After 1 min of stimulation at 200 Hz, the arterial pressure fall was reduced to 13.6 +/- 5.2% (P less than 0.001), whereas peak effects, reached after 15-40 s, were near maximal. The reduction of the depressor response emerged after about 10 s of stimulation and was complete after about 1 min. Peak RR-interval prolongation occurred within 10 s and showed no adaptation. Comparison of continuous stimulation and R-wave-triggered intermittent stimulation in 3 subjects suggested that the reflex effect was determined by the mean stimulus frequency. We conclude that frequency limitation in the baroreceptor reflex begins when the stimulus rate exceeds about 80 Hz. It leads to a frequency-dependent, gradual adaptation of the reflex effects when the stimulus rate exceeds 120 Hz. This conclusion is limited to the part of the reflex that is subserved by myelinated afferent fibers.

Responses of the cardiovascular system to carotid sinus nerve stimulation

The changes in blood pressure, heart rate, cardiac output and blood flow in the femoral and common carotid arteries on carotid sinus nerve stimulation (CSNS) were studied in chloralose anaesthetized dogs, both with spontaneous heart rhythm and during atrial pacing. Stimulation of the sinus nerves with impulse trains and with impulses of constant frequency had almost equal effects on the blood pressure. The former had a greater effect on the heart rate; these findings verified earlier observations. The reductions in cardiac output followed those in heart rate. During atrial pacing the stroke volume was reduced on CSNS. The total peripheral resistance, regional peripheral resistances and input impedance of the vascular bed of the femoral artery were calculated. The initial effects of CSNS varied in relation to the prestimulation total peripheral resistance and to stimulation frequency. Differences between stimulation with a constant frequency and the intermittent types, with the same number of impulses per cardiac cycle, were negligible as regards effects on stroke volume, blood flow and regional vascular resistances. CSNS caused changes in input impedance of the vascular bed of the femoral artery which were very similar to those observed earlier on intraarterial injection of vasodilator drugs. The different effects of intermittent and constant frequency CSNS on the heart rate in dogs with intact vagal nerves and no clamping of the common carotid arteries might be caused by asymmetries in the autonomic effects on the S.A. node. The neurophysiological mechanisms are discussed. It is deduced that greater reductions in blood pressure with intermittent stimulation are only obtained in the pre-existence of a high sympathetic tone.

Renal function in experimental chronic hydronephrosis. II. Backflow studies with radionuclides

Studies of backflow in experimental unilateral chronic hydronephrosis in dogs with a known residual renal function and renal pelvic volume were performed with radioactively labelled substances with molecular weights of ca. 600, 5 200 and 68 000 placed in the hydronephrotic renal pelvis. Three different pressure levels in the renal pelvis were used. Isotope concentrations were determined in the thoracic duct lymph, 0lood and urine from the contralateral kidney. The greater part of the test substances appeared to pass to the blood circulation, while a smaller part was found in the lymphatic ducts. The largest molecule, however, was only able to leave the renal pelvis to any appreciable extent at the highest pressure. In kidneys with a low grade of hydronephrosis and well preserved function, a relatively large amount of test substance left the renal pelvis (ca. 10-15%). In more advanced grades of hydronephrosis with greater impairment of renal function, the backflow was considerably less.