Stochastic vagus nerve stimulation affects acute heart rate dynamics in rats

Abstract

Vagus nerve stimulation (VNS) is an approved therapy for treatment of epilepsy and depression. While also shown to be promising in several preclinical and clinical studies to treat cardiovascular diseases, optimal therapeutic stimulation paradigms are still under investigation. Traditionally, parameters such as frequency, current, and duty cycle are used to adjust the efficacy of VNS therapy. This study explored the effect of novel stochastic VNS (S-VNS) on acute heart rate (HR) dynamics. The effect of S-VNS was evaluated in Sprague Dawley rats by comparing the acute HR and HR variability (HRV) responses to standard, periodic VNS (P-VNS) across different frequencies (FREQs, 10-30 Hz). Our results demonstrate that both S-VNS and P-VNS produced negative chronotropic effects in a FREQ-dependent manner with S-VNS inducing a significantly smaller drop in HR at 10 Hz and 20 Hz compared to P-VNS (p<0.05). S-VNS demonstrated a FREQ-dependent drop in the SD1/SD2 ratio, a measure of HRV, which was absent in P-VNS, suggesting that S-VNS may acutely modulate the nonlinear relationship between short- and long-term HRV. In conclusion, S-VNS is a novel stimulation procedure that may provide different physiological outcomes from standard P-VNS, as indicated by our analysis of HR dynamics. Our study provides a rationale for further detailed investigations into the therapeutic potential of S-VNS as a novel neuromodulation technique.

Fig 1. Detailed schematic of the experimental VNS protocol.

Standard P-VNS or S-VNS with different degree of stochasticity (STOCH, 10% and 20%) was administered across different frequencies (FREQ, 20, 30, and 10 Hz) with stabilization times between protocols and conditions. P-VNS, periodic vagus nerve stimulation; S-VNS, stochastic vagus nerve stimulation; STOCH, stochasticity; PRE, baseline recording; ON, continuous VNS; POST, recovery.

Fig 2. ECG recordings and corresponding HR responses.

Representative segments of ECG recordings and corresponding heart rate (HR) response for an anesthetized rat for PRE, ON, and POST during (A) P-VNS, (B) S-VNS (10%), and (C) S-VNS (20%) of the right cervical vagus nerve. Zoomed-in snapshots of PRE (black), ON (red), and POST (yellow) highlights VNS artifacts during stimulation. Here, VNS was continuously delivered at 20 Hz, 500 µs pulse width, and 1.0 mA for 2 minutes.

Fig 3. Effects of VNS FREQ on HR and heart period.

Mean percent drop in HR for (A) P-VNS, (B) S-VNS (10%), and (C) S-VNS (20%) at different FREQ. Mean percent drop in Heart Period for (D) P-VNS, (E) S-VNS (10%), and (F) S-VNS (20%) at different FREQ. Note data reported here for P-VNS is the mean and SEM of the average of P-VNS #1 and P-VNS #2 protocols (n = 8). (*p < 0.05).

Fig 4. Effects of STOCH on the chronotropic effects of VNS.

Mean percent drop in HR for (A) S-VNS (10%) and (B) S-VNS (20%) being compared to its immediate preceding P-VNS protocol at different FREQ. (C) Comparison of mean relative drop in HR between P-VNS protocols and (D) S-VNS (20%) and its subsequent P-VNS protocol at different FREQ. (*p < 0.05).

Fig 5. Effects of VNS on HRV using Poincaré analysis.

Representative Poincaré plots of (A) P-VNS and (B) S-VNS (10%) during VNS stimulation (ON) at 10 Hz and 30 Hz demonstrating the elliptical fitting of the beat-distribution cloud and the standard deviation of short-term (SD1) and long-term (SD2) variability. (C) Mean SD1/SD2 ratio for PRE, ON and POST across different FREQ. (*p < 0.05).

Fig 6. Effects of VNS FREQ on HR recovery.

(A) Schematic representation of different scenarios of HR recovery based on the value of HR_{POST Ratio}. Mean HR_{POST Ratio} values for (B) P-VNS, (C) S-VNS (10%), and (D) S-VNS (20%) at different FREQ. # p<0.05 compared to a mean theoretical value of 1.