Sustained Improvement of Heart Rate Variability
in Patients Undergoing a Program of Chiropractic Care:
A Retrospective Case SeriesThis section is compiled by Frank M. Painter, D.C.
Send all comments or additions to: Frankp@chiro.org
FROM: Chiropractic Journal of Australia 2018; 45 (4): 338–358 ~ FULL TEXT
OPEN ACCESS Amy Louise Haas, PhD, DC, David Russell, BSc (Psych), BSc (Chiro), Cert TT
Private Practice,
Nashua, NH, USAObjective: The purpose of this study was to report the sustained changes in heart rate variability (HRV) observed in 6 patients undergoing continuous chiropractic care for the correction of vertebral subluxations.
Clinical Features: Six patients between 25 to 55 years of age all presented with primarily musculoskeletal complaints for chiropractic care in a private practice setting. All patients were nonsmokers with no reported cardiac pathology. All patients were initially assessed for indicators of vertebral subluxation before being accepted for chiropractic care, and were monitored for changes in HRV scores over time.
Intervention and Outcomes: Chiropractic care, using Diversified and Thompson techniques to correct vertebral subluxations, was provided for an initial period of 10 to 52 weeks at a frequency of 2 to 3 visits per week. HRV, measured by SSDN, increased over the early part of their course of chiropractic care, and these increases were sustained whilst the patient remained under long term continuous care in all 6 patients. Improvements in SDNN ranged from 50% to greater than 300% as compared to pre-care values.
Conclusion: Patients receiving continuous chiropractic care to correct vertebral subluxation demonstrated a sustained improvement in heart rate variability (HRV). This novel finding objectively demonstrates long-term change consistent with improved neurophysiological regulation, adaptability and resilience in patients undergoing chiropractic care, and suggests the utility of chiropractic care for outcomes greater than only musculoskeletal improvements.
Key Indexing Terms: Chiropractic; Heart Rate Variability
Introduction
The primary objective of chiropractic care is to optimize health and wellbeing through the enhancement of nervous system function by reducing nerve interference caused by vertebral subluxations. [1–3] The Australian Spinal Research Foundation defines vertebral subluxation as “a diminished state of being, comprising of a state of reduced coherence, altered biomechanical function, altered neurological function and altered adaptability.” [4] A vertebral subluxation has been recognised as a complex of functional and/or structural changes in the articulations of the spine and pelvis that compromise neural integrity and may influence organ system function and general health. [5] The correction of vertebral subluxations is achieved through chiropractic adjustments that are a typically manually performed. [1, 2, 6] Research over the past 2 decades has shown that the chiropractic adjustment (also referred to as chiropractic spinal manipulation in the chiropractic research literature) results in changes in spinal biomechanics and structure [7–11], central nervous system function [12–16], motor output [17–20], and autonomic output. [21, 22]
Using objective measures of spinal and neurological function provides the means to quantitatively observe the effects of the chiropractic adjustment. Objective measures used in chiropractic clinical practice to identify the site of intervention and/or to measure outcomes often include both musculoskeletal assessments such as pre- and post-adjustment x-ray, leg length inequality, posture or gait changes, sEMG, algometry, range of motion, motion and static palpation [23], and non-musculoskeletal metrics such as paraspinal thermal balance, heart rate, blood pressure, respiration, reaction time, head repositioning sense, and balance testing. [23, 24] While each measure represents a unique and targeted view on structural and/or physiological changes associated with the chiropractic adjustment, employing alternative outcomes assessment technology allows for the opportunity to expand our understanding of the effects of the adjustment in a way that reflects global changes in autonomic nervous system function and adaptability, and also interfaces with multiple healthcare disciplines. [25]
Measurement of heart rate variability (HRV) has been documented as an effective method to objectively measure improvement in nervous system function. Originally conceived as an assessment tool for cardiac physiology [26], HRV reflects the influence of the Vagus nerve and the sympathetic nervous system on intrinsic heart rhythm [25–27] and therefore HRV monitoring represents a unique window into autonomic nervous system function.
Studies of HRV in the past 2 decades have established that decreased HRV, or "vagal tone", correlates and/or predicts pathological conditions such ascardiovascular disease [26–28],
inflammation [29–31],
diabetic neuropathy [32],
emotional dysregulation and post-traumatic stress disorder [33–37],
sleep disorders [38], and
cancer. [38–41]Conversely, high HRV has been associated with healthy longevity [42–44], and is used by elite athletes to predict their ability to function optimally in an upcoming workout. [45]
According to McCraty et al [25], an optimal level of HRV within an organism reflects healthy function and an inherent self-regulatory capacity, adaptability or resilience. Too little variation indicates age-related system depletion, chronic stress, pathology or inadequate functioning in various levels of self-regulatory control systems. McCraty’s assessment echoes the ideas of Hans Selye, the Nobel laureate who described the General Adaptation Syndrome [46], the body’s mal-adaptation to sustained emotional, physical or chemical stressors. HRV is increasingly emerging as a way to assess and predict multiple health outcomes, from disease to thriving health. [25] Accordingly, interventions that increase HRV "vagal tone" would be expected to improve health outcomes and promote vitality and wellness.
The chiropractic profession, as well as the professions of acupuncture and osteopathy, have increasingly used HRV for research purposes [47–58]; however, none of these studies have approached the effect of long-term chiropractic care on HRV. The current study chronicles the improvements in HRV in 6 patients undergoing long-term continuous chiropractic care (of 3 months to 3 years’ duration) for the correction of vertebral subluxation.
Case Series
This retrospective case series compares recorded baseline and ongoing progress HRV measurements of 6 patients following a program of chiropractic care for the correction of vertebral subluxation. The patients (3 female and 3 male) were 25 to 55 years of age, nonsmokers with no cardiac, hypertensive, or frank pathological conditions reported. As per criteria recommended by Nunan et al [59], patients whose data were included for the study reported no use of of statins, beta blockers, drugs that affect baroreceptor activity, neuroactive drugs, or SSRIs. Patient data were excluded if the patient reported unusual physical, chemical, or emotional stress in the day preceding testing.
Assessment for Vertebral Subluxation Complex
All patients were assessed for eligibility for chiropractic care using multiple objective methods to identify the locations and components of vertebral subluxation [23, 25], including posture or movement asymmetry, supine or prone leg length inequality, tissue texture/tonal/temperature changes or tenderness, pain or sensation changes, static and motion palpation findings, sEMG and paraspinal thermography, x-ray analysis of weightbearing spinal integrity and structure [7–11], biotensegrity-based posture and movement assessment [60], cerebellar testing including Rhomberg’s and/or Mittelmeyer’s test, proprioceptive/balance deficit and reaction time via the BESS (Balance Error Scoring System) based mobile SWAY® app [61, 62] (SWAY Medical, LLC, Tulsa, OK).
Outcome Measure: HRV
HRV data were collected using Pulse Wave ProfilerTM (PWP) instrument. Patients were assessed for baseline HRV at their initial appointment, prior to the start of care, and reassessed every 12 visits for 3–months, at 6–months, and every 6–months thereafter throughout their program of chiropractic care. HRV test-test reliability was confirmed by recording a set of 3 consecutive HRV measurements on non-adjusted control patients. For each HRV assessment during the course of a patient’s care program, the following protocol was followed. Patients were seated comfortably in a closed room. No breathing instructions were given, and patients were instructed to refrain from movement, conversation, or using digital devices. The left hand was used for data collection, which consisted of 1 minute of resting measurement followed by 5 minutes of data collection. After data collection, the resultant heart rate graph was visually inspected and SDNN data were accepted unless anomalies such as random spikes or missed data capture were noted (less than 10% of samples). Results for each case were compared to normal HRV scores for 5’ SDNN measurements per age and gender as per Voss et al [63], though conflicting reports suggest that normative 5’ SDNN values may be higher [44, 59] or lower. [55]
Chiropractic Management
Initial chiropractic care programs ranged from 2 to 3 visits weekly for 10 to 52 weeks based on patient presentation. [64, 65] Visit frequency transitioned to weekly after improvement or normalization of objective testing as well as posture and movement patterns. Exercises to facilitate neuromuscular re-education were selected per patient and included Pettibon Cervical TractionTM, use of a Pettibon Therapeutic Wobble ChairTM, home use of Pettibon HeadweightsTM or a Chiropractic BioPhysics® Denneroll, “pointer” spinal stabilizing exercise, and isometric head retraction exercises against a wall or car headrest.
All patients were managed using Diversified and Thompson Terminal Point techniques. Chiropractic adjustments were primarily manual full-spine or drop-assisted, with some instrument-assisted adjusting using an ActivatorTM instrument. Diversified is the most widely used chiropractic technique and system of adjusting that uses primarily motion and static palpation to locate levels of vertebral subluxation, and focuses on the restoration of proper biomechanics within the spine [66] and improved nervous system function. The Thompson Terminal Point Technique is a full-spine analysis and adjusting technique that utilizes a drop table to assist in the delivery of high-velocity, low amplitude chiropractic adjustments. [67] Pre- and post-adjustment assessments of each level of vertebral subluxation were noted, as well as subjective statements made by the patient. The force administered during a chiropractic adjustment was modified individually to a patient’s size, frame and spinal integrity.
Patient Responses to Chiropractic CareCase 1 A 52–year-old female presented with a chief complaint of pain in her right lower back and a concern with lack of progress in recovering from multiple pneumonias. Her baseline HRV recording indicated an SDNN of 21.6 msec, below the normal SDNN of 36.9 ± 13.8 msec reported for her gender and age. [63] An initial program of chiropractic care consisted of 3 visits a week for 10–weeks, and progressed to 2 scheduled visits per week when muscle activity and balance improved as measured by sEMG. Subjective outcomes included improvement of lung and low back pain, improved immune system function as reflected by reported reduced frequency of respiratory infections, and improvement of athletic endurance.
Objective radiographic improvements, [64, 65] after 4–months of care, included reduction of forward head posture from 2.1 cm to 0.7 cm, a 66.7% improvement, as well as improvement of C2–C7 sagittal curve (from 30° to 38.7° as measured by X-ray, representing a 22.5% improvement) and atlas plane to horizontal (from 22.8° to 29.9°, representing a 31.1% improvement). Her baseline SDNN of 21.6 msec improved to 125.2 msec after 6–weeks of care (a 479.6% increase from initial testing) and to 143.2 msec after 9–months of care (a 563% increase from her initial testing). After 21–months of care, her SDNN settled to 87.5 msec, an increase of 305.1%, and after another 13–months of weekly visits, her SDNN measured 88.9 msec (an increase of 311.6%) while also dealing with increased work and family stressors.
Case 2 A 43–year-old, athletic female presented with a chief complaint of right arm/elbow pain that inhibited her from lifting heavy weights in the gym, and goals of being active and fit. Her baseline HRV recording indicated an SDNN of 86.2 msec, above the normal of 45.4 ± 20.5 msec for her gender and age [63] but consistent with an SDNN for an aerobically-trained athlete. [69–71]
An initial program of chiropractic care consisted of 2 visits per week for 52–weeks. Outcomes of care included elimination of arm and elbow pain with the first 12 visits, improved muscle balance and activity as measured by sEMG after 8–months of care, reported improved weight lifting and athletic performance through the course of her care, and improved measured sagittal cervical spine structure (atlas plane to horizontal improved from 24° to 32.8°) after 18–months of care. [64, 65] HRV improved after 2–months of care to an SDNN of 152.4 msec, and after 15–months her SDNN was 143.6 msec. The patient continued ongoing care at a frequency of weekly visits. Her HRV recordings showed an SDNN of 142.5 after 28 continuous months of care, a 72.5% improvement from her initial pre-care HRV measurement.
Case 3 A 31–year-old, athletic male presented with a chief complaint of left upper back and shoulder pain, and goals of increased fitness, strength, and endurance. His baseline HRV recording indicated an SDNN of 69.6 msec, within the reported normal SDNN of 50.0 ± 20.9 msec for his gender and age. [63]
An initial program of chiropractic care consisted of 28 visits over 3–months, and progressed to 2 scheduled visits per week when muscle activity and balance improved as measured by sEMG, and once per week when the patient’s symptoms fully abated, at which time he terminated care. His baseline SDNN of 69.6 msec improved over 8–months to a sustained measurement of greater than 116 msec, a 69.4% improvement over his initial HRV measurement.
Case 4 A 48–year-old male presented with occasional headaches, poor sleep patterns, fatigue, and right shoulder pain, with goals of increased cardiovascular fitness. His baseline HRV recording indicated an SDNN of 24.3 msec, at the low end of the normal SDNN of 36.8 ± 14.6 msec reported for his gender and age. [63]
An initial program of chiropractic care consisted of 3 visits a week for 12–weeks, and was ongoing at the time of manuscript preparation. Subjective outcomes include improvement of sleep and decreased fatigue, and elimination of shoulder pain. Objective outcomes include increased cervical range of motion and improved overall posture as assessed by visual observation, as well as an improvement of reaction time as measured by SWAY® testing from a baseline of 427 ms before care to 309 ms after 3–months of continuous care. Normal reaction time for a patient of this age is 280 ± 40 msec. [61] His baseline SDNN of 24.3 msec improved to 30.9 msec within the first month of care, with a further improvement to 90.2 msec after 3–months of care, an improvement sustained at 4.5 months of care with an SDNN of 88.0 representing a 262.1% increase in HRV as compared to his pre-care score.
Case 5 A 25–year-old female referred by her gastroenterologist presented for chiropractic care seeking improvement of lower back pain, gastrointestinal dysfunction, and anxiety. Her baseline HRV recording indicated an SDNN of 50.6 msec, within the reported normal range of 48.7 ± 19.0 msec for her gender and age. [63]
An initial program of chiropractic care consisted of 3 visits a week for 12–weeks, and was ongoing at the time of manuscript preparation. Subjective outcomes after 3–months of care include resolution of anxiety, improvement of elimination function from 4 times per week to twice daily, and reduction of low back pain. Objective outcomes include improvement of muscle activity and balance as measured by sEMG, and improvement in single leg raise balance as measured by SWAY® (baseline proprietary SWAY score 78.2/100 before care to 92.7/100 after 3–months of care. [62] Her pre-care HRV baseline SDNN of 50.6 msec improved to 62 msec after one month of care, and to 110.6 after 2–months of care, and slightly diminished to 75.9 msec after 3–months of care (a sustained 50% increase over her pre-care score), possibly due to reported life stressors.
Case 6 A 42–year-old male presented with right neck and shoulder pain, with goals of maintaining strength and increasing flexibility. His baseline HRV recording indicated an SDNN of 59.3 msec, within the reported normal range of 44.6 ± 16.8 msec for his age and gender. [63]
An initial program of chiropractic care consisted of 3 visits per week for 12– weeks, and was ongoing at the time of manuscript preparation. Subjective outcomes include reduced neck and shoulder pain, improved athletic performance, and improvement of gait and flexibility. Objective improvements include an improvement of proprietary SWAY balance score from 60.1/100 before care to 95.2/100 after 3–months of care. [65] His baseline SDNN of 59.3 msec increased to 93.6 msec after 1–month of care, and was maintained at 95.6 msec after 3–months of care, a 61.2% increase from his pre-care score.
This work was approved by the IRB of the Foundation for Vertebral Subluxation.
Results
Figure 1
Figure 2
Figure 3
Data from unadjusted patients were collected to confirm protocol test-test reliability. While the individual baseline SDNN differed between individuals as predicted by Pinna’s HRV reliability analysis [68], the HRV trials for each unadjusted individual in Figure 1 demonstrate good test-test reliability, with standard deviations of less than 10%. The test-test reliability observed indicates that changes observed in Figures 2 and 3 are due to physiological change rather than random chance. Previous studies have demonstrated that HRV is not affected in non-adjusted controls and is not affected by the placebo effect. [55]
Figure 2 shows 6 retrospective case studies of patients whose baseline SDNN measured lower at their initial intake, prior to administration of care, and progressively increased over the course of their care, with a sustained improvement. HRV assessment for these data points was performed prior to any adjustment performed during that day's visit. Trend lines on each graph represent the progressive increase of baseline HRV in these patients over time, which is indicated on the X-axis in number of months.
Figure 3 contains comparative PWP representations of autonomic activity and balance for the 6 case studies, prior to initiation of chiropractic care and at their most recent assessment.
Discussion
This case series chronicles HRV changes of 6 adult patients receiving chiropractic care using Diversified and Thompson adjusting techniques for the correction of vertebral subluxation. The data described here are consistent with immediate and long-lasting neurophysiological changes effected by chiropractic case management. While the absolute values of baseline and post-care HRV differed between individuals, each individual’s outcome improved to values more consistent with younger or athletic individuals [69–71], and the improvements in HRV of 2 of these individuals are consistent with a transition from predicted worse health outcomes to greatly improved health outcomes [27].
One concern central to the use of HRV in clinical outcomes data is that HRV data is reported using many different units of measure as time domain versus frequency domain, power spectrum, high-frequency, low-frequency, very-lowfrequency, and ratios of these various units, which are intended to represent balance and coherence within the autonomic nervous system. Of the data analysis methods available, SDNN is thought to reflect both parasympathetic and sympathetic activity and to provide an index of total HRV [27]. Multiple studies have shown that of the data set derived from HRV measurement, SDNN has stronger reliability [68] and decreases linearly with age [44, 55], and therefore is simplest and most useful for cross-study comparison. Further, age normalized standards for SDNN obtained using 5–minute sampling times are readily available from multiple sources [59, 63] making SDNN data readily comparable across healthcare disciplines. Therefore, while all HRV metrics are reported in appendix 1, this study focuses on changes in SDNN in particular.
Pinna’s review of reliability of HRV measurements states that “short-term HRV measures are subject to large day-to-day variations…[and that] differences between individuals mostly reflect differences in the subject’s error-free values rather than random error”. [68] Intra-subject variability is also due to an intrinsic lability of HRV parameters, probably because they are under the influence of such factors as mood, alertness and mental activity, which are very difficult to control for in any study, as well as changes associated with frequency and depth of respiration. [68] Intrinsic intra-individual HRV variation complicates crossgroup comparison and may indeed underlie the variation and wide standard deviation in SDNN ranges reported. [43, 45, 56, 60, 63] Since individual physiology can vary according to the effect of various stressors on dynamic physiology [37, 45], the magnitude of response to various interventions, including chiropractic adjustment, may vary per individual per day. Therefore, we hypothesized that while any individual data point may not hold conclusive data, a collection of data points over time may show a trend indicative of HRV change. Furthermore, since changes in HRV 5 minute readings after interventions such as exercise are known to be transient [45], changes observed pre- versus post-adjustment cannot be interpreted to signify that the individual adjustments performed confer permanent physiological change. The sustained nature of the HRV improvements reported in these 6 cases may therefore reflect neuroplastic changes associated with long-term chiropractic care.
Sustained improvement of HRV over a course of chiropractic care may have implications for prediction of multiple health outcomes. For cardiac cases, data from 24–hour sampling times demonstrate a 5.3 times higher (34%) mortality for individuals with abnormally low SDNN. [29] Strikingly, in 1 report, patients with moderate 24–hour sampling time SDNN values (50–100 msec) have a 400% lower risk of mortality than those with low values (0–50 msec). [25] For the purposes of comparison of SDNN obtained under the conditions used in the current study, Bilchick et al [27] suggest a "cutoff" of 30 msec for 5–minute sampling-time SDNN for separation of better or worse health outcomes in cardiac cases. It is of interest that cases 3 and 4 in the current case series show a preadjustment SDNN value close to 30 msec and a post-adjustment SDNN value greatly increased past this "cutoff." Abnormally low SDNN has implications for non-cardiac health outcomes as well: low HRV has been shown to be predictive of worsened prognosis in cancer cases [39, 40], and conversely, higher HRV may increase resilience to cancer. [40] The case studies presented here are consistent with immediate and lasting improvement in health outcomes, and these data suggest the possibility that sustained improvement of HRV after a course of chiropractic care may have positive implications for the body’s ability to overcome pathologies such as cancer and cardiovascular disease.
Several of the post-care SDNN values in these case studies described herein are above the normative values established in the literature. [59, 63] Though clear lower limits have been established for 5’ HRV measurements, no clear upper limits have been identified as yet, therefore the significance of these findings is unclear. One possible explanation may be extrapolated from Stein et al’s study on 24–hour Holter monitoring of older adults, which suggested that some values of HRV may be mildly exaggerated by erratic rhythms found more prevalently in this age group. However, SDNN in particular was less affected than the other HRV metrics reported. [72] Without the ability or expertise to evaluate the individual PQRST waveforms used to generate the data in these case studies, the authors cannot exclude the possibility of an undetected erratic rhythm in cases 1 and 3 in this study generating an artificially elevated SDNN. Alternatively, because SDNN values used as comparable normative data per age and gender for the 6 case studies were generated from a “normal” population [59, 63], it is possible that a higher reference range would be identifiable for highly functional and healthy individuals, and that the SDNN values observed for cases 1 and 3 may simply be reflective of physiology improved above what is expected from a “normal” population. Indeed, several studies have shown that trained aerobic athletes have a higher baseline SDNN than their untrained counterparts. Aubert et al [69] reported elevated 10’ SDNN values of 97.9 ± 15.7 msec for aerobic trained athletes as compared to 65.4 ± 38.9 for sedentary individuals; similarly, Martinelli et al reported elevated 5’ SDNN values of 89.9 ± 24.8 msec for endurance-trained cyclists as compared to 59.1 ± 36.5 for their untrained counterparts [70], and for young (ages 18–25) individuals, Corrales et al report elevated 5’ SDNN values for male athletes of 101.2 ± 37.4 msec, and for female athletes of 106.6 ± 38.1 msec, significantly higher than for their untrained male (83.1 ± 31.7) or female (71.8 ± 24.5) counterparts. [71] The observation of significantly higher reference ranges for aerobically trained athletes suggests the possibility that the unexplained higher SDNN values reported for cases 1 and 3 could be consistent with improved cardiovascular physiology, as is found in trained athletes. Further study will be necessary to explore this possibility.
Improvement of HRV may reflect a means by which the chiropractic adjustment affects human physiology and is not exclusive of other potential physiological effects of the chiropractic adjustment as theorized by Kent [73] and Pickar [74], or Ingber [75]. Rather, improvement of HRV may represent readout of the physical and physiological effects that is related to and interconnected with these theories and others.
While the neurophysiological basis for the effect of the chiropractic adjustment on HRV remains to be further elucidated, recent research suggests several plausible mechanisms by which incoming sensory information from joints in the spine, especially of the head and neck, may affect cardiac regulation. [76] The fastigial nucleus (FN), an evolutionarily-conserved structure, receives input from spinocerebellar tracts, in particular somatosensory information from the spinal joints of the head and upper body. [77] Projections have been identified reaching from the FN of the cerebellum to several different structures that may affect cardiac regulation, including the amygdala, the hypothalamus, and medullary nuclei including the cardiovascular centre. [78] Further, the FN sends projections to the nucleus tractus solitarius (NTS), which contains the intermedius nucleus of the medulla (InM). The InM contains neurochemically diverse neurons and sends both excitatory and inhibitory projections to the NTS. [79, 80]. These data provide a novel pathway that may underlie possible reflex changes in autonomic variables after neck muscle spindle afferent activation. Incoming information from either the spinocerebellar tracts or directly from cervical spine afferents may therefore relay somatosensory input through the FN, the NTS, and the InM, both of which may in turn affect sympathetic as well as cardiac control centers.
Alternatively, input created via a chiropractic adjustment relayed through the FN to the amygdala could influence the integration of signals from inside and outside the body, thus affecting the body’s adaptive capacity. Thayer’s neurovisceral integration model [81] holds that a core set of neural structures provides an organism with the ability to integrate signals from inside and out the body and adaptively regulate cognition, perception, action, and physiology. Thayer’s recent meta-analysis of fMRI and PET data has demonstrated association of activation of the the amygdala and the prefrontal cortex with changes in HRV. [81] Consistent with this hypothesis, Lelic et al. [82] have demonstrated that manipulation of dysfunctional spinal joints affects sensorimotor integration in the prefrontal cortex. Therefore, reduced or abnormal vertebral motion or position, intersegmentally or globally, could result in alteration of somatosensory input through the spinocerebellar tracts to structures that influence neurovisceral integration, as measured by HRV. Much research will be needed to test this hypothesis; however, the existence of neural pathways leading from the spine through the brain to the modulators of cardiac activity is a promising start.
Thayer’s neurovisceral integration model is reminiscent of modulation of physiology via a central pattern generator (CPG), a cooperative set of neurons that generate rhythmic patterns such as gait, breathing, and swallowing. [83] A particular property of a CPG-modulated system is that sensory input to CPGs leads to adaptive changes [84], such a pebble in a shoe will lead to a limping gait pattern. A third possible underlying neurophysiological mechanism by which chiropractic care may affect HRV is that changes in sensory input as generated by a chiropractic adjustment, when perceived by any of the neural apparati discussed above, may affect a CPG that modulates heart rate. If this is the case, changes in HRV with chiropractic care could be viewed as evidence of adaptive changes executed by a CPG in response to sensory input. Consistent with this possibility, Senzon et al have demonstrated that sensory input generated by Network Spinal Analysis care results in generation of rhythmic muscle contraction that when measured by sEMG shows mathematical properties of CPG-modulated activity. [85] Indeed, if vertebral subluxation was to create afferent sensory changes delivered to neural structures that either comprise or influence a CPG, reduction or resolution of vertebral subluxation by the chiropractic adjustment may represent a means by which adaptive capacity may be modulated. Much further study will be necessary to explore this intriguing possibility.
The 6 case studies presented here contradict multiple studies that have clearly established a linear decline in SDNN with age. [44, 55, 63] The observed improvement in SDNN in the 6 cases presented suggests the possibility that the cumulative effect of regular chiropractic care may reverse a diminished HRV, and indeed, may be protective against the predicted age-related decline in HRV. Further longitudinal study will be necessary to confirm this observation and to explore the physiology that may underpin these improvements.
Limitations
As with any case series there are a number of inherent limitations. Although all patients demonstrated objective improvements in HRV, the inability to define whether these improvements were due to natural progression, pain reduction, unreported home care and self-medication, adjunct therapies administered during the program of care, or vertebral subluxation based chiropractic care makes these factors limitations to the study and causal effect cannot be determined. It’s clear that further clinical research is required to evaluate the relationship between chiropractic care for the correction of vertebral subluxation and improvement in HRV in adult patients.
Conclusion
The data presented demonstrate a sustained improvement in HRV over a course of chiropractic care that is consistent with improved health outcomes. While no definitive conclusion can be made from this study, these data show objective, non-musculoskeletal outcomes that are consistent with neurophysiological effects associated with reduction or resolution of vertebral subluxation including improvements in coherence, spinal biomechanical function, neurological function, resilience, and adaptability. Future directions for HRV research in the chiropractic research arena should include expanding upon the current research using a sample size large enough for statistical analysis and longitudinal study, exploration of potential effects of the chiropractic adjustment on EEG activity of cardiac and cardiac-related nuclei, and exploration of whether sensory input generated by the chiropractic adjustment may affect nuclei in the medulla in a way that directs HRV changes.
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