Parasympathetic dysfunction and antiarrhythmic effect of vagal nerve stimulation following myocardial infarction

Abstract

Myocardial infarction causes sympathetic activation and parasympathetic dysfunction, which increase risk of sudden death due to ventricular arrhythmias. Mechanisms underlying parasympathetic dysfunction are unclear. The aim of this study was to delineate consequences of myocardial infarction on parasympathetic myocardial neurotransmitter levels and the function of parasympathetic cardiac ganglia neurons, and to assess electrophysiological effects of vagal nerve stimulation on ventricular arrhythmias in a chronic porcine infarct model. While norepinephrine levels decreased, cardiac acetylcholine levels remained preserved in border zones and viable myocardium of infarcted hearts. In vivo neuronal recordings demonstrated abnormalities in firing frequency of parasympathetic neurons of infarcted animals. Neurons that were activated by parasympathetic stimulation had low basal firing frequency, while neurons that were suppressed by left vagal nerve stimulation had abnormally high basal activity. Myocardial infarction increased sympathetic inputs to parasympathetic convergent neurons. However, the underlying parasympathetic cardiac neuronal network remained intact. Augmenting parasympathetic drive with vagal nerve stimulation reduced ventricular arrhythmia inducibility by decreasing ventricular excitability and heterogeneity of repolarization of infarct border zones, an area with known proarrhythmic potential. Preserved acetylcholine levels and intact parasympathetic neuronal pathways can explain the electrical stabilization of infarct border zones with vagal nerve stimulation, providing insight into its antiarrhythmic benefit.

Keywords: Cardiology.

Figure 1. Cardiac voltage mapping and neurotransmitter analysis.

(A) Bipolar voltage electroanatomic maps of infarcted hearts were obtained and used to identify viable, border zone, and scar regions. Examples of bipolar electrical recordings from viable, border zone, and scar regions are shown. Examples of uinpolar electrograms obtained from similar regions are also shown. This map and bipolar voltage measurements were used to obtain samples for neurotransmitter analysis from appropriate locations. The scale bar shows the voltage from 0–10 mV. (B) In normal hearts (n = 12 normal animals, 3 samples per animal), acetylcholine (ACh) levels were not significantly different on the LV apex, anterior, lateral walls (linear mixed effects model). (C) ACh levels from paired LV epicardium and endocardium samples were also not significantly different (n = 9 normal animals, 2–3 samples per animal). (D) In infarcted hearts (n = 14), scar regions have the lowest ACh content compared with viable and border zone regions (P < 0.01, linear mixed effects model, 1–3 samples per region per animal). However, ACh content between viable and border zone regions of infarcted animals (n = 14) compared with similar regions of normal hearts (n = 15 animals) was not statistically significant (P = 0.4, linear mixed effects model). There is a suggestion that scar ACh may be somewhat reduced compared with normal hearts; however, this difference did not reach statistical significance. (E) Similar to ACh levels, there was no difference in norepinephrine (NE) content across the LV apex, anterior, or lateral wall (linear mixed effects model, n = 12 normal animals, 3 samples per animal). (F) Also, similar to ACh, NE levels in the endocardium were similar to epicardium (linear mixed effects model, n = 9 animals, 2–3 samples per animal). (G) Unlike ACh, the NE content in scar and border zone regions was significantly reduced compared with viable myocardium in infarcted hearts (P < 0.05 for border zone vs. viable, and P < 0.01 for scar vs. viable, linear mixed effects model, n = 5 animals, 1–3 samples per region per animal) and was significantly less than similar regions in normal hearts (P < 0.01, linear mixed effects model). There was no difference in NE content of viable regions of infarcted hearts compared with normal hearts. MI, myocardial infarction; EGM, myocardial electrogram; LV, left ventricle; Epi, epicardium; Endo, endocardium.

Figure 2. Recordings from cardiac neurons that are activated by VNS.

(A) Example of direct neuronal recordings from the ventral interventricular (VIV) fat pad ganglia of a normal heart showing 2 neurons that increased their firing activity with VNS and were therefore classified as postganglionic parasympathetic neurons. Each neuron was also identified and classified by its unique neuronal waveform (inset). (B) Example of neuronal recordings from the VIV fat pad of a heart with myocardial infarction (MI) demonstrating 2 neurons that also increased their firing frequency with VNS. Basal activity in the minute prior to VNS compared with during VNS was used to identify parasympathetic neurons. (C) The basal (prestimulation) activity of parasympathetic neurons that were activated by left VNS is reduced in infarcted hearts compared with normal hearts (n = 54 neurons from 15 normal and n = 33 neurons from 10 infarcted animals, P < 0.05, unpaired Student’s t test). (D) Basal activity of neurons that respond to right VNS is unchanged in normal vs. infarcted hearts (n = 56 neurons from 15 normal and n = 33 neurons from 10 infarcted animals, P = 0.4, unpaired Student’s t test). LVP, left ventricular pressure; VNS, vagal nerve stimulation.

Figure 3. Recordings from cardiac neurons that are suppressed by VNS.

(A) Example of direct neuronal recordings from the VIV fat pad ganglia of a normal heart showing 2 neurons that decrease their firing activity with VNS and were therefore classified as postganglionic parasympathetic neurons. Each neuron was identified and classified by its unique neuronal waveform (inset). (B) Example of neuronal recordings from the VIV fat pad of an infarcted heart demonstrating 2 neurons that also decreased their firing frequency with VNS. Basal activity in the minute prior to VNS compared with during VNS was used to identify parasympathetic neurons. (C) The basal (prestimulation) activity of parasympathetic neurons that were suppressed by left VNS is reduced in MI compared with normal hearts (P < 0.05, unpaired Student’s t test, n = 84 neurons from 15 normal and n = 18 neurons from infarcted animals). (D) Basal activity of neurons that are suppressed by right VNS is unchanged in normal vs. infarcted hearts (P = 0.4, unpaired Student’s t test, n = 86 neurons from 15 normal and n = 28 neurons from 10 infarcted animals). LVP, left ventricular pressure; MI, myocardial infarction; VNS, vagal nerve stimulation.

Figure 4. Classification and relationships of parasympathetic neurons in normal and infarcted hearts.

(A) Functional classification of parasympathetic neurons that receive inputs from the left or right vagus is shown in normal hearts. Overall, percentage of convergent parasympathetic neurons that responded to left VNS and at least 1 afferent stimulus (therefore classified as convergent) was 81% (109 of 135 left VNS responsive neurons from 15 normal animals), and the percentage of convergent neurons that responded to right VNS was 82% (109 of 133 right VNS responsive neurons from 15 normal animals). Of these convergent neurons, approximately 30% also received sympathetic input in normal hearts. (B) Type of parasympathetic neurons that receive input from the left or right vagus is shown in hearts with myocardial infarction (MI). Percentage of convergent neurons that responded to left VNS was 79% (42 of 53 left VNS responsive neurons from 10 infarcted animals), and percentage of convergent neurons that responded to right VNS was 82% (41 of 50 total neurons from 10 infarcted animals). These values were not different than normal hearts (P = 0.8, χ² test). However, percentage of convergent neurons also receiving sympathetic input was significantly increased in infarcted (n = 15 animals) compared with normal hearts (n = 10 animals), representing 65% (27 of 47 convergent neurons) and 68% (28 of 50 convergent neurons) for left and right VNS in infarcted animals, respectively, compared with 31% (34 of 109 convergent neurons) and 30% (33 of 109 convergent neurons) for left and right VNS in normal hearts, respectively (P < 0.05, χ² test). (C) Conditional probability analysis (probability that a VNS neuron that responded to one stimulus [X] also responded to another stimulus [Y]) for neurons that respond to VNS is shown in normal hearts. Significant relationships are noted for parasympathetic neurons that transduce preload and sensory inputs from the RV and LV due to touch, as well as those that transduce preload and receive sympathetic input. (D) Graphical representation of the conditional probability analysis is shown for VNS-responsive neurons of normal hearts, where significant relationships (those with a value ≥ 0.6) are delineated. (E) Conditional probability analysis for neurons that respond to VNS in infarcted hearts is shown. Significant relationships are noted for parasympathetic neurons that transduce preload and sensory inputs from the RV and LV due to touch, as well as those convergent neurons that transduce preload and receive sympathetic input. However, additional interactions of convergent neurons with sympathetic ganglion neurons exist in infarcted hearts. (F) Graphical representation of the conditional probability analysis is shown for parasympathetic neurons of infarcted hearts, where significant relationships (those with a value ≥ 0.6) are delineated. Eff, efferent; Symp, sympathetic; RV, right ventricle; LV, left ventricle; IVCo, IVC occlusion; Ao, aortic occlusion; SGS, sympathetic ganglion stimulation; VNS, vagal nerve stimulation; MI, myocardial infarction.

Figure 5. Electrophysiological response to VNS.

(A) Examples of unipolar electrograms obtained from the sock electrode and used for ARI analysis from normal hearts and from viable, border zone, and scar regions of infarcted hearts are shown. (B) Global ARI (mean ARI from all 56 electrodes) in normal animals (n = 12 animals) increased with VNS (left panel, P < 0.01, Wilcoxon signed rank test). Example of a polar map (right panel) from 1 normal animal shows that increases were observed in all regions. (C) The global ARI in infarcted hearts (n = 18 animals) also significantly increased with VNS (left panel, P < 0.01, Wilcoxon signed rank test). Example of polar maps obtained from 1 infarcted animal is shown (right panel). In infarcted hearts, VNS has a significant impact on regional ARIs, particularly noticeable along peri-infarct regions (location of scar is marked with dashed lines on the polar maps of this animal). (D) Scar regions, at baseline, demonstrated the greatest mean ARI (**P < 0.001, linear mixed effects model, n = 18 infarcted animals). In addition, all regions, even scar, demonstrate a significant increase in mean ARI from baseline (*P < 0.01, linear mixed effects model, n = 18 infarcted animals). Furthermore, when the percentage increase in ARI with VNS is compared across various regions of infarcted hearts, consistent with the ACh findings, scar shows the least response compared with viable and border zone regions (^§P < 0.01, linear mixed effects model, n = 18 infarcted animals). However, as with ACh results, there was no statistical difference in the response to VNS of scar compared with normal hearts (^¥P = 0.1, linear mixed effects model, n = 18 infarcted animals). ARI, activation recovery interval; MI, myocardial infarction; VNS, vagal nerve stimulation; LV, left ventricle.

Figure 6. Arrhythmia inducibility and dispersion of repolarization.

(A) An example of VT/VF induction using extrastimulus pacing from the RV endocardium is shown. Prior to VNS, this infarcted animal was inducible for VT, which degenerated into VF and required defibrillation. During intermittent VNS, VT/VF was no longer inducible with triple extrastimuli, and effective refractory period was reached. (B) Intermittent VNS significantly reduced the proportion of animals that were inducible for VT (n = 21 infarcted, P = 0.04, McNemar’s test). (C) Epicardial activation map during VT in an infarcted animal that was inducible at baseline shows significant area of slowed conduction (multiple contours, zig-zag arrow) with “early” activation meeting “late activation” along the anterolateral border zone, likely contributing to reentry. (D) The dispersion in ARI at baseline and during VNS is shown across scar, border zone, and viable regions in infarcted hearts. Regional analysis showed that the dispersion in ARI in border zone regions was greater than scar regions (P < 0.05, linear mixed effects model, n = 18 infarcted animals) at baseline. (E) Intermittent VNS significantly reduced the dispersion in ARI of border zone regions compared with scar and viable areas (*P < 0.01, linear mixed effects model, n = 18 infarcted animals). ARI, activation recovery interval; MI, myocardial infarction; VNS, vagal nerve stimulation; VT, ventricular tachyarrhythmia.

Figure 7. Delineation of scar via MRI, electroanatomic mapping, and histology.

(A) Location and extent of scar on epicardial rendering of cardiac MRI correlated well with electroanatomic mapping. Black arrows indicate course of the left anterior descending coronary artery. (B) Multiple high-resolution ex vivo cardiac MRI demonstrated areas of viable myocardium within “scar” regions. Areas with delayed enhancement (scar) are often heterogenous. Images shown are from 3 different infarcted hearts. Arrows indicate regions of viable myocardium within regions of delayed enhancement. (C) Histological examination using Masson’s Trichrome staining demonstrated viable islands of myocardium in areas demarcated as “scar” based on voltage criteria on electroanatomic mapping. Original magnification, 10×. Asterisk delineates viable islands of myocardium. LAD, left anterior descending coronary artery; LV, left ventricle; RV, right ventricle.