Interactions between the cardiovascular and pain regulatory systems: an updated review of mechanisms and possible alterations in chronic pain

Musculoskeletal pains and cardiovascular autonomic function in the general Northern Finnish population

BACKGROUND

Heart rate variability (HRV) and baroreflex sensitivity (BRS) measurements provide means for the objective assessment of cardiovascular autonomic function. As previous studies have associated chronic pain with abnormal autonomic function, we aimed to characterize the relationship between the number of musculoskeletal pain sites (NPS), pain intensity, and cardiovascular autonomic function among the population-based Northern Finland Birth Cohort 1966.

METHODS

At the age of 46, cohort members self-reported their musculoskeletal pains (enabling the determination of NPS [0-8] and pain intensity [Numerical Rating Scale, NRS, 0-10]) and underwent clinical assessments of cardiovascular autonomic function in seated and standing positions (HRV variables: heart rate [HR] and root mean square of successive differences in beat-to-beat intervals [rMSSD] for the entire cohort; BRS variables: low-frequency systolic blood pressure variability [SBPV] and cross-spectral baroreflex sensitivity [BRS] for those attending the examination in Oulu, Finland). Extensive confounder data were also collected (body mass index, physical activity, smoking, Hopkins Symptom Checklist-25, comorbidities, and medications). The full samples included 4186 and 2031 individuals (HRV and BRS samples, respectively). Three subanalyses focused on individuals with intense and frequent pain, individuals with symptoms of depression and anxiety, and the relationship between pain intensity and autonomic parameters.

RESULTS

Linear regression models showed varying associations between NPS, pain intensity, and cardiovascular autonomic parameters. However, after all adjustments NPS was only associated with one outcome among women (BRS, standing: beta = - 0.015, p = 0.048) and two among men (HR, seated: beta = - 0.902, p = 0.003; HR, standing: beta = - 0.843, p = 0.014). Pain intensity was not associated with any outcome after full adjustments. Significant sex*pain interactions were found in the data.

CONCLUSIONS

Our data suggest that musculoskeletal pain has, at most, a limited independent association with cardiovascular autonomic function. Future studies should carefully account for the potential confounders and sex interactions that this study revealed.

Taking rejection to heart: Associations between blood pressure and sensitivity to social pain

A reliable finding from the physical pain literature is that individuals with higher resting (i.e., tonic) blood pressure experience relatively less pain in response to nociceptive stimuli. Converging lines of evidence suggest that biological factors that influence the experience of physical pain may also relate to social pain. An open question, however, is whether higher blood pressure per se is a biological factor associated with lower sensitivity to social pain. This possible association was tested in three studies. Consistent with prior findings on physical pain, higher resting blood pressure was associated with lower self-reported sensitivity to social pain across individuals (Study 1 r = -.303, Study 2 r = -.262, -.246), even after adjusting for confounding factors related to blood pressure (Study 3 r = -.222). Findings suggest a previously unknown biological correlate of sensitivity to social pain, providing further evidence for possible shared substrates for physical and social pain.

Blood pressure-related pain modulation in fibromyalgia: Differentiating between static versus dynamic pain indicators

INTRODUCTION

Resting blood pressure (BP) has been found to be inversely associated with evoked pain responsiveness in healthy populations. However, some reports suggest that BP-related pain modulation may be dysfunctional in chronic pain patients. This study examined whether BP-related pain modulation, indexed by both static and dynamic evoked pain responses, is altered in fibromyalgia (FM) patients compared to pain-free individuals.

METHOD

Pain threshold and tolerance as static evoked pain measures and slowly repeated evoked pain (SREP) as a dynamic evoked pain index were measured in 30 FM patients and 27 healthy controls. BP was continuously recorded throughout a 5 minute pre-pain rest period.

RESULTS

SREP sensitization was observed only in the FM group. Higher BP predicted elevated pain threshold and tolerance in healthy individuals, but not in FM. Conversely, BP was inversely associated with SREP sensitization in FM whereas no association was found in healthy controls.

CONCLUSIONS

Static evoked pain measures suggested BP-related pain inhibitory dysfunction in FM. In contrast, for pain sensitization as indexed by SREP, FM displayed the expected BP-related inhibitory effects. BP-related pain modulation is manifested in FM differentially for static versus dynamic pain indicators. Use of both types of evoked pain measures may be valuable in the study of mechanisms underlying altered pain modulatory systems in FM.

Heartbeat-enhanced immersive virtual reality to treat complex regional pain syndrome

Objectives

To develop and test a new immersive digital technology for complex regional pain syndrome (CRPS) that combines principles from mirror therapy and immersive virtual reality and the latest research from multisensory body processing.

Methods

In this crossover double-blind study, 24 patients with CRPS and 24 age- and sex-matched healthy controls were immersed in a virtual environment and shown a virtual depiction of their affected limb that was flashing in synchrony (or in asynchrony in the control condition) with their own online detected heartbeat (heartbeat-enhanced virtual reality [HEVR]). The primary outcome measures for pain reduction were subjective pain ratings, force strength, and heart rate variability (HRV).

Results

HEVR reduced pain ratings, improved motor limb function, and modulated a physiologic pain marker (HRV). These significant improvements were reliable and highly selective, absent in control HEVR conditions, not observed in healthy controls, and obtained without the application of tactile stimulation (or movement) of the painful limb, using a readily available biological signal (the heartbeat) that is most often not consciously perceived (thus preventing placebo effects).

Conclusions

Next to these specific and well-controlled analgesic effects, immersive HEVR allows the application of prolonged and repeated doses of digital therapy, enables the automatized integration with existing pain treatments, and avoids application of painful bodily cues while minimizing the active involvement of the patient and therapist.

Classification of evidence

This study provides Class III evidence that HEVR reduces pain and increases force strength in patients with CRPS.

Pain and respiration: a systematic review

Breathing techniques are commonly used to alleviate pain. Despite their frequent use, surprisingly little is known about their efficacy as well as their underlying physiological mechanisms. The purpose of this systematic review is to summarize and critically appraise the results of existing studies on the association between respiration and pain, and to highlight a potential physiological mechanism underlying the respiration-pain connection. A total of 31 publications from between 1984 and 2015 were retrieved and analyzed. These articles were classified into 4 groups: experimental and clinical studies of the effect of pain on respiration, clinical studies of the effects of breathing techniques on pain, and experimental studies of the influence of various forms of respiration on laboratory-induced pain. The findings suggest that pain influences respiration by increasing its flow, frequency, and volume. Furthermore, paced slow breathing is associated with pain reduction in some of the studies, but evidence elucidating the underlying physiological mechanisms of this effect is lacking. Here, we focus on the potential role of the cardiovascular system on the respiratory modulation of pain. Further research is definitely warranted.

Poor emotional responsiveness in clinical hypertension: Reduced accuracy in the labelling and matching of emotional faces amongst individuals with hypertension and prehypertension

Psychological factors are known to play an important part in the origin of many medical conditions including hypertension. Recent studies have reported elevated blood pressure (even in the normal range of variation) to be associated with a reduced responsiveness to emotions or 'emotional dampening'. Our aim was to assess emotional dampening in individuals with more extreme blood pressure levels including prehypertensives (N = 58) and hypertensives (N = 60) by comparing their emotion recognition ability with normotensives (N = 57). Participants completed novel facial emotion matching and facial emotion labelling tasks following blood pressure measurement and their accuracy of emotion recognition and average response times were compared. The normotensives demonstrated a significantly higher accuracy of emotion recognition than the prehypertensives and the hypertensives in labelling of facial emotions. This difference generalised to the task where two facial halves (upper & lower) had to be matched on the basis of emotions. In neither the labelling nor matching emotion conditions did the groups differ in their speed of emotion processing. Findings of the present study extend reports of 'emotional dampening' to hypertensives as well as to those at-risk for developing hypertension (i.e. prehypertensives) and have important implications for understanding the psychological component of such medical conditions as hypertension.

[Baroreflex sensitivity and chronic pain : Pathogenetic significance and clinical implications]

The interaction of cardiovascular dynamics and pain perception is an important component of intrinsic pain regulation. In healthy subjects acute pain stimuli cause increased sympathetic arousal and increased mean arterial pressure. Arterial baroreceptors sense phasic blood pressure changes and relay the information to the lower brainstem via the dorsomedial nucleus tractus solitarius (dmNTS). Projections in the brainstem and also higher cortical areas result in elevation of blood pressure as part of the autonomic nervous system as well as modulation of sleep, anxiety and pain. In healthy subjects there is an inverse relationship between blood pressure and pain sensitivity but this relationship is impaired in chronic pain patients. Persistent stress, pain behavior and classical and operant conditioning mechanisms reduce baroreflex sensitivity (BRS) and dmNTS activity in a subgroup of patients. This leads to a decrease of autonomic regulatory function as well as reduced pain inhibition. Importantly, baroreflex function can be modulated by cognitive and affective processes. This article reviews the role of the baroreflex arc as a possible crucial factor in the development and maintenance of chronic pain. The importance of learning mechanisms is described. Mechanism-based individualized treatment approaches for patients with hypertensive stress reactivity are also critically discussed.

Exploring the relationship between frontal asymmetry and emotional dampening

Cardiovascular emotional dampening is the term used to describe the inverse relationship between resting blood pressure and emotional responsivity which extends from normotensive to hypertensive ranges. Little is known about its underlying physiological mechanisms, but it is thought to involve some disruption in emotion processing. One area that has yet to be explored in the literature is the relationship between emotional dampening and frontal asymmetry, a psychophysiological indicator for motivational direction and emotional valence bias. The present study explored that relationship using data from a sample of 48 healthy college students. Measures of baseline resting blood pressure and frontal cortical activity were recorded, after which participants completed a series of emotion-related tasks. Results revealed a significant relationship between resting systolic blood pressure and left frontal activity. Likewise, left frontal activity was associated with neutral appraisal of emotionally valenced stimuli within the tasks. The findings from the present study yield support for a link between emotional dampening and left frontal activity. Implications are discussed.

Cardiovascular autonomic regulation, inflammation and pain in rheumatoid arthritis

Background

Rheumatoid arthritis (RA) is a chronic inflammatory condition characterised by reduced heart rate variability (HRV) of unknown cause. We tested the hypothesis that low HRV, indicative of cardiac autonomic cardiovascular dysfunction, was associated with systemic inflammation and pain. Given the high prevalence of hypertension (HTN) in RA, a condition itself associated with low HRV, we also assessed whether the presence of hypertension further reduced HRV in RA.

Methods

In RA-normotensive (n = 13), RA-HTN (n = 17), normotensive controls (NC; n = 17) and HTN (n = 16) controls, blood pressure and heart rate were recorded. Time and frequency domain measures of HRV along with serological markers of inflammation (high sensitivity C-reactive protein [hs-CRP], tumour necrosis factor-α [TNF-α] and interleukins [IL]) were determined. Reported pain was assessed using a visual analogue scale.

Results

Time (rMSSD, pNN50%) and frequency (high frequency power, low frequency power, total power) domain measures of HRV were lower in the RA, RA-HTN and HTN groups, compared to NC (p = 0.001). However, no significant differences in HRV were noted between the RA, RA-HTN and HTN groups. Inverse associations were found between time and frequency measures of HRV and inflammatory cytokines (IL-6 and IL-10), but were not independent after multivariable analysis. hs-CRP and pain were independently and inversely associated with time domain (rMMSD, pNN50%) parameters of HRV.

Conclusions

These findings suggest that lower HRV is associated with increased inflammation and independently associated with increased reported pain, but not compounded by the presence of HTN in patients with RA.

•

Rheumatoid arthritis (RA) is a chronic inflammatory condition accompanied by low heart rate variability (HRV).

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Important autonomic-immune interactions are suggested, but have not been thoroughly examined in RA.

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We show that low HRV in RA is associated with increased serum inflammatory cytokine levels and patient-reported pain.

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In our patients with RA, reductions in HRV were not compounded by the presence of hypertension.

Autonomic function and pressure pain threshold following thoracic mobilization in asymptomatic subjects: A randomized controlled trial

OBJECTIVE

To compare the effects of two different mobilization techniques and a placebo intervention applied to the thoracic spine on heart rate variability (HRV) and pressure pain threshold (PPT) in asymptomatic individuals.

METHODS

Sixty healthy asymptomatic subjects aged between 18 and 40 years old were randomized to a single session of one of the three interventions: posterior-to-anterior (PA) rotatory thoracic passive accessory intervertebral mobilization (PAIVM) (PA group), unilateral thoracic PA in slump position (SLUMP group) or placebo intervention (Placebo group). HRV and PPT at C7 and T4 spinous process, first dorsal interossei muscles bilaterally, and muscle belly of tibialis anterior bilaterally were measured before and immediately after the intervention. A univariate analysis of covariance (ANCOVA) adjusted for baseline values assessed the effect of "Group". Pairwise comparisons with Bonferroni adjustment for multiple comparisons were performed.

RESULTS

There were no significant between-group differences for HRV. A significant between-group difference for PPT in the ipsilateral tibia was found favoring the SLUMP group in comparison with the PA group. There were no significant between-group differences for PPT in the other landmarks.

CONCLUSION

A single treatment of thoracic PAIVM in prone lying and slump position did not alter PPT and HRV compared to placebo in asymptomatic subjects.

Increased Sympathetic Outflow Induces Adaptation to Acute Experimental Pain

BACKGROUND

There are interrelationships between the autonomic nervous system and pain. This study aims to explore the effect of different autonomic manipulations on pain perception and modulation.

METHODS

Twenty healthy subjects (10 men and 10 women, mean age 25 ± 3 years) participated in this single-blinded, semi-randomized, controlled study, which included 2 study visits. Warm detection thresholds, heat pain thresholds, conditioned pain modulation (CPM), and pain adaptation were tested before and after administration of phenylephrine, clonidine, yohimbine, and saline.

RESULTS

Changes in heart rate and blood pressure were found after all the pharmacological interventions. The only effect on pain measures was that yohimbine enhanced pain adaptation capacity while phenylephrine reduced it (P = 0.032). Several significant correlations were found between autonomic and pain parameters; greater decreases in heart rate after phenylephrine were associated with reduced pain ratings (r² = 0.288, P = 0.018). In addition, enhanced pain adaptation was associated with higher total vascular resistance (r² = 0.442, P = 0.01).

CONCLUSIONS

Different effects of acute autonomic manipulations on experimental pain were found: an increase in sympathetic tone induced by yohimbine led to reduced pain sensitivity; a decrease in sympathetic tone with no effect on vagal-parasympathetic tone induced by phenylephrine led to reduction in pain adaptation capacity; and a decrease in sympathetic tone and increase in vagal parasympathetic tone by clonidine led to no change in pain adaptation capacity. While increased sympathetic outflow does facilitate pain adaptation, activation of either the sympathetic or parasympathetic limbs of the autonomic nervous system does not affect pain thresholds or CPM. Finally, a correlation exists between nociception and cardiovascular parameters only due to baroreflex activation.

Autonomic components of Complex Regional Pain Syndrome (CRPS) are favourably affected by Electrical Twitch-Obtaining Intramuscular Stimulation (ETOIMS): effects on blood pressure and heart rate

INTRODUCTION

Favourable pain relief results on evoking autonomous twitches at myofascial trigger points with Electrical Twitch Obtaining Intramuscular Stimulation (ETOIMS).

AIM

To document autonomic nervous system (ANS) dysfunction in Complex Regional Pain Syndrome (CRPS) from blood pressure (BP) and pulse/heart rate changes with ETOIMS.

METHODS AND MATERIALS

A patient with persistent pain regularly received serial ETOIMS sessions of 60, 90, 120 or ≥150 min over 24 months. Outcome measures include BP: systolic, diastolic, pulse pressure and pulse/heart rate, pre-session/immediate-post-session summed differences (SDPPP index), and pain reduction. His results were compared with that of two other patients and one normal control. Each individual represented the following maximal elicitable twitch forces (TWF) graded 1-5: maximum TWF2: control subject; maximum TWF3: CRPS patient with suspected ANS dysfunction; and maximum TWF4 and TWF5: two patients with respective slow-fatigue and fast-fatigue twitches who during ETOIMS had autonomous twitching at local and remote myotomes simultaneously from denervation supersensitivity. ETOIMS results between TWFs were compared using one-way analysis of variance test.

RESULTS

The patients showed immediate significant pain reduction, BP and pulse/heart rate changes/reduction(s) except for diastolic BP in the TWF5 patient. TWF2 control subject had diastolic BP reduction with ETOIMS but not with rest. Linear regression showed TWF grade to be the most significant variable in pain reduction, more so than the number of treatments, session duration and treatment interval. TWF grade was the most important variable in significantly reducing outcome measures, especially pulse/heart rate. Unlike others, the TWF3 patient had distinctive reductions in SDPPP index.

CONCLUSIONS

Measuring BP and pulse/heart rate is clinically practical for alerting ANS dysfunction maintained CRPS. SDPPP index (≥26) and pulse/heart rate (≥8) reductions with almost every ETOIMS treatment, plus inability to evoke autonomous twitches due to pain-induced muscle hypertonicity, are pathognomonic of this problem.

Vagally Mediated Heart Rate Variability Promotes the Perception of Paradoxical Pain

Abstract. Self-regulation mechanisms are governed by prefrontal inhibitory processes and play a crucial role in the modulation of pain. In the present study the thermal grill paradigm was used to investigate the association of vagally mediated resting heart rate variability, a psychophysiological marker of trait self-regulatory capacity, with paradoxical pain sensations induced by non-noxious stimulation. This thermal grill illusion is only perceived by part of the tested individuals. The mechanisms underlying the observed interindividual differences in paradoxical pain sensitivity are largely unknown. During the experimental task, a temperature combination of 15 °C and 41 °C was set at the glass tubes of the thermal grill. The 52 healthy participants placed their dominant hand on the grill for a duration of one min. The magnitude of sensory and affective pain sensations perceived during stimulation was assessed with numerical rating scales. Before stimulation, a short-term electrocardiogram was recorded to compute vagally mediated heart rate variability at rest. Logistic regression analyses revealed that participants with higher vagal tone were significantly more likely to perceive the thermal grill illusion than subjects displaying lower resting heart rate variability. Paradoxical pain sensations were primarily predicted by normalized respiratory sinus arrhythmia. Our results confirm that the magnitude of vagally mediated resting heart rate variability is associated with the individual disposition to illusive pain perceptions. Since the latter is considered to be a marker of trait self-regulation ability, the present findings may corroborate and complement previous evidence for an impact of psychological characteristics on paradoxical pain sensitivity.

Conditioned pain modulation among young, healthy, and physically active African American and non-Hispanic White adults

OBJECTIVE

Research shows that African American (AA) adults experience more severe and frequent pain compared to non-Hispanic White (NHW) adults. Additionally, experimental studies demonstrate that AA adults exhibit less efficient central pain inhibition compared to NHW adults, which may partially explain the racial/ethnic disparities in pain. Evidence suggests that regular physical activity (PA) may help improve central pain inhibition, but research shows that AA adults engage in less PA, and are less likely to meet PA guidelines for health promotion compared to NHW adults. These observations suggest that PA levels may help better understand the racial/ethnic difference in central pain inhibition. Therefore, this study compared central pain inhibition and PA levels among AA and NHW adults.

METHODS

Young and healthy participants were recruited on campus, and 27 AA and 27 NHW adults completed this study. Central pain inhibitory processing was assessed using conditioned pain modulation (CPM), where changes in electrical pain ratings were quantified during and after exposure to pressure pain compared to baseline. PA levels were assessed using self-report questionnaires and accelerometer.

RESULTS

The participants were generally physically active, and most participants in both groups met the public recommendation of PA for health promotion. Electrical pain ratings were significantly reduced during and after exposure to pressure pain compared to baseline. There was no racial/ethnic difference in a magnitude of changes in electrical pain ratings.

CONCLUSION

Young, healthy, and physically active AA and NHW adults exhibit similar CPM responses. Regular PA may help attenuate the racial/ethnic difference in CPM responses.

Anatomical changes correlated with chronic pain in forensic medicine

This study was performed to determine the relationships between chronic pain and anatomic changes that may occur in the body. Autopsies were performed on fatalities that required death investigation in Linn County, IA, or adjacent and nearby areas. Persons with chronic pain were older than the control population at the time of death. Diabetes, hypertension and depression were more common in persons with chronic pain. Certain causes of death may also have been related to chronic pain. The heart, lungs, liver, spleen and kidneys were significantly heavier in persons with chronic pain; emphysema and pleural and abdominal adhesions were more common in persons with chronic pain. There appear to have been diffuse changes in the body related to chronic pain. These changes may have been mediated by a number of systemic mechanisms that are involved with chronic pain, including cardiovascular activity, the immune system, the neuroendocrine system and others.

Cervical Decompression Surgery for Cervical Spondylotic Myelopathy and Concomitant Hypertension: A Multicenter Prospective Cohort Study

STUDY DESIGN

We performed decompression surgery or conservative treatments on 135 cervical spondylotic myelopathy (CSM) patients with concomitant hypertension and did follow-up assessments up to 1 year to examine the change of blood pressure, spinal cord function, and cervical pain.

OBJECTIVE

The aim of this study was to determine whether concomitant hypertension is relieved after decompression surgery, and whether it is related to the improvement of spinal cord function or cervical pain.

SUMMARY OF BACKGROUND DATA

In clinical practice, we often found that some patients with CSM have concomitant hypertension. Interestingly, after CSM was treated successfully by decompression surgery, some patients' high blood pressure returned to normal range even without oral medications.

METHODS

We enrolled 135 CSM patients with hypertension, 103 of whom received decompression surgery, and remaining 32 patients accepted conservative treatments. We did follow-up assessments at 3, 6, and 12 months. The primary endpoints were changes of blood pressure, and secondary endpoints were changes of modified Japanese Orthopedic Association (mJOA) score and cervical pain visual analogue scale (VAS). Spearman correlation coefficients were calculated between changes in systolic blood pressure (SBP) and mJOA scores, VAS scores.

RESULTS

In patients with decompression surgery, the significant decrease in both SBP and diastolic blood pressure (DBP) that was seen 3 months and sustained through subsequent visit at 12 months. Paired-samples t test showed that both SBP and DBP were significantly lower than baseline blood pressure at all time points after procedure (P < 0.001). Significant correlation was found between the improvement rates of mJOA score and changes in SBP (r = -0.579, P < 0.001). But the correlation between changes in VAS score and changes in SBP was not significant (r = 0.58, P = 0.571).

CONCLUSION

Cervical decompression surgery could reduce concomitant high blood pressure in CSM patients, indicating a significant association between the decrease in blood pressure and the improvement of spinal cord function.

LEVEL OF EVIDENCE

2.

Psychobiological response to pain in female adolescents with nonsuicidal self-injury

BACKGROUND

Nonsuicidal self-injury (NSSI) is associated with reduced pain sensitivity and alterations in top-down processing of nociceptive information. The experience of acute pain is characterized by reactivity of the autonomic nervous system (ANS) and hypothalamic-pituitary-adrenal (HPA) axis, which to our knowledge has not been systematically investigated in the context of NSSI.

METHODS

Adolescents fulfilling DSM-5 diagnostic criteria for NSSI and matched healthy controls received cold pain stimulation. We obtained self-reports on psychological distress and measured blood pressure, heart rate variability (HRV) and saliva cortisol. Regression analyses were used to investigate group differences on observed difference scores, adjusting for confounding variables.

RESULTS

We included 30 adolescents engaging in NSSI and 30 controls in our study. Adolescents in the NSSI group showed a greater pain threshold. Groups significantly differed in their psychological response to pain. In patients with NSSI, mood and body awareness increased after painful stimulation; in controls it decreased. Tension increased in controls only. The HPA axis response to painful stimulation was increased in the NSSI compared with the control group. Analysis of ultra-short-term recordings of HRV revealed significant group differences during the anticipation of pain and recovery.

LIMITATIONS

Future studies should incorporate multiple measures of saliva cortisol and replicate the present findings in a naturalistic setting.

CONCLUSION

Compared with controls, individuals engaging in NSSI show psychological benefits in response to pain. Biological findings highlight decreased physiologic arousal before and prolonged arousal (ANS and HPA axis response) after painful stimulation in adolescents engaging in NSSI. Greater pain-inflicted autonomic arousal and cortisol secretion may counteract dissociative states, reduce negative affect and increase body awareness in adolescents engaging in NSSI, lending support for a neurobiological pathomechanism underlying the intraindividual and antisuicide functions of NSSI.

Nonpainful wide-area compression inhibits experimental pain

Compression therapy, a well-recognized treatment for lymphoedema and venous disorders, pressurizes limbs and generates massive non-noxious afferent sensory barrages. The aim of this study was to study whether such afferent activity has an analgesic effect when applied on the lower limbs, hypothesizing that larger compression areas will induce stronger analgesic effects, and whether this effect correlates with conditioned pain modulation (CPM). Thirty young healthy subjects received painful heat and pressure stimuli (47°C for 30 seconds, forearm; 300 kPa for 15 seconds, wrist) before and during 3 compression protocols of either SMALL (up to ankles), MEDIUM (up to knees), or LARGE (up to hips) compression areas. Conditioned pain modulation (heat pain conditioned by noxious cold water) was tested before and after each compression protocol. The LARGE protocol induced more analgesia for heat than the SMALL protocol (P < 0.001). The analgesic effect interacted with gender (P = 0.015). The LARGE protocol was more efficient for females, whereas the MEDIUM protocol was more efficient for males. Pressure pain was reduced by all protocols (P < 0.001) with no differences between protocols and no gender effect. Conditioned pain modulation was more efficient than the compression-induced analgesia. For the LARGE protocol, precompression CPM efficiency positively correlated with compression-induced analgesia. Large body area compression exerts an area-dependent analgesic effect on experimental pain stimuli. The observed correlation with pain inhibition in response to robust non-noxious sensory stimulation may suggest that compression therapy shares similar mechanisms with inhibitory pain modulation assessed through CPM.

Pain Catastrophizing, rather than Vital Signs, Associated with Pain Intensity in Patients Presenting to the Emergency Department for Pain

This study examined the relationships of self-reported pain intensity with vital signs, pain catastrophizing, and state anxiety in patients presenting to the emergency department (ED) for acute pain, exacerbations of chronic pain, or acute pain with concurrent chronic (combined) pain, comparing the pattern of relationships among these three pain groups. One hundred fifty-eight patients presenting to the ED for pain were recruited. Vital signs and self-reported pain intensity were obtained at triage, then participants completed self-report measures of pain catastrophizing, state anxiety, and demographic information. No significant associations were found between vital signs and pain intensity at triage in any of the pain groups. Pain catastrophizing was significantly associated with self-reported pain intensity in the acute pain group (r = .34, p < .05) and combined pain group (r = .30, p < .05), and state anxiety was significantly associated with self-reported pain intensity in with the acute pain group (r = .27, p < .05). When pain catastrophizing and state anxiety were used in a stepwise multiple regression analysis to predict self-reported pain intensity in the acute pain group, only pain catastrophizing emerged as a unique predictor (β = .405, p < .01). Consistent with previous research, vital signs were not associated with self-reported pain intensity in patients presenting to the ED for pain, including those with chronic pain. Given the significant association of pain catastrophizing and pain intensity among patients presenting to the ED for acute pain, brief measurement of pain catastrophizing may inform pain treatment in the ED.

Affective impairment in chronic low blood pressure

OBJECTIVE

Physical complaints such as faintness, dizziness, cold limbs and headaches have been well-established in chronic low blood pressure (hypotension). This study investigated the occurrence of adverse emotional states and the symptoms of depression in this condition. As autonomic dysregulation, particularly diminished sympathetic tone, is believed to be involved in the etiology of hypotension, the impact of different facets of autonomic cardiovascular control on mood and depressive symptoms was also explored.

METHODS

Forty individuals with chronic hypotension and forty normotensive control persons were presented with the Mood Scale and Beck Depression Inventory. Stroke volume, cardiac output, pre-ejection period, Heather index and aortic peak blood flow velocity were recorded under resting conditions as indices of beta-adrenergic inotropic drive. Respiratory sinus arrhythmia and baroreflex sensitivity were additionally obtained.

RESULTS

Hypotensive individuals scored markedly higher on both questionnaire scales than controls, indicating an adversely affected emotional state and more severe depressive symptoms. In the entire sample, cardiac output, Heather index, and aortic peak blood flow velocity correlated negatively with the questionnaire scores; according to regression analysis, the Heather index explained the largest proportion of test score variance.

CONCLUSION

Although hypotension does not constitute a serious medical condition, the findings of an adverse affective state and increased burden with depressive symptoms corroborate the view that it can have a considerable impact on wellbeing and quality of life. The correlations of the beta-adrenergic indices with the questionnaire scales indicate that cardiac sympathetic regulation plays a key role in the psychophysiological mediation of hypotension-related mood impairment.

Upregulation of fatty acid amide hydrolase in the dorsal periaqueductal gray is associated with neuropathic pain and reduced heart rate in rats

Nerve damage can induce a heightened pain response to noxious stimulation, which is termed hyperalgesia. Pain itself acts as a stressor, initiating autonomic and sensory effects through the dorsal periaqueductal gray (dPAG) to induce both sympathoexcitation and analgesia, which prior studies have shown to be affected by endocannabinoid signaling. The present study addressed the hypothesis that neuropathic pain disrupts autonomic and analgesic regulation by endocannabinoid signaling in the dPAG. Endocannabinoid contents, transcript levels of endocannabinoid signaling components, and catabolic enzyme activity were analyzed in the dPAG of rats at 21 days after painful nerve injury. The responses to two nerve injury models were similar, with two-thirds of animals developing hyperalgesia that was maintained throughout the postinjury period, whereas no sustained change in sensory function was observed in the remaining rats. Anandamide content was lower in the dPAG of rats that developed sustained hyperalgesia, and activity of the catabolic enzyme fatty acid amide hydrolase (FAAH) was higher. Intensity of hyperalgesia was correlated to transcript levels of FAAH and negatively correlated to heart rate and sympathovagal balance. These data suggest that maladaptive endocannabinoid signaling in the dPAG after nerve injury could contribute to chronic neuropathic pain and associated autonomic dysregulation. This study demonstrates that reduced anandamide content and upregulation of FAAH in the dPAG are associated with hyperalgesia and reduced heart rate sustained weeks after nerve injury. These data provide support for the evaluation of FAAH inhibitors for the treatment of chronic neuropathic pain.

Influence of exercise on visceral pain: an explorative study in healthy volunteers

Background and objectives

Contradictory results have been found about the effect of different exercise modalities on pain. The aim of this study was to investigate the early effects of aerobic and isometric exercise on different types of experimental pain, including visceral pain, compared to an active control condition.

Methods

Fifteen healthy subjects (6 women, mean [standard deviation] age 25 [6.5] years) completed 3 interventions consisting of 20 minutes of aerobic cycling, 12 minutes of isometric knee extension and a deep breathing procedure as active control. At baseline and after each intervention, psychophysical tests were performed, including electrical stimulation of the esophagus, pressure pain thresholds and the cold pressor test as a measure for conditioned pain modulation. Participants completed the Medical Outcome Study Short-Form 36 and State-Trait Anxiety Inventory prior to the experiments. Data were analyzed using two-way repeated measures analysis of variance.

Results

No significant differences were found for the psychophysical tests after the interventions, compared to baseline pain tests and the control condition.

Conclusion

No hypoalgesic effect of aerobic and isometric exercise was found. The evidence for exercise-induced hypoalgesia appears to be not as consistent as initially thought, and caution is recommended when interpreting the effects of exercise on pain.

The Role of Chronic Psychosocial Stress in Explaining Racial Differences in Stress Reactivity and Pain Sensitivity

OBJECTIVE

To examine the role of psychosocial factors in mediating the relationship between African American (AA) race and both increased pain sensitivity and blunted stress reactivity.

METHODS

Participants included 133 AA and non-Hispanic white (nHW) individuals (mean [SD] age, 37 [9]) matched for age, sex, and socioeconomic status. Participants underwent mental stress testing (Trier Social Stress Test) while cardiovascular, hemodynamic, and neuroendocrine reactivity were measured. Participants completed questionnaires assessing potential sources of psychosocial stress and were tested for pain responses to cold pain and the temporal summation of heat pulses. Mediation analyses were used to determine the extent to which exposure to psychosocial stress accounted for the observed racial differences in stress reactivity and pain.

RESULTS

Chronic stress exposure and reactivity to mental stress was largely similar among AAs and nHWs; however, AAs exhibited heightened pain to both cold (p = .012) and heat (p = .004). Racial differences in the relationship between stress reactivity and pain were also observed: while greater stress reactivity was associated with decreased pain among nHWs, reactivity was either unrelated to or even positively associated with pain among AAs (e.g., r = -.21 among nHWs and r = .41 among AAs for stroke volume reactivity and cold pressor intensity). Adjusting for minor racial differences in chronic psychosocial stress did not change these findings.

CONCLUSIONS

Accounting for psychosocial factors eliminated racial differences in stress reactivity but not racial differences in sensitivity to experimental pain tasks. Increased exposure to chronic stress may not explain AAs' increased pain sensitivity in laboratory settings.

Increased sympathetic nerve activity and reduced cardiac baroreflex sensitivity in rheumatoid arthritis

KEY POINTS

Rheumatoid arthritis (RA) is a chronic inflammatory condition associated with an increased risk of cardiovascular mortality. Increased sympathetic nerve activity and reduced cardiac baroreflex sensitivity heighten cardiovascular risk, althogh whether such autonomic dysfunction is present in RA is not known. In the present study, we observed an increased sympathetic nerve activity and reduced cardiac baroreflex sensitivity in patients with RA compared to matched controls. Pain was positively correlated with sympathetic nerve activity and negatively correlated with cardiac baroreflex sensitivity. The pattern of autonomic dysfunction that we describe may help to explain the increased cardiovascular risk in RA, and raises the possibility that optimizing pain management may resolve autonomic dysfunction in RA.

ABSTRACT

Rheumatoid arthritis (RA) is a chronic inflammatory condition associated with increased cardiovascular morbidity/mortality and an incompletely understood pathophysiology. In animal studies, central and blood borne inflammatory cytokines that can be elevated in RA evoke pathogenic increases in sympathetic activity and reductions in baroreflex sensitivity (BRS). We hypothesized that muscle sympathetic nerve activity (MSNA) was increased and BRS decreased in RA. MSNA, blood pressure and heart rate (HR) were recorded in age- and sex-matched RA-normotensive (n = 13), RA-hypertensive patients (RA-HTN; n = 17), normotensive (NC; n = 17) and hypertensive controls (HTN; n = 16). BRS was determined using the modified Oxford technique. Inflammation and pain were determined using serum high sensitivity C-reactive protein (hs-CRP) and a visual analogue scale (VAS), respectively. MSNA was elevated similarly in RA, RA-HTN and HTN patients (32 ± 9, 35 ± 14, 37 ± 8 bursts min^-1 ) compared to NC (22 ± 9 bursts min^-1 ; P = 0.004). Sympathetic BRS was similar between groups (P = 0.927), whereas cardiac BRS (cBRS) was reduced in RA, RA-HTN and HTN patients [5(3-8), 4 (2-7), 6 (4-9) ms mmHg^-1 ] compared to NC [11 (8-15) ms mmHg^-1 ; P = 0.002]. HR was independently associated with hs-CRP. Increased MSNA and reduced cBRS were associated with hs-CRP although confounded in multivariable analysis. VAS was independently associated with MSNA burst frequency, cBRS and HR. We provide the first evidence for heightened sympathetic outflow and reduced cBRS in RA that can be independent of hypertension. In RA patients, reported pain was positively correlated with MSNA and negatively correlated with cBRS. Future studies should assess whether therapies to ameliorate pain and inflammation in RA restores autonomic balance and reduces cardiovascular events.

Exercise prevents development of autonomic dysregulation and hyperalgesia in a mouse model of chronic muscle pain

Chronic musculoskeletal pain (CMP) conditions, like fibromyalgia, are associated with widespread pain and alterations in autonomic functions. Regular physical activity prevents the development of CMP and can reduce autonomic dysfunction. We tested if there were alterations in autonomic function of sedentary mice with CMP, and whether exercise reduced the autonomic dysfunction and pain induced by CMP. Chronic musculoskeletal pain was induced by 2 intramuscular injections of pH 5.0 in combination with a single fatiguing exercise task. A running wheel was placed into cages so that the mouse had free access to it for either 5 days or 8 weeks (exercise groups) and these animals were compared to sedentary mice without running wheels. Autonomic function and nociceptive withdrawal thresholds of the paw and muscle were assessed before and after induction of CMP in exercised and sedentary mice. In sedentary mice, we show decreased baroreflex sensitivity, increased blood pressure variability, decreased heart rate variability, and decreased withdrawal thresholds of the paw and muscle 24 hours after induction of CMP. There were no sex differences after induction of the CMP in any outcome measure. We further show that both 5 days and 8 weeks of physical activity prevent the development of autonomic dysfunction and decreases in withdrawal threshold induced by CMP. Thus, this study uniquely shows the development of autonomic dysfunction in animals with chronic muscle hyperalgesia, which can be prevented with as little as 5 days of physical activity, and suggest that physical activity may prevent the development of pain and autonomic dysfunction in people with CMP.

Effects of closure versus non-closure of the visceral and parietal peritoneum at cesarean section: does it have any effect on postoperative vital signs? A prospective randomized study

OBJECTIVE

To investigate the effect of peritonization at cesarean section on postoperative vital signs which was thought to be an indirect finding secondary to increased sympathetic activity originated from pain caused by stretched peritoneum.

METHODS

One hundred and thirty-three pregnant women were randomized to four groups; Closure of parietal peritoneum only (group 1; n = 32), closure of visceral and parietal peritoneums (group 2; n = 32), no closure of peritoneums (group 3; n = 32) and closure of the visceral peritoneum only (group 4; n = 32). All participants were monitored for blood pressure, pulse activity and hourly urinary output during the first postoperative 24 h. Postoperative pain was measured using a Visual Analogue Scale 6th and 24th hours after surgery. Return of bowel function was measured from the end of the operation to the first passage of flatus. Operating time, pre- and postoperative hemoglobin, postoperative complications, length of hospital stay and postoperative urine osmolarity were noted.

RESULTS

The mean surgery duration was significantly longer in group 2. Diuresis was found significantly decreased in group 2. Pulse rate and systolic and diastolic blood pressure were significantly higher in group 2. Closure of both peritoneums was associated with higher post-operative pain as assessed using Visual analogue scale score analyses in group 2.

CONCLUSION

Both visceral and parietal membrane closure in cesarean section should be avoided in women with hypertensive disorders, renal function abnormalities and autonomic dysfunction because of increased postoperative pain and associated sympathetic overactivity.

Short-term assessment of autonomic nervous system as a potential tool to quantify pain experience

Various pain conditions may result in altered autonomic nervous system regulation. We evaluated whether autonomic regulation, analyzed with short-term heart rate variability (HRV), differs between low back pain (LBP) patients and healthy controls. Additionally, we tested if short-term HRV recordings are feasible and informative in planning home monitoring of LBP patients. The study population consisted of 16 volunteers (8 LBP patients and 8 healthy subjects) (age 42±10 years, body mass index 26±4 kg/m2, 7 men and 9 women). Usually 3- to 5-minute R-R interval recordings have been used as short-term recordings of HRV, but recent evidence supports even shorter R-R interval recording procedure for short-term HRV assessment. We collected R-R interval data for 1 minute in sitting, standing and bending down tasks. Mean heart rate (HR) and vagally mediated beat-to-beat variability (SD1 and rMSSD) were analyzed. The results showed that autonomic nervous system function assessed with the short-term measurement HRV method differentiates LBP patients from healthy controls in sitting and standing. Vagally mediated SD1 and rMSSD were significantly lower and the HR was higher among the patients compared to the controls. A novel finding was also the feasibility of 1-minute measurement of HRV, which may open entirely new opportunities to assess accurately concomitant changes in autonomic nervous system function and self-reported individual pain experience. This could lead to a more personalized pain treatment and more efficient health care resource allocation as the new measurement methods is more suitable for home monitoring than the previously used ones.

Resting blood pressure differentially predicts time course in a tonic pain experiment

Resting blood pressure (BP) shows a negative relationship with pain sensitivity (BP-related hypoalgesia). In chronic pain conditions, this relationship is inverted. The precise mechanisms responsible for the inversion are unknown. Using a tonic pain protocol, we report findings closely resembling this inversion in healthy participants. Resting BP and state measures of anxiety and mood were assessed from 33 participants (21 female). Participants then immersed their dominant hand in painfully hot water (47 °C) for five trials of 1-min duration, with 30-s intertrial intervals. Throughout the trials, participants continually registered their pain. After a 35-min intermission, the trial sequence was repeated. A disassociation of the negative relationship of resting systolic BP (as per Trial 1) was found using hierarchical linear modeling (p < .001, R(2)  = .07). The disassociation unfolds over each consecutive trial, with an increasingly positive relationship. In Sequence 2, the initially negative relationship is almost completely absent. Furthermore, the association of BP and pain was found to be moderated by anxiety, such that only persons with low anxiety exhibited BP hypoalgesia. Our findings expand the existing literature by incorporating anxiety as a moderator of BP hypoalgesia. Furthermore, the protocol emulates the changing relationship between BP and pain observed in chronic pain patients. The protocol has potential as a model for chronic pain; however, future research should determine if similar physiological systems are involved. The finding holds potential diagnostic or prognostic relevance for certain clinical pain conditions, especially those involving dysfunction of the descending modulation of pain.

Relationships of abdominal pain, reports to visceral and temperature pain sensitivity, conditioned pain modulation, and heart rate variability in irritable bowel syndrome

BACKGROUND

Irritable bowel syndrome (IBS) is a heterogeneous condition with a number of pathophysiological mechanisms that appear to contribute to symptom chronicity. One of these is altered pain sensitivity.

METHODS

Women between ages 18-45 were recruited the community. Of those enrolled, 56 had IBS and 36 were healthy control (HC) women. Participants completed questionnaires, kept a 4-week symptom diary and had a 12-h Holter placed to assess nighttime heart rate variability including high frequency power (HF), low frequency power (LF), and total power (TP). At mid-follicular phase approximately 80% of women completed a thermal pain sensitivity test with conditioned pain modulation and visceral pain sensitivity using a water load symptom provocation (WLSP) test.

KEY RESULTS

As expected, daily abdominal pain was significantly higher in the IBS compared to HC group. There were no differences between the bowel pattern subgroups (IBS-diarrhea [IBS-D], IBS-constipation plus mixed [IBS-CM]). Thermal pain sensitivity did not differ between the IBS and the HC groups, but was significantly higher in the IBS-CM group than the IBS-D group. In the WLSP test, the IBS group experienced significantly more symptom distress than HCs and the IBS-CM group was higher than the IBS-D group. Heart rate variability indicators did not differ between the groups or IBS subgroups. Daily abdominal pain was positively correlated with LF and TP in the IBS group.

CONCLUSIONS & INFERENCES

Despite similar levels of abdominal pain in IBS, the IBS-CM group demonstrated greater sensitivity to both thermal and visceral testing procedures.

TMD and chronic pain: a current view

This review aims at presenting a current view on the physiopathologic mechanisms associated with temporomandibular disorders (TMDs). While joint pain is characterized by a well-defined inflammatory process mediated by tumor necrosis factor-α and interleukin, chronic muscle pain presents with enigmatic physiopathologic mechanisms, being considered a functional pain syndrome similar to fibromyalgia, irritable bowel syndrome, interstitial cystitis and chronic fatigue syndrome. Central sensitization is the common factor unifying these conditions, and may be influenced by the autonomic nervous system and genetic polymorphisms. Thus, TMDs symptoms should be understood as a complex response which might get worse or improve depending on an individual's adaptation.

Alterations in the coupling functions between cortical and cardio-respiratory oscillations due to anaesthesia with propofol and sevoflurane

The precise mechanisms underlying general anaesthesia pose important and still open questions. To address them, we have studied anaesthesia induced by the widely used (intravenous) propofol and (inhalational) sevoflurane anaesthetics, computing cross-frequency coupling functions between neuronal, cardiac and respiratory oscillations in order to determine their mutual interactions. The phase domain coupling function reveals the form of the function defining the mechanism of an interaction, as well as its coupling strength. Using a method based on dynamical Bayesian inference, we have thus identified and analysed the coupling functions for six relationships. By quantitative assessment of the forms and strengths of the couplings, we have revealed how these relationships are altered by anaesthesia, also showing that some of them are differently affected by propofol and sevoflurane. These findings, together with the novel coupling function analysis, offer a new direction in the assessment of general anaesthesia and neurophysiological interactions, in general.

Inter-individual responses to experimental muscle pain: Baseline anxiety ratings and attitudes to pain do not determine the direction of the sympathetic response to tonic muscle pain in humans

We have recently shown that intramuscular infusion of hypertonic saline, causing pain lasting ~60min, increases muscle sympathetic nerve activity (MSNA) in one group of subjects, yet decreases it in another. Across subjects these divergent sympathetic responses to long-lasting muscle pain are consistent over time and cannot be foreseen on the basis of baseline MSNA, blood pressure, heart rate or sex. We predicted that differences in anxiety or attitudes to pain may account for these differences. Psychometric measures were assessed prior to the induction of pain using the State and Trait Anxiety Inventory (STAI), Pain Vigilance and Awareness Questionnaire (PVAQ), Pain Anxiety Symptoms Scale (PASS) and Pain Catastrophising Scale (PCS); PCS was also administered after the experiment. MSNA was recorded from the common peroneal nerve, before and during a 45-minute intramuscular infusion of hypertonic saline solution into the tibialis anterior muscle of 66 awake human subjects. Forty-one subjects showed an increase in mean burst amplitude of MSNA (172.8±10.6%) while 25 showed a decrease (69.9±3.8%). None of the measured psychological parameters showed significant differences between the increasing and the decreasing groups. We conclude that inter-individual anxiety or pain attitudes do not determine whether MSNA increases or decreases during long-lasting experimental muscle pain in healthy human subjects.

Temporal patterns of sitting at work are associated with neck-shoulder pain in blue-collar workers: a cross-sectional analysis of accelerometer data in the DPHACTO study

Background

Our aim was to examine the extent to which temporal patterns of sitting during occupational work and during leisure-time, assessed using accelerometry, are associated with intense neck–shoulder pain (NSP) in blue-collar workers.

Methods

The population consisted of 659 Danish blue-collar workers. Accelerometers were attached to the thigh, hip, trunk and upper dominant arm to measure sitting time and physical activity across four consecutive days. Temporal sitting patterns were expressed separately for work and leisure by the proportion of total time spent sitting in brief bursts (0–5 min), moderate (>5–20 min) and prolonged (>20 min) periods. The peak NSP intensity during the previous 3 months was assessed using a numerical rating scale (range 0–10) and dichotomized into a lower (≤4) and higher (>4) NSP score. Logistic regression analyses with multiple adjustments for individual and occupational factors were performed to determine the association between brief, moderate and prolonged sitting periods, and NSP intensity.

Results

Time in brief bursts of occupational sitting was negatively associated with NSP intensity (adjusted OR 0.68, 95 % CI 0.48–0.98), while time in moderate periods of occupational sitting showed a positive association with NSP (adjusted OR 1.32, 95 % CI 1.04–1.69). Time in prolonged periods of occupational sitting was not associated with NSP (adjusted OR 0.78, 95 % CI 0.78–1.09). We found no significant association between brief, moderate or prolonged sitting periods during leisure, and NSP.

Conclusion

Our findings indicate that the association between occupational sitting time and intense NSP among blue-collar workers is sensitive to the temporal pattern of sitting.

Pain Modulation and Autonomic Function: The Effect of Clonidine

OBJECTIVE

The α2-agonist clonidine is an analgesic agent, whose yet uncertain action may involve either increase in pain modulation efficiency, change in autonomic function, and/or decrease in anxiety level. The present study aimed to examine the effect of oral clonidine on pain perception in healthy subjects in order to reveal its mode of action.

DESIGN

Randomized, double-blind, placebo-controlled study.

SUBJECTS

Forty healthy subjects.

METHODS

Subjects received either 0.15 mg oral clonidine or placebo. We measured pain parameters of heat pain thresholds, tonic heat stimulus, mechanical temporal summation, offset analgesia (OA) and conditioned pain modulation (CPM); autonomic parameters of deep breathing ratio and heart rate variability indices obtained before, during, and after tonic heat stimulus; and psychological parameters of anxiety and pain catastrophizing.

RESULTS

Clonidine decreased systolic blood pressure (P = 0.022) and heart rate (P = 0.004) and increased rMSSD (P = 0.020), though no effect was observed on pain perception, pain modulation, and psychological parameters. Autonomic changes were correlated with pain modulation capacity; for OA, the separate slope model was significant (P = 0.008); in the clonidine group, more efficient OA was associated with lower heart rate (r = 0.633, P = 0.005), unlike in the placebo group.

CONCLUSIONS

The change in autonomic function that was related to the increase in pain modulation capacity, and the lack of change in anxiety, suggest a combined modulatory-autonomic mode of analgesic action for clonidine.

The Relationships Between Parasympathetic Function and Pain Perception: The Role of Anxiety

BACKGROUND

Previous studies have identified relationships between autonomic function and pain perception. Anxiety was found to influence both autonomic and pain responses. We examined the effect of anxiety level on parasympathetic function and pain perception as well as on the relationships between these 2 systems.

METHODS

Thirty healthy females were divided into high- and low-anxiety groups according to their trait anxiety levels. Parasympathetic function was obtained using heart rate variability, deep breathing, and Valsalva ratios. Pain perception parameters of heat pain thresholds, pain rating of supra-thresholds stimulus, mechanical temporal summation, and conditioned pain modulation response were examined.

RESULTS

The low-anxiety and high-anxiety groups exhibited no significant differences in the parasympathetic function and pain perception parameters. Assessment of the associations revealed that in the high-anxiety group, higher mean ratings of the tonic heat pain stimulus were significantly correlated with higher rMSSD (r² = 0.358, P = 0.019), but this was not found for the low-anxiety group (P = 0.282). In addition, in the high-anxiety group, efficient conditioned pain modulation response was correlated with higher deep breathing ratio (r² = 0.363, P = 0.023); however, in the low-anxiety group, the correlation did not reach significance (P = 0.109).

CONCLUSIONS

This study demonstrates the role of anxiety level on the relationships between parasympathetic function and pain perception. We suggest that a situation of high anxiety leads to higher norepinephrine levels that can influence both parasympathetic function and pain perception, thus explaining the significant relationships found between these 2 systems only in subjects with high anxiety.

Respiratory hypoalgesia? Breath-holding, but not respiratory phase modulates nociceptive flexion reflex and pain intensity

Several observations suggest that respiratory phase (inhalation vs. exhalation) and post-inspiratory breath-holds could modulate pain and the nociceptive reflex. This experiment aimed to investigate the role of both mechanisms. Thirty-two healthy participants received supra-threshold electrocutaneous stimulations to elicit both the Nociceptive Flexion Reflex (NFR) and pain, either during spontaneous inhalations or exhalations, or during three types of instructed breath-holds: following exhalation, at mid-inhalation and at full-capacity inhalation. Whether the electrocutaneous stimulus was applied during inhalation or exhalation did not affect the NFR or pain. Self-reported pain was reduced and the NFR was increased during breath-holding compared to spontaneous breathing. Whereas the type of breath-hold did not impact on self-reported pain, breath-holds at full-capacity inhalation and following exhalation were associated with a lower NFR amplitude compared to breath-holds at mid-inhalation. The present findings confirm that breath-holding can modulate pain (sensitivity) and suggest that both attentional distraction and changes in vagal activity may underlie the observed effects.