Continuous low-level vagus nerve stimulation reduces stellate ganglion nerve activity and paroxysmal atrial tachyarrhythmias in ambulatory canines

Abstract

Background: We hypothesize that left-sided low-level vagus nerve stimulation (LL-VNS) can suppress sympathetic outflow and reduce atrial tachyarrhythmias in ambulatory dogs.

Methods and results: We implanted a neurostimulator in 12 dogs to stimulate the left cervical vagus nerve and a radiotransmitter for continuous recording of left stellate ganglion nerve activity, vagal nerve activities, and ECGs. Group 1 dogs (N=6) underwent 1 week of continuous LL-VNS. Group 2 dogs (N=6) underwent intermittent rapid atrial pacing followed by active or sham LL-VNS on alternate weeks. Integrated stellate ganglion nerve activity was significantly reduced during LL-VNS (7.8 mV/s; 95% confidence interval [CI] 6.94 to 8.66 versus 9.4 mV/s [95% CI, 8.5 to 10.3] at baseline; P=0.033) in group 1. The reduction was most apparent at 8 am, along with a significantly reduced heart rate (P=0.008). Left-sided low-level vagus nerve stimulation did not change vagal nerve activity. The density of tyrosine hydroxylase-positive nerves in the left stellate ganglion 1 week after cessation of LL-VNS were 99 684 μm(2)/mm(2) (95% CI, 28 850 to 170 517) in LL-VNS dogs and 186 561 μm(2)/mm(2) (95% CI, 154 956 to 218 166; P=0.008) in normal dogs. In group 2, the frequencies of paroxysmal atrial fibrillation and tachycardia during active LL-VNS were 1.4/d (95% CI, 0.5 to 5.1) and 8.0/d (95% CI, 5.3 to 12.0), respectively, significantly lower than during sham stimulation (9.2/d [95% CI, 5.3 to 13.1]; P=0.001 and 22.0/d [95% CI, 19.1 to 25.5], P<0.001, respectively).

Conclusions: Left-sided low-level vagus nerve stimulation suppresses stellate ganglion nerve activities and reduces the incidences of paroxysmal atrial tachyarrhythmias in ambulatory dogs. Significant neural remodeling of the left stellate ganglion is evident 1 week after cessation of continuous LL-VNS.

Figure 1

Study protocols. A, For dogs without rapid atrial pacing (Group 1, N=6), one week of baseline recording was performed two weeks after surgery. Chronic low-level vagus nerve stimulation (LL-VNS) was then performed for one week. After LL-VNS terminated, nerve activities were recorded for another week before euthanasia. B, For dogs undergoing intermittent rapid atrial pacing (Group 2, N=6), after 2 weeks of postoperative recovery, nerve activities and heart rhythm were recorded for 1 day before the onset of pacing. High-rate (640 bpm) atrial pacing was then given for 6 days, followed by 1 day of pacing-free recording. During this recording period only, active or sham stimulation sessions were given on alternate weeks. This protocol was repeated until sustained AF was documented. LL-VNS, low-level vagus nerve stimulation; AF, atrial fibrillation.

Figure 2

Immediate effects of LL-VNS. The administration of LL-VNS (without affecting sinus rate) immediately suppressed SGNA for approximately 10 sec (from point a to b) before sporadic SGNA reappeared. Upward dashed arrows point to SGNA. Except for the first 5 sec during programmer transmission, chronic LL-VNS did not produce any stimulus artifacts. ECG, electrocardiogram; SGNA, stellate ganglion nerve activity; VNA, vagal nerve activity; SLGPNA, superior left ganglionated plexi nerve activity.

Figure 3

Effects of LL-VNS on SGNA and heart rate (HR). A, Chronic LL-VNS significantly reduced SGNA over 24 hrs. The SGNA normalized to baseline level after cessation of LL-VNS. B, Hourly averages of SGNA show that the reduction in integrated SGNA was particularly striking at 8 AM. All values are averaged over 5 days and 6 dogs. C, The administration and cessation of chronic LL-VNS did not change the overall heart rate. D, Hourly averages of heart rate reveal that the morning surge of heart rate (arrowhead) was markedly attenuated during LL-VNS. HR, heart rate. * P<0.05.

Figure 4

Effects of LL-VNS on SGNA-induced heart rate changes. A, Graph of statistical dot plot shows that the SGNA-induced heart rate acceleration was markedly attenuated during LL-VNS compared to baseline. B, An example at baseline shows a burst of SGNA (black arrowhead) led to heart rate increase from 67 to 196 bpm (129 bpm increment). C, An example during LL-VNS shows a burst of SGNA (white arrowhead) led to heart rate increase from 63 to 160 bpm (97 bpm increment).

Figure 5

Daily changes of SGNA in response to the LL-VNS. The SGNA was significantly decreased on the D1 of LL-VNS and was further decreased on the D4 and D5 of LL-VNS. After the cessation of LL-VNS, the SGNA remained suppressed for more than 24 hrs before gradually normalizing to baseline level. * P<0.05 compared to the D5 of baseline. † P<0.05 compared to the D3 during LL-VNS.

Figure 6

Tyrosine hydroxylase (TH) immunostaining of left stellate ganglion. Upper panel shows the left stellate ganglion (LSG) of three representative dogs with vagus nerve stimulation (all belong to Group 1). Lower panel shows the LSG of three representative normal control dogs. In experimental dogs, there was a significantly decreased density of TH-positive nerve structures in the LSG and significantly more ganglion cells lacking immunoreactivity to TH (unfilled arrows), which were less common in the control dogs (solid arrows). Scale bar=50 µm.

Figure 7

Effects of LL-VNS on paroxysmal atrial tachyarrhythmias. A, Chronic LL-VNS significantly prevented the occurrence of paroxysmal atrial fibrillation (PAF). B, An example of PAF that shows fast and irregular ventricular responses and fractionated atrial electrograms (inset). The PAF episodes were following burst firings of SGNA, VNA and SLGPNA (arrowheads). C, LL-VNS also significantly prevented the occurrence of paroxysmal atrial tachycardia (PAT). D, An example of PAT shows that following burst firings of SGNA, VNA and SLGPNA (arrowheads), the atrial rate abruptly accelerated to 228 bpm and lasted for more than 20 sec. As contrasted to PAF, the atrial local electrograms in PAT were regular (inset). * P<0.05 comparing active LL-VNS with sham. PAF, paroxysmal atrial fibrillation; PAT, paroxysmal atrial tachycardia.