European Journal of Cardiovascular Medicine

Chronic pelvic pain (CPP) is a complex and often debilitating condition that significantly impacts the quality of life of women worldwide. Defined as non-cyclic pain persisting for more than six months in the lower abdomen or pelvis, CPP affects approximately 15–20% of women during their reproductive years. It poses a considerable challenge to clinicians due to its multifactorial etiology, which may involve gynecological, urological, gastrointestinal, musculoskeletal, and psychological components.^1-3

The pathophysiology of chronic pelvic pain is not yet fully understood; however, recent research suggests that autonomic nervous system (ANS) dysregulation may play a critical role. One promising non-invasive tool for assessing autonomic function is Heart Rate Variability (HRV)—a measure of the variation in time intervals between consecutive heartbeats. HRV reflects the dynamic balance between sympathetic and parasympathetic activity and is increasingly used as a biomarker of stress, pain perception, and overall autonomic health.^4-7

Previous studies have indicated altered HRV patterns in various chronic pain conditions such as fibromyalgia, irritable bowel syndrome, and endometriosis.^8,9 However, there is a scarcity of research focusing specifically on HRV in women with chronic pelvic pain, particularly within the Indian population. Understanding HRV patterns in this group may offer insights into the autonomic alterations associated with CPP and help identify potential diagnostic or therapeutic targets.

In this context, the present study aims to assess heart rate variability in females with chronic pelvic pain and compare it with age-matched healthy controls. By evaluating both time domain and frequency domain parameters of HRV, this study seeks to explore potential autonomic imbalances in women suffering from CPP and contribute to the growing body of literature on its underlying mechanisms.

Study Design and Duration

This was a hospital-based, cross-sectional observational study conducted over a duration of 18 months at  the Department of Physiology in collaboration with the Gynaecology Outpatient Department (OPD) of Indira Gandhi Institute of Medical Sciences (IGIMS), Patna. The design was chosen to explore the differences in autonomic function, as measured by heart rate variability, between females with chronic pelvic pain and healthy controls at a single point in time.

Study Population

The study included a total of 200 female participants, divided equally into two groups. The study group comprised 100 females diagnosed with chronic pelvic pain, while the control group consisted of 100 healthy, age-matched women from the general community who did not report any history of pelvic pain or chronic illness. All participants were within the age range of 18 to 60 years.

Participants for the study group were selected from among patients attending the gynaecology OPD at IGIMS. Diagnosis of chronic pelvic pain was confirmed by a gynecologist based on clinical evaluation and patient history. Controls were recruited through community outreach programs and health awareness camps, ensuring they matched the study group in terms of age and general sociodemographic background.

Inclusion Criteria

Participants were included in the study based on the following criteria:

• Female sex, aged between 18 and 60 years.
• For the study group: confirmed diagnosis of chronic pelvic pain persisting for at least six months, not attributable to cyclical menstrual pain.
• For the control group: no history of pelvic pain, chronic disease, or use of medications affecting autonomic function.

Exclusion Criteria

Participants were excluded from either group if they met any of the following conditions:

• Pregnancy at the time of assessment.
• History of cardiovascular diseases, diabetes mellitus, thyroid disorders, or other systemic illnesses that may affect autonomic nervous system activity.
• Known psychiatric illness or current use of antidepressants, anxiolytics, or autonomic-modulating drugs.
• History of pelvic surgery within the past six months.
• Inability to comprehend instructions or cooperate with study procedures.

Assessment of Chronic Pelvic Pain

Evaluation and objectivation of chronic pelvic pain in the study group were conducted during their clinical visit to the Physiology Department and the gynaecology OPD. Pain assessment was performed using the Short-Form McGill Pain Questionnaire (SF-MPQ), a validated tool that includes sensory and affective descriptors of pain, along with a visual analog scale (VAS) for rating pain intensity. This helped in both qualifying and quantifying the subjective experience of pain among participants.

Measurement of Heart Rate Variability (HRV)

Heart rate variability, serving as an indicator of autonomic nervous system function, was measured using a standard, non-invasive digital recording system from AD Instruments (Australia) in the Department of Physiology. All participants were advised to avoid caffeine, nicotine, and vigorous physical activity for at least 12 hours prior to the assessment. Measurements were taken in a quiet, temperature-controlled room with the participant lying in a relaxed supine position.

A lead II electrocardiogram (ECG) was recorded for a continuous period of five minutes under resting conditions. The ECG signal was digitized and analyzed using LabChart software provided by AD Instruments. Both time domain parameters (such as SDNN: standard deviation of NN intervals, RMSSD: root mean square of successive differences) and frequency domain parameters (including low-frequency [LF], high-frequency [HF], and LF/HF ratio) were evaluated to assess sympathovagal balance.

Ethical Considerations

The study was conducted following ethical guidelines for human research. Approval was obtained from the Institutional Ethics Committee of IGIMS, Patna, before initiating the study. All participants were fully informed about the purpose, procedures, and potential risks of the study, and written informed consent was obtained prior to enrollment. Confidentiality and privacy of the participants were strictly maintained throughout the study.

Statistical Analysis

The collected data were compiled and analyzed using appropriate statistical software. Quantitative data were expressed as mean ± standard deviation (SD). Comparison between the study and control groups was performed using the two-tailed Student’s t-test for independent samples. A p-value of less than 0.05 was considered statistically significant. If data did not meet the assumptions of normality, alternative non-parametric tests were considered. Additional statistical techniques were applied as needed based on the distribution and pattern of the data to ensure accurate interpretation of results.

Table 1 presents the demographic and clinical characteristics of the study participants. The mean age and body mass index (BMI) were comparable between the chronic pelvic pain (CPP) group and the control group, with no statistically significant differences observed (p = 0.58 and p = 0.12, respectively). Marital status, educational attainment, and employment status were also similar across groups. Notably, the mean duration of pain in the CPP group was 18.4 ± 9.6 months. A significantly higher prevalence of associated conditions such as endometriosis (32%), irritable bowel syndrome (25%), and urinary symptoms (18%) was observed in the CPP group, with all comparisons yielding highly significant p-values (p < 0.001), confirming a greater comorbidity burden in affected individuals.

Table 1: Demographic and Clinical Characteristics of Study Participants

Table 2 summarizes the findings from the Short-Form McGill Pain Questionnaire (SF-MPQ) and related pain measures. As expected, the CPP group demonstrated significantly higher scores across all pain domains, including the Visual Analog Scale (VAS), sensory, affective, and total SF-MPQ scores (all p < 0.001). Pain was reported as daily in 72% of CPP subjects, with a mean pain duration of 5.4 ± 2.8 hours per day. Additionally, 58% of participants in the CPP group reported regular use of analgesics, while none of the controls reported any pain, underscoring the severity and chronicity of pain in the affected group.

Table 2: Pain Assessment Using Short-Form McGill Pain Questionnaire (SF-MPQ) and Additional Pain Metrics

Table 3 illustrates the time domain heart rate variability (HRV) parameters. Compared to controls, the CPP group had a significantly higher mean heart rate (78.5 ± 8.6 bpm vs. 72.3 ± 7.1 bpm, p < 0.001) and showed reduced HRV as indicated by lower SDNN (28.4 ± 9.3 ms vs. 42.7 ± 10.5 ms, p < 0.001), RMSSD (22.1 ± 7.8 ms vs. 35.6 ± 9.2 ms, p < 0.001), pNN50 (6.8% vs. 15.4%, p < 0.001), and HRV triangular index (7.5 ± 2.9 vs. 11.2 ± 3.5, p < 0.001). These findings suggest a marked reduction in parasympathetic activity and overall autonomic flexibility in women with chronic pelvic pain.

Table 3: Time Domain HRV Parameters with Additional Metrics

Table 4 shows frequency domain HRV parameters. The CPP group had significantly lower total power (980 ± 290 ms² vs. 1420 ± 350 ms², p < 0.001), VLF power, and HF power, indicating decreased overall autonomic activity and vagal tone. Interestingly, LF power in absolute terms was slightly higher in the CPP group (p = 0.002), and LF normalized units (nu) were markedly elevated (60.5 ± 8.7 vs. 42.3 ± 7.9, p < 0.001), while HF normalized units were significantly reduced (39.5 ± 8.7 vs. 57.7 ± 7.9, p < 0.001). The LF/HF ratio, a marker of sympathovagal balance, was significantly increased in the CPP group (1.65 ± 0.55 vs. 0.78 ± 0.32, p < 0.001), indicating autonomic imbalance with relative sympathetic dominance.

Table 4: Frequency Domain HRV Parameters with Normalized Units

Table 5 explores HRV differences within the CPP group based on pain severity and the presence of endometriosis. Participants with severe pain (VAS > 6) exhibited significantly lower SDNN and RMSSD values, higher LF power, and reduced HF power, compared to those with mild or moderate pain (VAS ≤ 6), with all differences reaching statistical significance. The LF/HF ratio was also higher in the severe pain group (1.85 ± 0.56 vs. 1.38 ± 0.48, p < 0.001), indicating greater autonomic dysregulation. Similarly, participants with endometriosis had significantly lower RMSSD and HF power, and higher LF/HF ratios compared to those without endometriosis, suggesting more pronounced sympathetic overactivity and vagal withdrawal in this subgroup.

Table 5: HRV Parameters by Pain Severity and Associated Conditions (CPP Group Only)

Table 6 presents the correlation analysis between pain intensity, duration, HRV parameters, and selected clinical variables within the CPP group. There was a significant negative correlation between VAS and total SF-MPQ scores with key HRV measures including SDNN, RMSSD, and HF power (all p < 0.001), indicating that greater pain intensity is associated with reduced parasympathetic activity. Conversely, LF power and the LF/HF ratio showed positive correlations with pain scores, highlighting a sympathetic shift in individuals with more severe pain. Duration of daily pain also correlated similarly with HRV metrics. BMI showed weak and mostly non-significant correlations, suggesting that autonomic changes were more strongly associated with pain parameters than body composition.

Table 6: Correlations Between Pain Scores, HRV Parameters, and Clinical Variables (CPP Group Only)

The present study aimed to investigate autonomic nervous system function through heart rate variability (HRV) analysis in women with chronic pelvic pain (CPP) and compare the findings with age-matched healthy controls. The results demonstrate a significant alteration in HRV parameters among women with CPP, indicating autonomic dysregulation characterized predominantly by decreased parasympathetic activity and relative sympathetic dominance. These findings are consistent with the hypothesis that autonomic nervous system imbalance plays a critical role in the pathophysiology of chronic pain disorders, including CPP.

The demographic data confirmed that the CPP and control groups were well matched in terms of age, body mass index (BMI), marital status, educational level, and employment status. However, a significantly higher prevalence of comorbid conditions such as endometriosis, irritable bowel syndrome (IBS), and urinary symptoms was observed in the CPP group. These findings echo previous reports suggesting that CPP is frequently associated with a spectrum of chronic overlapping pain conditions (COPCs), all of which are known to involve central and autonomic dysregulation.^4,6,9

Pain assessment using the Short-Form McGill Pain Questionnaire (SF-MPQ) revealed significantly higher sensory, affective, and total pain scores in the CPP group. Most participants reported daily pain and regular analgesic use, highlighting the chronic and disabling nature of the condition. These results are in line with those of past studies, who reported elevated pain scores and reduced functional quality of life in CPP patients, emphasizing the need for multidimensional pain assessment.^7,10,11

In terms of HRV analysis, our findings revealed significantly reduced time domain indices such as SDNN, RMSSD, and pNN50 in the CPP group. These indices are well-established markers of parasympathetic activity, and their reduction is suggestive of vagal withdrawal and reduced heart rate adaptability. These results closely align with findings from studies on other chronic pain syndromes.^5-7

Frequency domain analysis further corroborated these findings. While total power, very low frequency (VLF), and high-frequency (HF) components were significantly lower in the CPP group, low-frequency (LF) power in normalized units and the LF/HF ratio were markedly higher. The LF/HF ratio, commonly used to estimate the balance between sympathetic and parasympathetic inputs, was almost twice as high in the CPP group compared to controls, indicating a pronounced shift toward sympathetic predominance. This pattern is consistent with the findings of previous studies who reported elevated LF/HF ratios in women with endometriosis-related pelvic pain, and in chronic visceral pain conditions such as IBS.^12-15

Furthermore, intra-group comparisons within the CPP cohort revealed that patients with severe pain (VAS > 6) exhibited significantly lower HRV parameters compared to those with mild-to-moderate pain. This suggests a dose-response relationship between pain intensity and autonomic dysfunction. These results support earlier work, who observed that individuals with higher subjective pain ratings displayed lower HRV, independent of age and comorbid conditions.^15-17

Interestingly, women with endometriosis in our study had significantly lower RMSSD and HF power, and a higher LF/HF ratio than those without endometriosis. This may suggest that endometriosis, beyond being a source of nociceptive input, might also exacerbate autonomic imbalance. These findings are supported by past research, who demonstrated that endometriosis is associated with heightened stress response, anxiety, and decreased vagal tone, potentially compounding the autonomic dysregulation seen in chronic pelvic pain.^5,10,11

The correlation analysis further strengthened the association between pain and autonomic dysfunction. Negative correlations were found between HRV parameters (SDNN, RMSSD, HF power) and pain metrics (VAS, SF-MPQ scores, and pain duration per day), while LF power and LF/HF ratio positively correlated with these pain measures. These relationships confirm that reduced vagal activity and increased sympathetic modulation are closely linked with greater pain perception and longer daily pain duration. This mirrors the conclusions of past studies, who noted that low parasympathetic tone may not only reflect pain-related stress but could also impair endogenous pain modulation mechanisms.^16-18

Despite the robustness of the findings, this study has some limitations. First, the cross-sectional design precludes establishing causality between HRV changes and CPP. Longitudinal studies are needed to determine whether autonomic dysfunction precedes the onset of chronic pain or emerges as a consequence. Second, although efforts were made to exclude confounding factors, the presence of comorbid conditions like endometriosis and IBS in some CPP participants could have influenced HRV independently. Future studies with larger sample sizes and stratified analyses are recommended. Additionally, psychological variables such as depression, anxiety, and sleep disturbances, which are known to affect HRV, were not formally assessed in this study.

Nonetheless, the strengths of the present study include a well-characterized clinical sample, the use of validated tools for pain assessment, and comprehensive HRV analysis using both time and frequency domain parameters. Importantly, this study contributes valuable data from an Indian population, addressing a gap in the literature and offering culturally relevant insights into the autonomic underpinnings of CPP.

In conclusion, this study demonstrates significant autonomic dysfunction in women with chronic pelvic pain, characterized by reduced parasympathetic activity and relative sympathetic dominance, as evidenced by both time and frequency domain parameters of heart rate variability. These alterations were more pronounced in individuals with severe pain and in those with comorbid conditions such as endometriosis, suggesting a strong link between autonomic imbalance and pain severity. The findings underscore the potential of HRV as a non-invasive biomarker for assessing autonomic involvement in chronic pelvic pain and open new avenues for integrative, neurophysiologically informed approaches to diagnosis and management. Further longitudinal and interventional studies are warranted to elucidate causality and to explore whether modulation of autonomic function can serve as a therapeutic target in this challenging and multifactorial condition.

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