Pain sensitivity in offspring of hypertensives at rest and during baroreflex stimulation

Hypertensive Hypoalgesia in a Complex Chronic Disease Population

Hypertension-related hypoalgesia, defined as lower pain sensitivity in individuals with high blood pressure, has yet to be examined in a large-scale study of complex care residents. Here, the Continuing Care Reporting System database, which contains health information on residents of Canadian complex chronic care facilities, was used for assessment. Hypertension was reported among 77,323 residents (55.5%, total N = 139,920). Propensity score matching, with a 1:1 ratio, was used to identify a control record without hypertension for each case. Multinomial logistic regression was used to quantify the effects of hypertension and sex on four-level ordinal pain variables, controlling for potential confounders. The matched dataset included n = 40,799 cases with hypertension and n = 40,799 without hypertension, with 57% female. Residents with hypertension had significantly lower odds of reporting pain (yes/no) (OR = 0.85, 95% CI 0.81–0.90, p < 0.001), including on measures of severe pain (OR = 0.69, 95% CI 0.63–0.76, p < 0.001). A significant interaction between hypertension and sex (OR = 1.17, 95% CI 1.03–1.32, p = 0.014) indicated that a significantly greater proportion of females without hypertension reported severe pain (8.71%). The results confirm the relationship between hypertension and reduced pain sensitivity on a population level.

Blood pressure-related hypoalgesia: a systematic review and meta-analysis

OBJECTIVE

Spontaneous or experimentally induced high blood pressure (BP) is associated with reduced pain perception, known as BP-related hypoalgesia. Despite its clinical implications, such as the interference with early detection of myocardial infarction in 'at risk' groups, the size of the association between high BP and pain has not yet been quantified. Moreover, the distinct association between high BP and physiological or psychological components of pain has not yet been considered so far. The aim of this study was to overcome this gap by performing separate meta-analyses on nociceptive response versus quantifiable perceptual measures of pain in relation to high BP.

METHODS

PubMed and Web of Knowledge databases were searched for English language studies conducted in humans. Fifty-nine studies were eligible for the analyses. Pooled effect sizes (Hedges' g) were compared. Random effect models were used. Results show that higher BP is significantly associated with lower nociceptive response (g = 0.38; k = 6) and reduced pain perception, assessed by quantifiable measures (g = 0.48; k = 59).

RESULTS

The association between BP and pain perception, derived from highly heterogeneous studies, was characterized by significant publication bias. BP assessment, pain assessment, site of pain stimulation, percentage of female participants in the sample, and control for potential confounders were significant moderators.

CONCLUSION

Current meta-analytic results confirm the presence of BP-related hypoalgesia and point towards the need for a better understanding of its underlying mechanisms.

Contribution of Baroreceptor Function to Pain Perception and Perioperative Outcomes

Baroreceptors are mechanosensitive elements of the peripheral nervous system that maintain homeostasis by coordinating physiologic responses to external and internal stimuli. While it is recognized that carotid and cardiopulmonary baroreceptor reflexes modulate autonomic output to mitigate excessive fluctuations in arterial blood pressure and to maintain intravascular volume, increasing evidence suggests that baroreflex pathways also project to key regions of the central nervous system that regulate somatosensory, somatomotor, and central nervous system arousal. In addition to maintaining autonomic homeostasis, baroreceptor activity modulates the perception of pain, as well as neuroimmune, neuroendocrine, and cognitive responses to physical and psychologic stressors. This review summarizes the role that baroreceptor pathways play in modulating acute and chronic pain perception. The contribution of baroreceptor function to postoperative outcomes is also presented. Finally, methods that enhance baroreceptor function, which hold promise in improving postoperative and pain management outcomes, are presented.

Resting blood pressure modulates chest pain intensity in patients with acute myocardial infarction

Introduction and Objectives

Animal models and human studies show that resting blood pressure (BP) is inversely associated with pain sensitivity. The phenomenon of "hypertension-associated hypoalgesia" was proposed as a possible explanation for the intervariability in pain perception. Given that a portion of patients with acute myocardial infarction (AMI) do not experience significant pain, we used the model of severe cardiac ischemia to explore whether BP affects the intensity of chest pain.

Methods

Patients with AMI admitted to the cardiac intensive care unit with coronary catheterization-proven completely occluded coronary artery were included (n = 67). Resting BP at admission and 5 days after AMI was obtained. Participants reported chest pain intensity and underwent psychophysical evaluation including pain ratings for pressure, heat, and pinprick stimuli as well as temporal summation and conditioned pain modulation paradigms.

Results

Patients with lower systolic BP (≤120 mm Hg) vs higher (≥140 mm Hg) reported higher chest pain scores at symptom onset (82.3 vs 61.7, P = 0.048) and during peak AMI (82.8 vs 57.5, P = 0.019). Higher pain ratings in response to pinprick stimulus were associated with lower BP at admission (analysis of variance P = 0.036). Patients with hypertension demonstrated lower pain sensitivity in response to pressure stimulation (531.7 ± 158.9 kPa/s vs 429.1 ± 197.4). No significant associations were observed between BP and the other assessed psychophysical measures.

Conclusion

Study findings reinforce the phenomenon of hypertension-associated hypoalgesia through characterization of the association between BP and clinical pain experiences at onset and during AMI in a model of acute clinical pain.

Brain-Heart Pathways to Blood Pressure-Related Hypoalgesia

OBJECTIVE

High blood pressure (BP) is associated with reduced pain sensitivity, known as BP-related hypoalgesia. The underlying neural mechanisms remain uncertain, yet arterial baroreceptor signaling, occurring at cardiac systole, is implicated. We examined normotensives using functional neuroimaging and pain stimulation during distinct phases of the cardiac cycle to test the hypothesized neural mediation of baroreceptor-induced attenuation of pain.

METHODS

Eighteen participants (10 women; 32.7 (6.5) years) underwent BP monitoring for 1 week at home, and individual pain thresholds were determined in the laboratory. Subsequently, participants were administered unpredictable painful and nonpainful electrocutaneous shocks (stimulus type), timed to occur either at systole or at diastole (cardiac phase) in an event-related design. After each trial, participants evaluated their subjective experience.

RESULTS

Subjective pain was lower for painful stimuli administered at systole compared with diastole, F(1, 2283) = 4.82, p = 0.03. Individuals with higher baseline BP demonstrated overall lower pain perception, F(1, 2164) = 10.47, p < .0001. Within the brain, painful stimulation activated somatosensory areas, prefrontal cortex, cingulate cortex, posterior insula, amygdala, and the thalamus. Stimuli delivered during systole (concurrent with baroreceptor discharge) activated areas associated with heightened parasympathetic drive. No stimulus type by cardiac phase interaction emerged except for a small cluster located in the right parietal cortex.

CONCLUSIONS

We confirm the negative associations between BP and pain, highlighting the antinociceptive impact of baroreceptor discharge. Neural substrates associated with baroreceptor/BP-related hypoalgesia include superior parietal lobule, precentral, and lingual gyrus, regions typically involved in the cognitive aspects of pain experience.

Stress and Addiction: When a Robust Stress Response Indicates Resiliency

OBJECTIVE

Stress reactivity research has traditionally focused on the idea that exaggerated responses to stress may have adverse effects on health. Accumulating evidence suggests that attenuated responses to stress and delayed recovery may also be problematic.

METHODS

This review focuses on the role of the stress response of the hypothalamic-pituitary-adrenocortical axis, the endogenous opioid system, and the cardiovascular system in hypertension, pain perception, and addictive behaviors. Results from multiple methods of assessment and stress paradigms conducted in our laboratory over the past two decades are integrated with research from other investigators and with existing theories.

RESULTS

Research indicates that exaggerated biological and physiological responses to stress and attenuated pain perception are associated with hypertension and risk for cardiovascular diseases. This research complements work linking reduced stress responses with enhanced pain sensitivity and discomfort. Multiple studies have also demonstrated that an attenuated stress response is linked to exacerbation of withdrawal symptoms and relapse in nicotine addiction. Evidence indicates important moderators (i.e., sex, personality traits, and early life adversity) and hypothalamic-pituitary-adrenocortical- and endogenous opioid system-related mechanisms in the altered response to stress. I integrate these findings in a conceptual model emphasizing that robust stress responses in the context of addiction and relapse should be considered as a marker of resiliency.

CONCLUSIONS

A blunted stress response may indicate long-term physiological dysregulation that could usher harmful consequences for cardiovascular disease, pain perception, and addictive disorders. The impact of dysregulation is influenced by multiple individual and situational factors that should be considered in evaluating the clinical significance of stress response dysregulation.

The relationship between blood pressure and pain

The relationship between pain and hypertension is potentially of great pathophysiological and clinical interest, but is poorly understood. The perception of acute pain initially plays an adaptive role, which results in the prevention of tissue damage. The consequence of ascending nociception is the recruitment of segmental spinal reflexes through the physiological neuronal connections. In proportion to the magnitude and duration of the stimulus, these spinal reflexes cause the activation of the sympathetic nervous system, which increases peripheral resistances, heart rate, and stroke volume. The response also involves the neuroendocrine system, and, in particular, the hypothalamic-pituitary-adrenal axis, in addition to further activation of the sympathetic system by adrenal glands. However, in proportion to an elevation in resting blood pressure, there is a contemporary and progressive reduction in sensitivity to acute pain, which could result in a tendency to restore arousal levels in the presence of painful stimuli. The pathophysiological pattern is significantly different in the setting of chronic pain, in which the adaptive relationship between blood pressure and pain sensitivity is substantially reversed. The connection between acute or chronic pain and cardiovascular changes is supported observationally, but some of this indirect evidence is confirmed by experimental models and human studies. The pain regulatory process and functional interaction between cardiovascular and pain regulatory systems are briefly reviewed. Various data obtained are described, together with their potential clinical implications.

Association of elevated blood pressure with low distress and good quality of life: results from the nationwide representative German Health Interview and Examination Survey for Children and Adolescents

OBJECTIVE

Quality of life is often impaired in patients with known hypertension, but it is less or not at all reduced in people unaware of their elevated blood pressure. Some studies have even shown less self-rated distress in adults with elevated blood pressure. In this substudy of the nationwide German Health Interview and Examination Survey for Children and Adolescents (KIGGS), we addressed the question whether, also in adolescents, hypertensive blood pressure is linked to levels of distress and quality of life.

METHODS

Study participants aged 11 to 17 years (N = 7688) received standardized measurements of blood pressure, quality of life (using the Children's Quality of Life Questionnaire), and distress (Strengths and Difficulties Questionnaire).

RESULTS

Elevated blood pressure was twice as frequent as expected, with 10.7% (n = 825) above published age-, sex- and height-adjusted 95th percentiles. Hypertensive participants were more likely to be obese and to report on adverse health behaviors, but they showed better academic success than did normotensive participants. Elevated blood pressure was significantly and positively associated with higher self- and parent-rated quality of life (for both, p ≤ .006), less hyperactivity (for both, p < .005), and lower parent-rated emotional (p < .001), conduct (p = .021), and overall problems (p = .001). Multiple regression analyses confirmed these findings.

CONCLUSIONS

Our observation linking elevated blood pressure to better well-being and low distress can partly be explained by the absence of confounding physical comorbidity and the unawareness of being hypertensive. It also corresponds to earlier research suggesting a bidirectional relationship with repressed emotions leading to elevated blood pressure and, furthermore, elevated blood pressure serving as a potential stress buffer.

Cardiovascular-emotional dampening: the relationship between blood pressure and recognition of emotion

OBJECTIVE

Persons with elevated blood pressure (BP) show dampened emotional responses to affect-laden stimuli. We sought to further examine cardiovascular-emotional dampening by examination of the relationship between resting hemodynamic measures and recognition of emotion in an African American community-based sample.

METHODS

Participants were 106 African American men and women (55 women; mean age = 52.8 years), mainly low in socioeconomic status, and part of the Healthy Aging in Nationally Diverse Longitudinal Samples pilot study. Participants evaluated emotional expressions in faces and sentences using the Perception of Affect Test (PAT). Resting BP, total peripheral resistance (TPR), cardiac output, and heart rate were obtained continuously using a Portapres BP monitor.

RESULTS

Total PAT scores were inversely related to systolic (r = -0.30) and diastolic (r = -0.24) BPs, TPR (r = -0.36), and age (r = -0.31; p values < .01) and were positively related to cardiac output (r = 0.27) and education (r = 0.38; p values < .01), as well as with mental state (r = 0.25) and body mass index (r = -0.20; p values < .05). Accuracy of emotion recognition on the PAT tasks remained inversely related to TPR and BP after adjustment for demographic variables, medication, mental state, and body mass index.

CONCLUSIONS

Elevated BP and TPR were associated with reduced perception of affect. TPR was the most consistent independent hemodynamic correlate of emotional dampening for the PAT scores. These results suggest potentially important links among central nervous system regulation of emotions, hemodynamic processes, and hypertension development.

The impact of prenatal stress on basal nociception and evoked responses to tail-docking and inflammatory challenge in juvenile pigs

The consequences of tail-docking (at 2-4 days) and prenatal stress (maternal social stress during the 2nd third of pregnancy) on baseline nociceptive thresholds and responses to acute inflammatory challenge were investigated in juvenile pigs in two studies. Nociceptive thresholds were assessed on the tail root and on the hind foot using noxious mechanical and cold stimulation before and after acute inflammatory challenge by intradermal injection of 30 μg capsaicin (study 1) or 3% carrageenan (study 2) into the tail root. Four groups of 8 (study 1, n=14-16 pigs/treatment) or 5 (study 2, n=6 pigs/treatment/sex) week-old pigs were exposed to the main factors: maternal stress and treatment (docked vs. intact tails). In study 1, tail docking did not significantly alter thresholds to noxious mechanical stimulation, whilst prenatally stressed pigs had significantly higher baseline thresholds to noxious mechanical stimulation on the tail root and on the hind foot than unstressed pigs, whether tail-docked or intact. Capsaicin injection induced localised mechanical allodynia around the tail root in all treatment groups, but had no effect on noxious plantar mechanical responses; however prenatally stressed offspring exhibited significantly attenuated response thresholds to capsaicin compared to controls. In study 2 tail docking did not alter thresholds to either mechanical or noxious cold stimulation. Baseline response durations to noxious cold stimulation of the tail root were significantly shorter in both sexes of prenatally stressed pigs, whilst male but not female prenatally stressed pigs exhibited significantly higher baseline thresholds to mechanical stimulation than controls, although results in female pigs tended towards significance. Carrageenan injection into the tail root induced localised mechanical and cold allodynia in all treatment groups, effects that were attenuated in prenatally stressed pigs. Collectively, these findings indicate that prenatal stress can induce long-term alterations in nociceptive responses, manifest as a reduced sensitivity to noxious mechanical and cold stimulation and evoked inflammatory allodynia. Neonatal tail-docking does not lead to long-term alterations in nociception in pigs.

Chest pain is inversely associated with blood pressure during exercise among individuals being assessed for coronary heart disease

Acute and chronic increases in blood pressure have been related to decreases in pain perception. This phenomenon has been studied primarily using acute experimental pain stimuli. To extend the literature to naturalistic pain and in particular the problem of silent cardiac ischemia, this study examined the relationship between blood pressure and chest pain during exercise stress testing. Nine hundred seven (425 men, 482 women) individuals undergoing exercise stress testing for diagnosis of possible myocardial ischemia completed the McGill Pain Questionnaire (MPQ) immediately afterward and other questionnaires before and after testing. Blood pressure was measured before, during, and after exercise. Systolic blood pressure at the end of exercise was inversely related to a number of measures of pain such as total score on the MPQ. The relationship could not be explained by individual differences in exercise duration, medication use, sex, or other measured variable. In sum, the inverse relationship between blood pressure and sensitivity to pain that has been observed in other populations in experimental and naturalistic conditions was observed for chest pain during exercise. Blood pressure may contribute to episodes of silent ischemia.

Systolic inhibition of nociceptive responding is moderated by arousal

Inhibition of the nociceptive flexion reflex during systole could be due to activation of the arterial baroreceptors. Physiological arousal, characterized by raised blood pressure, increases afferent activity from the arterial baroreceptors but attenuates the baroreflex. This study examined the effects of arousal on systolic inhibition of the nociceptive flexion reflex in 38 adults. The threshold current to elicit the reflex in the leg was determined, and participants were stimulated at threshold intensity for 12 trials in two conditions: rest (low arousal) and mental arithmetic (high arousal). In each trial, stimulation was delivered 0 ms, 300 ms, or 600 ms after the R-wave of the electrocardiogram. Nociceptive responding was inhibited for stimulation at 300 ms after the R-wave during rest but not mental arithmetic. This moderation of systolic inhibition of nociception could be due to attenuation of the baroreflex with increased arousal.

Altered cardiovascular/pain regulatory relationships in chronic pain

In healthy individuals, there is an inverse relationship between resting blood pressure (BP) and pain sensitivity. This study examined possible dysregulation of this adaptive relation in chronic pain patients, and tested whether the extent of this dysregulation is a function of pain duration. Continuous resting BP's were assessed for 5 min after a 5-min rest period in 121 chronic benign pain patients. Unlike the inverse relationship observed previously in normals, mean resting diastolic. BPs during the assessment period were correlated positively with ratings of pain severity. A Pain Duration x Systolic BP interaction emerged (p < .05) such that the magnitude of the BP-pain relation was greatest in patients with the longest duration of pain, r(38) = .50, p < .001. A hypothesized progressive alteration in endogenous pain regulatory systems in chronic pain patients was supported. A possible role of endogenous opioid dysfunction in accounting for these alterations is discussed.

Opioid analgesia in persons at risk for hypertension

OBJECTIVE

Acute pain sensitivity is reduced in clinical hypertension, but the precise relationship between pain perception and altered blood pressure control is not well-characterized. A negative correlation between resting blood pressure and pain sensitivity is observed throughout the normotensive range, suggesting links between basic mechanisms of blood pressure control and pain regulation. The opioid peptides are important endogenous analgesic mechanisms, but their role in the hypoalgesia of blood pressure elevations has not been well-established. The current study sought to examine the effects of endogenous opioids on blood pressure-associated hypoalgesia in young adults at risk for hypertension development.

METHODS

The effects of the opioid receptor antagonist, naltrexone, on cold pressor pain sensitivity were assessed in young adult men (n = 49) and women (n = 76) with mildly elevated casual blood pressure.

RESULTS

Results indicate interactions between hypertension risk and the effects of opioid blockade on pain sensitivity.

CONCLUSIONS

These findings suggest exaggerated opioid analgesia in persons at enhanced risk for hypertension and point to important links between altered neuropeptide regulation of pain and altered blood pressure control mechanisms in the early stages of hypertension.

Effects of baroreceptor stimulation and opioids on the auditory startle reflex

We examined (a) whether carotid baroreceptor stimulation attenuates the auditory startle response and its modulation by preceding affective pictures, and (b) whether these effects are mediated by endogenous opioids. Seventy-eight young normotensive adults with or without a parental history of hypertension received brief exposures to affective pictures and noise bursts during phasic manipulation of the carotid baroreceptors. In each participant, opioids were blocked by naltrexone in half of the sessions. Baroreceptor stimulation had a strong dampening effect on the startle response. This effect was not influenced by opioid blockade, sex, or parental history of hypertension. No baroreceptor effects were obtained regarding ratings of the affective pictures or startle modulation by the pictures. The baroreceptor stimulation effects seem to be mediated by the basal primary acoustic startle circuit rather than by higher affective circuits.

Effects of artificial and natural baroreceptor stimulation on nociceptive responding and pain

The arterial baroreflex may mediate hypertensive hypoalgesia. Carotid baroreceptors can be artificially stimulated by neck suction and inhibited by compression. Effects of brief neck suction and compression on nociceptive responding and pain were studied in 25 normotensive adults. The sural nerve was electrocutaneously stimulated at threshold intensity during systole or diastole combined with neck suction, neck compression, or no pressure. Nociceptive responding was indexed by electromyographic activity elicited in the biceps femoris. Participants rated the intensity of sural stimulation. Although artificial baroreceptor stimulation (suction) did not affect nociceptive responding, baroreceptor inhibition (compression) reduced pain ratings. In contrast, natural baroreceptor stimulation during systole reduced nociceptive responding compared to diastole, but did not affect pain ratings. The data provide partial support for baroreflex modulation of pain.

Hypertension and appraisal of physical and psychological stressors

OBJECTIVE

In the operant conditioning of hypertension hypothesis, it is assumed that the frequently found diminished sensitivity to painful stimuli in hypertensives can be generalized to sensitivity to other stressors, including psychological stressors. The validity of this assumption is examined in the present study.

METHODS

Unmedicated hypertensives (42) and normotensive controls (21) of both sexes were exposed to a physical stressor (electric current) and psychological active coping (mental arithmetic, free speech) and passive coping (unpleasant films) tasks, while indices of prestressor anxiety and task appraisal were measured.

RESULTS

Hypertensive women, but not men, showed diminished pain sensitivity, together with lower prestressor anxiety and a tendency to lower negative appraisal of the tasks, compared to their normotensive counterparts. In addition, positive correlations were obtained between pain sensitivity and negative appraisal of psychological stressors involving interpersonal threat (speech) and passive coping (films).

CONCLUSION

Preliminary support has been obtained for extrapolation of diminished pain appraisal to appraisal of some psychological stressors (although for a part only in women); an important assumption in the operant conditioning hypothesis of hypertension.

Cardiopulmonary baroreflex stimulation and blood pressure-related hypoalgesia

Results from both experimental animals and humans suggest that baroreflex stimulation may be involved in blood pressure-related hypoalgesia. However, most of this research, especially in the area of human experimentation, has focused on sinoaortic baroreceptors. Cardiopulmonary baroreflex stimulation may also be an important moderator of pain. Sixty-six healthy male undergraduates varying in risk for hypertension participated in an experimental protocol in which painful mechanical finger pressure was presented three times in a counterbalanced fashion. One pain stimulus was preceded by 6 min of supine rest, another by a period of rest interspersed with periodic Valsalva manoeuvres, and another by a period in which cardiopulmonary baroreceptors were stimulated by passive leg elevation. Significantly lower pain was reported by men with relatively elevated systolic blood pressure following leg elevation but not the other conditions. Cardiopulmonary baroreflex stimulation was documented by increased forearm blood flow and other data obtained via impedance cardiography. These results suggest that blood pressure related hypoalgesia may be at least partially related to cardiopulmonary baroreflex stimulation.

Theoretical review: altered pain regulatory systems in chronic pain

This review synthesizes the existing literature regarding the relationship between resting blood pressure and pain sensitivity, and the literature indicating possible endogenous opioid dysfunction in chronic pain. Adaptive interactions between the cardiovascular and pain regulatory systems occur in healthy individuals, with greater blood pressure associated with decreased acute pain sensitivity. Endogenous opioids appear necessary for full expression of this relationship. There is ample evidence indicating diminished endogenous opioid CSF/plasma levels in chronic pain patients, yet little is known about the functional effects of these opioid changes. A theoretical model is proposed based upon the literature reviewed suggesting progressive dysfunction in endogenous opioid systems with increasing chronic pain duration. This dysfunction is hypothesized to result in dysregulation of normally adaptive relationships between the cardiovascular and pain regulatory systems, resulting in increased chronic pain intensity and increased acute pain sensitivity among chronic pain patients. Preliminary data are consistent with the hypothesis of progressive opioid changes resulting in dysfunctional alterations in the adaptive blood pressure-pain relationship. Clinical implications of this theory are discussed.

Relationship between a genetic predisposition to hypertension, blood pressure levels and pain sensitivity

INTRODUCTION

The aim of this study was to determine whether the degree of blood pressure elevation and/or a genetic predisposition to hypertension have a major role in determining a reduced pain perception in hypertensives. The reasons underlying the relationship between blood pressure elevation and pain perception mechanisms are not completely understood.

METHODS

One hundred and four untreated hypertensive patients (65 subjects with and 39 without a positive parental history of hypertension) together with a control group of 42 subjects (20 normotensive offspring of normotensive parents, and 22 normotensive offspring of hypertensive parents) were submitted to standard blood pressure evaluation, 24-h blood pressure monitoring and dental pain perception evaluation.

RESULTS

Both pain threshold and tolerance were found to be higher in hypertensive than normotensive subjects (P < 0.0001 and P < 0.015, respectively). Positive significant correlations were found between both 24-h systolic and diastolic pressure and the pain perception variables. When a 2 x 2 ANOVA test was performed, factoring for the effects of both blood pressure status and family history of hypertension on pain sensitivity, a significant effect was revealed only for blood pressure status. Moreover, after controlling for blood pressure by a covariate analysis, no significant difference was found between the subjects with or without hypertensive parents as regards pain perception variables.

CONCLUSIONS

Pain sensitivity is correlated to blood pressure levels whereas the parental history of hypertension per se does not affect the pain perception pattern. Thus, the degree of blood pressure elevation, more than a genetic predisposition to hypertension may influence the mechanisms leading to hypalgesia in hypertension.

Hypertension and pain sensitivity: effects of gender and cardiovascular reactivity

Repeatedly, hypertensives have been found to appraise physical stressors as less aversive than normotensives. The main aim of the present study was to examine the effects of gender and cardiovascular reactivity in the relationship between hypertension and appraisal of pain. Forty-two unmedicated hypertensives and 21 normotensive controls of both genders were exposed to an electric current stimulus, while various cardiovascular parameters and prestressor anxiety were measured. In general, hypertensive women, but not men, showed diminished pain sensitivity compared to their normotensive counterparts. When the analyses were repeated with controlling for cardiovascular reactivity, the between-group effects were no longer significant. The results indicate that (i) profound gender differences exist in hypertension-related pain sensitivity and (ii) these effects seem to be mediated, at least partly, by cardiovascular reactivity.

Risk for hypertension and diminished pain sensitivity in women: autonomic and daily correlates

Research suggests an association between risk for hypertension and decreased pain sensitivity. However, few studies have utilized non-behavioral indices of pain to corroborate subjective reports or sought to generalize these findings to women. Furthermore, it has not been established whether results obtained using well-controlled laboratory pain stimuli extend to naturalistic pain. In Study 1, 80 young adult women with (N = 40) and without (N = 40) a parental history of hypertension and with either normatively low or high resting systolic blood pressure (SBP) were exposed to two experimental pain stimuli, finger pressure and the cold pressor test. In addition to behavioral pain measures, respiratory sinus arrhythmia (RSA) reactions to pain were also assessed. Women with a parental history of hypertension and/or normatively high resting SBP experienced significantly less pain, as assessed by both behavioral and RSA measures. In Study 2, 37 of the participants from Study 1 monitored their behaviors, affect, and physical symptoms, three times a day for 32 days. Laboratory pain sensitivity was significantly correlated with daily reports of pain but not gastrointestinal symptoms. The present results confirm an association between risk for hypertension and hypoalgesia in women and suggest generalizability of this relationship to everyday life.

Blood pressure-related hypoalgesia in bulimia nervosa

OBJECTIVE

This study examined pain sensitivity and its relationship to arterial blood pressure in bulimia nervosa (BN).

METHODS

Fourteen women who met Diagnostic and Statistical Manual of Mental Disorders, 4th Edition, criteria for BN, purging subtype, and 14 controls were tested for ischemic pain sensitivity after an extended baseline period. Blood pressure, cardiac output, stroke volume, and total peripheral resistance were assessed during baseline, during ischemic pain testing, and at the point of voluntary tolerance.

RESULTS

Women with BN had significantly greater ischemic pain tolerance than controls. Additionally, only for BN women was blood pressure related to pain sensitivity. Systolic blood pressure during the pain procedure and at the point of tolerance was positively related to pain threshold and tolerance times and negatively related to rated unpleasantness of pain in BN, whereas no relationships involving blood pressure and pain sensitivity were observed in controls.

CONCLUSIONS

These results may have implications for maladaptive changes in central pain-cardiovascular regulatory systems for women with BN.

Effects of naloxone on hemodynamic and sympathetic nerve responses to pain in normotensive vs. borderline hypertensive men

Pain sensitivity decreases with increasing resting blood pressure. This blood pressure-pain interaction may be mediated by endogenous opioids which have been shown to affect both blood pressure and nociception. To test this hypothesis, we measured mean arterial blood pressure (MAP), central venous pressure (CVP), heart rate (HR), muscle sympathetic nerve activity (MSNA), serum catecholamines, and individual pain rating scales during 2 min periods of noxious mechanostimulation (skin fold pinching) in nine young (26 +/- 2 year), male normotensive (NT) subjects and in 12 age and weight matched males with borderline hypertension (BHT). Measurements were performed before and after the i.v. administration of naloxone (0.15 mg/kg) and placebo in a randomized double-blind cross-over trial. In the pre-naloxone trials, pain led to similar changes in MAP, CVP, MSNA and plasma catecholamines in the two groups except for a higher increase in HR in the BHT group as compared to the NT group (3 +/- 1 vs. 1 +/- 1 bpm; P < 0.005). Opioid blockade with naloxone increased MSNA responses to pain in the NT group (from 5 +/- 1 to 9 +/- 1 bursts/min, and, from 100 +/- 23 to 204 +/- 36 units/min, respectively; P < 0.05) but did not significantly affect the MSNA response to pain in the BHT group. Pain induced responses of MAP, CVP, and catecholamines were not altered by naloxone in either group. Overall, there was a highly significant inverse correlation between pain perception and resting blood pressure which was not significantly affected by naloxone. The BHT subjects exhibited a lower pain perception compared to the NT subjects (P < 0.005). Naloxone increased pain rating in the NT group (from 194 +/- 9 to 218 +/- 13; P < 0.005) but not in the borderline hypertensive group (160 +/- 8 vs. 168 +/- 10; P = 0.36). Except for a decreased HR response in the BHT group, placebo had no effect on the responses to pain. Our data do not indicate a major role of the endogenous opioid system for the blood pressure-pain interaction in man. Endogenous opioids affect pain perception and sympathetic nerve activity responses to pain in normotensive men but their activity seems to be attenuated in borderline hypertensive subjects. Therefore, the lower pain sensitivity in human essential hypertension is probably mediated by non-opioid mechanisms.

Empirical contributions

To evaluate the possible negative association between risk for hypertension and pain sensitivity by in women 24 healthy young adult women with a parental history of hypertension and 24 without a parental history of hypertension participated in 2 laboratory sessions in which electric shock and the cold-pressor test were administered To assess the possible role of stress-induced analgesia in blood pressure-related hypoalgesia, the sessions were identical with the exception of the fact that participants were exposed to a stressful 20-min videogame before the pain stimuli on one occasion and a nonstressful control task on the other. Women with a parental history of hypertension and high blood pressure reactivity to the videogame displayed a reduced sensitivity to electric shock on both days, suggesting that risk for hypertension is associated with reduced sensitivity to at least some pain stimuli in women. Blood pressure reactivity to stress-associated baroreceptor stimulation was not implicated as a mediator of decreased pain perception. However, other analyses revealed an effect of family history on shock pain only among women who report relatively high anxiety, suggesting that other aspects of the stress response may be involved in this phenomenon.

Blood pressure, gender, and parental hypertension are factors in baseline and poststress pain sensitivity in normotensive adults

We studied 38 men and 36 women to learn whether a brief speech stressor reduced normotensive humans' thermal pain sensitivity, whether baseline and poststress pain threshold and tolerance varied with blood pressure (BP) and hemodynamic measures, and whether these relations differed by gender and parental hypertension (PH). PH-women with low-resting BPs had lower baseline pain tolerance than did all the other groups (ps <.05), and this group alone exhibited stress-induced analgesia (p = .008). In women, pre- and poststress pain tolerance varied directly with rest and stress BP (ps <.05).

Resting systolic blood pressure, parental history of hypertension, and sensitivity to noxious stimuli

Increased nociceptive thresholds have been reported in hypertensive rats and humans, and also in normotensive humans at risk for developing hypertension by virtue of elevated resting blood pressure levels or a family history of hypertension. Because risk for hypertension may be more accurately predicted by a combination of risk factors, the present study examined the pain sensitivity of young adult males as a function of resting systolic blood pressure and family history of hypertension. Cold pressor and forearm ischemia tasks were administered in a counterbalanced order. Pain was assessed using numerical rating scales of intensity and unpleasantness during the tasks and the McGill Pain Questionnaire immediately following the tasks. Individuals with a parental history of hypertension obtained significantly lower Pain Rating Index scores on the McGill Pain Questionnaire for both tasks, and also reported significantly lower pain on both numerical rating scales during the ischemic task. No significant relationship was observed between resting systolic blood pressure and measures of pain sensitivity.

Pain perception and cardiovascular responses in men with positive parental history for hypertension

Evidence suggests a reduced pain sensitivity in hypertensive individuals. This study sought to extend this work to normotensive individuals with hypertensive parents. Men with a positive (PH+) or negative (PH-) parental history for hypertension rated their pain every 15 s during a 90-s hand cold pressor test and for 90 s after the cold pressor test. Systolic (SBP) and diastolic (DBP) blood pressures and heart rate were measured throughout. After the cold pressor test, the men recalled their pain using the McGill Pain Questionnaire. PH+ men showed greater SBP and DBP responses to the cold pressor test. Although pain ratings during the cold pressor test did not differ between groups, posttest reported pain receded faster in the PH+ than in the PH- men. The PH+ men also reported less total pain on the McGill. These findings support the hypothesis that risk for hypertension may be associated with attenuated pain responses to nociceptive stimuli.

Hypertension-associated hypalgesia. Evidence in experimental animals and humans, pathophysiological mechanisms, and potential clinical consequences

A behavioral hypalgesia (increased response threshold to noxious stimuli) has been consistently, although not invariably, reported in spontaneous and experimental acute and chronic hypertension in the rat. Studies in human hypertension have also demonstrated a diminished perception of pain, assessed as pain thresholds or ratings. The sensitivity to painful stimuli correlated inversely with blood pressure levels, and this relationship extended into the normotensive range. Evidence in humans and rats points to a role of the baroreflex system in modulating nociception. In the rat, blood pressure-related antinociception may be due to attenuated transmission of noxious stimuli at the spinal level secondary to descending inhibitory influences that are projected from brain stem sites involved in cardiovascular regulation and that may depend on baroreceptor activation and/ or on a central "drive." Both endorphinergic and noradrenergic central neurons (the latter acting through postsynaptic alpha 2-receptors) have been shown to be involved, and other mediators probably also play a role. Functionally, blood pressure-related antinociception may represent an aspect of a more-complex coordinated adaptive response of the body to "stressful" situations. It is still uncertain whether in human essential hypertension hypalgesia is secondary to elevated blood pressure or whether both depend on some common mechanism. Studies on the effect of hypotensive treatment are too few to allow conclusions. According to one hypothesis, the reduction in pain perception caused by baroreceptor activation secondary to blood pressure elevation may represent a rewarding mechanism that may be reinforced with repeated stress and may be involved in the development of hypertension in some individuals. Hypertension-associated hypalgesia may have clinically relevant consequences, especially in silent myocardial ischemia and unrecognized myocardial infarction, both of which are more prevalent in hypertensive individuals.

Exteroceptive suppression of temporalis and masseter muscle activity is enhanced in offspring of hypertensives

Exteroceptive suppression of temporalis and masseter muscle activity was examined in young men with and without a parental history of hypertension. Recent clinical studies suggest that the second exteroceptive suppression period is attenuated in several chronic pain disorders and that this brainstem reflex may serve as a noninvasive index of endogenous pain control. In the present study, offspring of hypertensives exhibited a significant protraction of the late exteroceptive suppression period for both muscle sites, suggesting that the decreased pain sensitivity previously observed in individuals at risk for hypertension may be related to enhanced central pain modulation.

Parental history of hypertension and enhanced cardiovascular reactivity are associated with decreased pain ratings

Ischemic pain was examined in adult males with and without a parental history of hypertension. Blood pressure and heart rate were recorded during baseline, cold pressor, and ischemia. Repeated pain ratings were obtained during cold pressor and ischemia, and the McGill Pain Questionnaire was completed after each stressor. A median split was used to identify high and low mean arterial pressure and heart rate reactors to cold pressor. Parental history of hypertension, high heart rate reactivity, and high mean arterial pressure reactivity were each associated with significantly lower ischemic pain ratings on the McGill Pain Questionnaire, suggesting that risk for hypertension is associated with hypoalgesia in normotensives.

Effects of PRES baroreceptor stimulation on thermal and mechanical pain threshold in borderline hypertensives and normotensives

Prior studies have noted a pain relieving effect of baroreceptor stimulation and of higher tonic blood pressure in animals and humans. The present study used a new technique for the controlled, noninvasive stimulation of human carotid baroreceptors (PRES). PRES baroreceptor manipulation was delivered to both normotensive subjects (n = 11) and medication-free labile hypertensive subjects (n = 10) during both thermal and mechanical pain. Consistent with prior research, hypertensives had a higher threshold for thermal pain than did normotensives. PRES baroreceptor manipulation had no significant effect on thermal pain threshold for either group. For the mechanical pain model, the opposite results were obtained; group pain thresholds did not differ, but there was a significant PRES baroreceptor stimulation effect of increasing pain threshold for both groups. Results are discussed in terms of specific features of the stimuli, dampening of pain in hypertensives, and adaptation to pain.

Do endogenous opioids mediate the relationship between blood pressure and pain sensitivity in normotensives?

Elevated resting blood pressure is associated with decreased pain sensitivity in both animals and humans. Recent evidence suggests that this relationship may be mediated by endogenous opioid peptides in hypertensives, but the precise mechanism has not been investigated in normotensives. We examined the effect of opioid receptor blockade with naloxone on the relationship between resting blood pressure and pain sensitivity in normotensive humans. Sixteen young adults were given cold pressor and handgrip challenges after treatment with either naloxone or saline in a placebo-controlled, within-subject design. Multiple regression procedures indicated that resting systolic blood pressure was a significant predictor of cold pain ratings even after the effects of naloxone were statistically controlled. The interaction between systolic blood pressure and opioid blockade was non-significant. These data suggest that the relationship between resting blood pressure and pain sensitivity in normotensive humans is mediated, at least in part, by non-opioid mechanisms.

The effects of sodium loading on blood pressure and pain responses to the cold pressor test

Two cold pressor tests were administered to 18 healthy normotensive males on two occasions, after 2 weeks of dietary sodium loading and after 2 weeks of maintaining their normal diet. While the addition of an extra 10 g/day of sodium to the diet had no overall effect on resting cardiovascular activity, it produced significantly greater diastolic blood pressure and smaller heart rate responses to the pain stimuli. As well, consistent with the large animal literature indicating a relationship between high blood pressure and diminished pain sensitivity, pain ratings were found to be significantly lower during the sodium loaded testing session. The ratings were corroborated by evidence of lower skin conductance reactivity to the second cold pressor test in the sodium loaded condition. Information concerning an elevation of risk for hypertension, perhaps provided by baroreceptors, may lead to compensatory reactions with hypoalgesia as one effect.