Pneumogastric (Vagus) Nerve Activity Indexed by Heart Rate Variability in Chronic Pain Patients Compared to Healthy Controls: A Systematic Review and Meta-Analysis

Background: A large body of scientific literature derived from experimental studies emphasizesthe vital role of vagal-nociceptive networks in acute pain processing. However, research onvagal activity, indexed by vagally-mediated heart rate variability (vmHRV) in chronic pain patients(CPPs), has not yet been summarized.Objectives: To systematically investigate differences in vagus nerve activity indexed by timeand frequency-domain measures of vmHRV in CPPs compared to healthy controls (HCs).Study Design: A systematic review and meta-analysis, including meta-regression on a varietyof populations (i.e., clinical etiology) and study-level (i.e., length of HRV recording) covariates.Setting: Not applicable (variety of studies included in the meta-analysis)Methods: Eight computerized databases (PubMed via MEDLINE, PsycNET, PsycINFO, Embase,CINAHL, Web of Science, PSYNDEX, and the Cochrane Library) in addition to a hand searchwere systematically screened for eligible studies based on pre-defined inclusion criteria. A metaanalysis on all empirical investigations reporting short- and long-term recordings of continuoustime- (root-mean-square of successive R-R-interval differences [RMSSD]) and frequency-domainmeasures (high-frequency [HF] HRV) of vmHRV in CPPs and HCs was performed. True effectestimates as adjusted standardized mean differences (SMD; Hedges g) combined with inversevariance weights using a random effects model were computed.Results: CPPs show lower vmHRV than HCs indexed by RMSSD (Z = 5.47, P < .0001; g =-0.24;95% CI [-0.33, -0.16]; k = 25) and HF (Z = 4.54, P < .0001; g = -0.30; 95% CI [-0.44,-0.17]; k = 61).Meta-regression on covariates revealed significant differences by clinical etiology,age, gender, and length of HRV recording.Limitations: We did not control for other potential covariates (i.e., duration of chronic pain,medication intake) which may carry potential risk of bias.Conclusion(s): The present meta-analysis is the most extensive review of the current evidenceon vagal activity indexed by vmHRV in CPPs. CPPs were shown to have lower vagal activity,indexed by vmHRV, compared to HCs. Several covariates in this relationship have been identified.Further research is needed to investigate vagal activity in CPPs, in particular prospective andlongitudinal follow-up studies are encouraged.Key words: Vagus nerve, heart rate variability, chronic pain, irritable bowel syndrome,fibromyalgia, primary headache disorders, meta-analysis, systematic review