The Effect of Anodal Transcranial Direct Current Stimulation Over Left and Right Temporal Cortex on the Cardiovascular Response: A Comparative Study

Unraveling the temporal interplay of slow-paced breathing and prefrontal transcranial direct current stimulation on cardiac indices of autonomic activity

The neurovisceral integration model proposes that information flows bidirectionally between the brain and the heart via the vagus nerve, indexed by vagally mediated heart rate variability (vmHRV). Voluntary reduction in breathing rate (slow-paced breathing, SPB, 5.5 Breathing Per Minute (BPM)) can enhance vmHRV. Additionally, prefrontal transcranial direct current stimulation (tDCS) can modulate the excitability of the prefrontal region and influence the vagus nerve. However, research on the combination of SPB and prefrontal tDCS to increase vmHRV and other cardiac (heart rate (HR) and blood pressure) and peripheral (skin conductance) indices is scarce. We hypothesized that the combination of 20 min of SPB and prefrontal tDCS would have a greater effect than each intervention in isolation. Hence, 200 participants were divided into four groups: active tDCS with SPB, active tDCS with 15 BPM breathing, sham tDCS with SPB, and sham tDCS with 15 BPM breathing. Regardless of the tDCS condition, the 5.5 BPM group showed a significant increase in vmHRV over 20 minutes and significant decreases in HR at the first and second 5-min epochs of the intervention. Regardless of breathing condition, the active tDCS group exhibited higher HR at the fourth 5-min epoch of the intervention than the sham tDCS group. No other effects were observed. Overall, SPB is a robust technique for increasing vmHRV, whereas prefrontal tDCS may produce effects that counteract those of SPB. More research is necessary to test whether and how SPB and neuromodulation approaches can be combined to improve cardiac vagal tone.

Anodal high-definition transcranial direct current stimulation reduces heart rate and modulates heart-rate variability in healthy young people: A randomized cross-controlled trial

OBJECTIVE

To investigate whether anodal high-definition transcranial current stimulation (HD-tDCS) over the left dorsolateral pre-frontal cortex (DLPFC) could modulate the heart rate (HR) and heart-rate variability (HRV) in healthy young people.

METHODS

Forty healthy young people were enrolled in this randomized crossover trial. The participants were randomized to receive anodal HD-tDCS (n = 20) or sham HD-tDCS (n = 20) over the left DLPFC with a washout period of 1 week. Electrocardiogram (ECG) data were continuously recorded 20 min before the stimulation, during the session (20 min), and 20 min after the session. HR and the time- and frequency-domain indices of the HRV were measured to investigate the activity of the sympathetic and parasympathetic nervous systems.

RESULTS

Anodal HD-tDCS over the left DLPFC induced a significant decrease in HR and a significant increase in the average of normal-to-normal intervals (AVG NN), low-frequency (LF) power, total power (TP), and LF/high-frequency (HF) ratio in comparison with the sham stimulation and the baseline. However, sham HD-tDCS over the left DLPFC had no significant effect on HR or HRV.

CONCLUSIONS

Anodal HD-tDCS over the left DLPFC could reduce HR and modulate the HRV in healthy young people. HD-tDCS may show some potential for acutely modulating cardiovascular function.

The effects of noninvasive brain stimulation on heart rate and heart rate variability: A systematic review and meta-analysis

Noninvasive brain stimulation (NIBS) techniques such as transcranial magnetic stimulation and transcranial direct current stimulation are widely used to test the involvement of specific cortical regions in various domains such as cognition and emotion. Despite the capability of stimulation techniques to test causal directions, this approach has been only sparsely used to examine the cortical regulation of autonomic nervous system (ANS) functions such as heart rate (HR) and heart rate variability (HRV) and to test current models in this regard. In this preregistered (PROSPERO) systematic review and meta-analysis, we aimed to investigate, based on meta-regression, whether NIBS represents an effective method for modulating HR and HRV measures, and to evaluate whether the ANS is modulated by cortical mechanisms affected by NIBS. Here we have adhered to the PRISMA guidelines. In a series of four meta-analyses, a total of 131 effect sizes from 35 sham-controlled trials were analyzed using robust variance estimation random-effects meta-regression technique. NIBS was found to effectively modulate HR and HRV with small to medium effect sizes. Moderator analyses yielded significant differences in effects between stimulation of distinct cortical areas. Our results show that NIBS is a promising tool to investigate the cortical regulation of ANS, which may add to the existing brain imaging and animal study literature. Future research is needed to identify further factors modulating the size of effects. As many of the studies reviewed were found to be at high risk of bias, we recommend that methods to reduce potential risk of bias be used in the design and conduct of future studies.

Postexercise blood pressure and autonomic responses after aerobic exercise following anodal tDCS applied over the medial prefrontal cortex

Transcranial direct current stimulation (tDCS) is acknowledged to modulate autonomic cardiac activity and hemodynamic responses at rest and during exercise. However, its potential to optimize postexercise hypotension (PEH) has not been investigated. This study investigated the effects of anodal tDCS applied over the medial prefrontal cortex (mPFC) upon blood pressure (BP) and heart rate variability (HRV) throughout 60 min following acute aerobic exercise. Fifteen young men (27.5 ± 5.2 yrs; 72.9 ± 8 kg; 170 ± 0.1 cm; 124.1 ± 1.9/67.7 ± 2.1 mmHg) underwent three counterbalanced experimental sessions: a) anodal tDCS + exercise (tDCS); b) sham stimulation + exercise (SHAM); c) non-exercise control (CONT). Exercise consisted in 50-min cycling at 65-70% heart rate reserve. BP and HRV were assessed during 60-min postexercise. Mean reduction in systolic BP occurred after tDCS vs. SHAM (-4.1 mmHg; P=0.03) and CONT (-5.8 mmHg; P=0.003), and in MAP vs. CONT (-3.0 mmHg, P=0.03). Parasympathetic activity lowered after tDCS and SHAM vs. CONT, as respectively reflected by R-R intervals (-328.1% and -396.4%; P = 0.001), SDNN (-155.7% and -193.4%; P = 0.006), and pNN50 (-272.3% and -259.1%; P = 0.021). There was a clear tendency of increased sympatho-vagal balance vs. CONT (P = 0.387) after SHAM (+246.3%), but not tDCS (+25.9%). In conclusion, an aerobic exercise bout preceded by tDCS applied over mPFC induced PEH in normotensive men. Parasympathetic activity lowered, while sympatho-vagal balance increased after both tDCS and SHAM vs. CONT. However, these responses seemed to be tempered by anodal stimulation, which might help explaining the occurrence of PEH after tDCS and not SHAM. These findings warrant further research on the role of tDCS within exercise programs aiming at BP management.