Men Show Reduced Cardiac Baroreceptor Sensitivity during Modestly Painful Electrical Stimulation of the Forearm: Exploratory Results from a Sham-Controlled Crossover Vagus Nerve Stimulation Study

Abstract

This paper presents data from a transcutaneous vagus nerve stimulation experiment that point towards a blunted cardiac baroreceptor sensitivity (cBRS) in young males compared to females during electrical stimulation of the forearm and a rhythmic breathing task. Continuous electrocardiography, impedance cardiography and continuous blood-pressure recordings were assessed in a sex-matched cohort of twenty young healthy subjects. Electrical stimulation of the median nerve was conducted by using a threshold-tracking method combined with two rhythmic breathing tasks (0.1 and 0.2 Hz) before, during and after active or sham transcutaneous vagus nerve stimulation. Autonomic and hemodynamic parameters were calculated, and differences were analyzed by using linear mixed models and post hoc F-tests. None of the autonomic and hemodynamic parameters differed between the sham and active conditions. However, compared to females, male participants had an overall lower total cBRS independent of stimulation condition during nerve stimulation (females: 14.96 ± 5.67 ms/mmHg, males: 11.89 ± 3.24 ms/mmHg, p = 0.031) and rhythmic breathing at 0.2 Hz (females: 21.49 ± 8.47 ms/mmHg, males: 15.12 ± 5.70 ms/mmHg, p = 0.004). Whereas vagus nerve stimulation at the left inner tragus did not affect the efferent vagal control of the heart, we found similar patterns of baroreceptor sensitivity activation over the stimulation period in both sexes, which, however, significantly differed in their magnitude, with females showing an overall higher cBRS.

Keywords: heart rate variability; nerve stimulation; sex differences; spontaneous baroreceptor sensitivity; stress; transcutaneous vagus nerve stimulation.

Figure 1

Experimental protocol.

Figure 2

Main sex effects in cBRS during different phases of the experimental protocol independent of active and sham stimulation. Men displayed an overall significant lower total cBRS than women during nerve stimulation and rhythmic breathing at 0.2 Hz and non-significantly at 0.1 Hz. The dark-colored boxes depict 50% of all the data points. Together with the light-colored boxes, they represent 75% of all the measured data. While the black dots represent the means averaged over all conditions (baseline, tVNS and post-stimulation) for the indicated tasks, the gray lines within the boxes show the respective medians; * p < 0.05; ** p < 0.01.

Figure 3

(a) Average fluctuations of total cBRS between genders over the time course of the sham measurement day. (b) Average fluctuation of total cBRS between genders over the time course of the active measurement day. Depicted are means ± SD for each condition and task. NS: nerve stimulation. RB: rhythmic breathing.