The Role of Heart Rate Variability in Mindfulness-Based Pain Relief

Abstract

Mindfulness meditation is a self-regulatory practice premised on sustaining nonreactive awareness of arising sensory events that reliably reduces pain. Yet, the specific analgesic mechanisms supporting mindfulness have not been comprehensively disentangled from the potential nonspecific factors supporting this technique. Increased parasympathetic nervous system (PNS) activity is associated with pain relief corresponding to a number of cognitive manipulations. However, the relationship between the PNS and mindfulness-based pain attenuation remains unknown. The primary objective of the present study was to determine the role of high-frequency heart rate variability (HF HRV), a marker of PNS activity, during mindfulness-based pain relief as compared to a validated, sham-mindfulness meditation technique that served as a breathing-based control. Sixty-two healthy volunteers (31 females; 31 males) were randomized to a 4-session (25 min/session) mindfulness or sham-mindfulness training regimen. Before and after each group's respective training, participants were administered noxious (49°C) and innocuous (35°C) heat to the right calf. HF HRV and respiration rate were recorded during thermal stimulation and pain intensity and unpleasantness ratings were collected after each stimulation series. The primary analysis revealed that during mindfulness meditation, higher HF HRV was more strongly associated with lower pain unpleasantness ratings when compared to sham-mindfulness meditation (B = -.82, P = .04). This finding is in line with the prediction that mindfulness-based meditation engages distinct mechanisms from sham-mindfulness meditation to reduce pain. However, the same prediction was not confirmed for pain intensity ratings (B = -.41). Secondary analyses determined that mindfulness and sham-mindfulness meditation similarly reduced pain ratings, decreased respiration rate, and increased HF HRV (between group ps < .05). More mechanistic work is needed to reliably determine the role of parasympathetic activation in mindfulness-based pain relief as compared to other meditative techniques. Perspective: Mindfulness has been shown to engage multiple mechanisms to reduce pain. The present study extends on this work to show that higher HRV is associated with mindfulness-induced reductions in pain unpleasantness, but not pain intensity ratings, when compared to sham-mindfulness meditation. These findings warrant further investigation into the mechanisms engaged by mindfulness as compared to placebo.

Keywords: Mindfulness meditation; heart rate variability; pain; placebo.

FIGURE 1.. Overview of Experimental Design. First column, Pre-intervention session.

We collected baseline respiration rate and HF HRV while participants were asked to “rest quietly” in a supine position. Participants then underwent psychophysical training (PT) where they were familiarized with the Visual Analog Scales (VAS) and the range of thermal stimuli. All participants were administered two neutral and two heat thermal stimulation series in the order neutral, heat, neutral, heat. Pain intensity and pain unpleasantness ratings were collected after each thermal series. Participants were then randomly assigned to a mindfulness meditation or a sham-mindfulness meditation group after completion of the pre-intervention session. Second column, Sessions 2–5. Subjects participated in four sessions (25m/d) of mindfulness meditation or sham-mindfulness meditation training. Third column, Post-intervention session. Baseline measures of respiration rate and HF HRV were collected while participants were instructed to “rest quietly” in the supine position. VAS ratings of pain intensity and pain unpleasantness were collected after all thermal series. Two neutral and two heat series were administered in the order neutral, heat, neutral, heat while subjects were instructed to “rest comfortably” and “not to meditate” (i.e., Rest + Stimulation). Subjects in both groups were then instructed to “begin meditating and continue meditating for the remainder of the experiment.” Participants were provided ten minutes to meditate (Meditation). Subsequently, all participants were administered two neutral and two heat series in the order neutral, heat, neutral, heat while they continued to meditate (Meditation + Stimulation). Participants were provided two minutes to meditate in between each of the thermal series.

FIGURE 2.

Participant inclusion flow diagram. Seventy-five participants provided informed consent for the current study, and 38 individuals were randomized to the mindfulness meditation group whereas 37 participants were randomized to the sham-mindfulness meditation group. Eight participants (4 mindfulness; 4 sham-mindfulness) voluntarily withdrew after signing consent due to scheduling conflicts or unknown reasons and were replaced. One mindfulness meditator was excluded due to disclosure of a psychiatric illness after signing consent. Sixty-six participants (33 mindfulness; 33 sham-mindfulness) completed all study procedures. Data corresponding to 3 individuals (1 mindfulness; 2 sham-mindfulness) were removed from the data set due to significant outlier detection. One mindfulness meditator was removed due to a miscommunication between the participant and researcher leading to the participant meditating during the post-intervention session Rest + Stimulation condition. Consequently, sixty-two participants (31 mindfulness; 31 sham-mindfulness) are included in the final analyses.

FIGURE 3.. The relationship between HF HRV and pain unpleasantness ratings.

There was a significant (p = 0.04) group difference on the relationship between HF HRV and pain unpleasantness. A) Post-hoc analyses revealed that there was a marginally significant relationship between increased HF HRV and decreased pain unpleasantness ratings (r = −0.46; p = 0.07) for the mindfulness meditation group after controlling for age, pain ratings during rest, respiration rate, and HF HRV during rest. B) HF HRV was not significantly (r = 0.42; p = 0.11) associated with pain unpleasantness after accounting for age, pain ratings during rest, respiration rate, and HF HRV during rest in the sham-mindfulness meditation group.

FIGURE 4.. Post-intervention session psychophysical pain intensity (A) and pain unpleasantness (B) ratings (mean ± SEM).

Both groups significantly reduced pain intensity (p = 0.02; left) and unpleasantness (p = 0.002; right) ratings when compared to rest and when controlling for pre-intervention pain ratings. There was no significant group × manipulation interaction on pain intensity (p = 0.51) or unpleasantness (p = 0.64) ratings.