Heart rate variability biofeedback and implications for tinnitus, PTSD and chronic pain in veterans: a scoping review

Introduction

Chronic conditions are highly prevalent among veterans, with tinnitus, post-traumatic stress disorder (PTSD), and chronic pain among the most common (Carlson et al., 2019). These conditions frequently co-occur and are associated with significant impacts on quality of life, including sleep disturbances, emotional distress, and functional impairment (Carlson et al., 2019). Veterans are particularly affected due to noise exposure (Jokel et al., 2019), trauma (Institute of Medicine, 2014), and physical injury during military service (Roy et al., 2013). Each of these conditions shows considerable variability in clinical presentation across individuals.

Tinnitus, usually defined as the perception of a sound in the absence of an external sound source, can present with or without measurable hearing loss and may reflect heterogeneous underlying mechanisms, including sensorineural, somatosensory, or vascular contributions (Genitsaridi et al., 2020). Its clinical presentation varies in perceptual characteristics, symptom severity, and associated comorbidities, potentially reflecting differences in underlying pathophysiology and individual responses to the condition (Genitsaridi et al., 2020). Beyond its auditory manifestations, tinnitus may affect sleep, concentration, emotional functioning, and coping (Genitsaridi et al., 2020). Anxiety and depressive symptoms are also frequently reported among affected individuals (Patil et al., 2023).

PTSD is characterized by intrusive symptoms, avoidance, and alterations in mood and arousal following trauma (American Psychiatric Association, 2022). For some, fear-based reexperiencing may be prominent, including intrusive memories, trauma-related dreams, and flashbacks. Others may experience disturbances in mood and social functioning, such as persistent negative emotional states, a reduced capacity to experience positive emotions, and social detachment. Avoidance may extend to internal experiences or external reminders associated with the traumatic event(s). In other cases, alterations in arousal and reactivity may predominate, including irritability and anger, hypervigilance, exaggerated startle responses, and concentration and sleep disturbances.

Chronic pain persists beyond normal healing time and often involves complex biopsychosocial processes (Rahman et al., 2023). Pain perception reflects interactions among central neural and sensory processes, cognitive and emotional factors, stress, and previous traumatic experiences (Rahman et al., 2023). According to Berry et al. (2014), chronic pain, among other conditions, may contribute to sustained activation of the body's stress response, potentially affecting autonomic regulation. It may also coexist with negative emotional states, perceived stress, and limitations in physical functioning. Furthermore, sleep disturbances are clinically significant in half of individuals with chronic pain (Rahman et al., 2023). The high comorbidity rate among these conditions in veterans suggests overlapping contributing factors, although the underlying mechanisms remain incompletely understood.

Several psychosocial factors may contribute to the maintenance or exacerbation of these conditions. Exposure to physical and psychological stressors may elicit neural and hormonal responses with either adaptive or maladaptive effects (Abdallah and Geha, 2017). In PTSD, symptom persistence may be influenced by negative appraisals, maladaptive coping, and subsequent stressful events, whereas social support may contribute to more favorable outcomes (American Psychiatric Association, 2022). In chronic pain, fear of pain exacerbation may promote avoidance and inactivity (Abdallah and Geha, 2017), while stress, thoughts, emotions, and persistent poor sleep quality may further influence pain perception and contribute to symptom maintenance (Rahman et al., 2023). In tinnitus, self-focused attention, somatic awareness, and poor sleep quality have been associated with greater emotional distress and perceived disability (Newman et al., 1997). In veterans with comorbid PTSD, tinnitus loudness was reported to increase during periods of stress (Moring et al., 2018). Interactions between tinnitus and PTSD have also been reported, as trauma-related sounds may exacerbate tinnitus (Fagelson, 2007), while greater tinnitus-related distress has been associated with increased PTSD reexperiencing (Moring et al., 2022). Overall, these findings highlight the potential role of cognitive, emotional, behavioral, and social factors, as well as interactions between co-occurring conditions, in symptom persistence or exacerbation across these conditions.

PTSD, chronic pain, and tinnitus show clinical overlap and may interact through psychological and physiological processes. Stress may represent a common factor across these conditions, with stress-related responses involving autonomic nervous system activation (Abdallah and Geha, 2017). PTSD and tinnitus-related distress share manifestations across sleep, attentional, emotional, and arousal-related domains, while tinnitus intrusiveness and perceptual characteristics have been associated with multiple PTSD symptom clusters (Moring et al., 2022). Neurobiological findings further suggest overlapping alterations in neural systems involved in auditory vigilance in PTSD and tinnitus (Moring et al., 2022). PTSD and chronic pain may also be mutually reinforcing, with each condition potentially exacerbating the impact of the other, while sleep disturbances may further contribute to this relationship (Hadlandsmyth et al., 2024). The relationship between chronic pain and tinnitus appears less well-documented; however, evidence involving post-traumatic headache (PTH) suggests a potential connection, as tinnitus and hyperacusis have been associated with PTH (Moring et al., 2022). Together, these findings support potential interrelationships among PTSD, tinnitus, and chronic pain.

The association between PTSD and tinnitus has been further documented specifically in veteran populations. According to Fagelson (2007), 34% of the first 300 patients enrolled in the Veterans Affairs Medical Center Tinnitus Clinic also had a diagnosis of PTSD. The two conditions may share mechanisms affecting auditory responses, including exaggerated startle responses and reduced sound tolerance. Furthermore, veterans with both conditions reported greater difficulty coping with tinnitus and greater perceived handicap than those without PTSD, despite comparable tinnitus loudness and awareness (Fagelson and Smith, 2016). These findings suggest that comorbid PTSD may be particularly associated with the functional and emotional impact of tinnitus rather than its perceptual characteristics.

Large-scale cohort data provide further evidence of associations among these conditions in veterans. Swan et al. (2017) conducted a retrospective observational study involving more than 570,000 Iraq and Afghanistan veterans receiving health care through the U.S. Department of Veterans Affairs. PTSD alone was significantly associated with hearing loss and/or tinnitus, with veterans in this group having greater odds of tinnitus [adjusted odds ratio = 1.61, 95% CI (1.55, 1.68)] than those without PTSD, depression, or traumatic brain injury (TBI). Several pain-related conditions were also associated with tinnitus, including headache, neck pain, and jaw pain. Although chronic headache may constitute a chronic pain syndrome according to Rahman et al. (2023), headache was not specifically characterized as chronic in Swan et al. (2017). Migraine attacks are described as an example of pain that may be triggered by stress and followed by maladaptive physiological responses associated with greater vulnerability to pain persistence (Abdallah and Geha, 2017). In addition, Jaramillo et al. (2016) identified a relationship between headache and PTSD in a cohort of more than 38,000 Iraq and Afghanistan veterans. Overall, these findings support associations among PTSD, tinnitus, and pain-related conditions in veterans, although causality cannot be established. Studies have also examined PTSD and tinnitus alongside pain-related manifestations (e.g., PTH; Moring et al., 2022). However, such manifestations were not necessarily categorized as chronic pain in the original studies. Thus, the available evidence does not appear to have examined PTSD, tinnitus, and chronic pain together.

Alterations in autonomic nervous system (ANS) functioning have been reported in PTSD (Sack et al., 2004; Kurt Gleichauf, 2013; Schneider and Schwerdtfeger, 2020) and chronic pain (Tracy et al., 2016), particularly reduced parasympathetic (vagal) activity. Heart rate variability (HRV), which reflects variability in the time intervals between heartbeats, is commonly used as a non-invasive index of autonomic regulation (Laborde et al., 2017). According to Berry et al. (2014), chronic pain–related activation of the stress response has been associated with reduced HRV power and lower resilience and self-regulatory capacity. However, evidence linking tinnitus to ANS dysfunction is more limited and remains inconsistent (Datzov et al., 1999; Ylikoski et al., 2017).

HRV biofeedback (HRVB) is a non-pharmacological intervention that aims to modulate autonomic function (Lehrer and Gevirtz, 2014). This technique involves training individuals to breathe at their resonance frequency, typically around six breaths per minute, with slightly longer exhalations than inhalations (approximately 5.5 and 4.5 s, respectively), thereby enhancing vagal activity and increasing HRV. It has been shown to modulate baroreflex sensitivity, a key mechanism underlying autonomic regulation. It has been proposed to support autonomic balance and, in turn, improve emotional regulation, stress resilience, and physiological flexibility (Lehrer and Gevirtz, 2014). However, many of these physiological effects appear to be driven by acute responses occurring during paced breathing, including transient increases in HRV and baroreflex engagement. While some studies report improvements across repeated sessions, evidence for sustained adaptations over time remains inconsistent.

A recent systematic review on the efficacy of HRV biofeedback for the management of various chronic conditions prevalent in the general population, including pain, anxiety, and sleep disturbances, reported that most studies found improvements in clinical outcomes co-occurring with improvements in HRV, suggesting a possible regulatory effect of HRVB on autonomic function (Fournié et al., 2021). Similar findings have been reported for the management of chronic pain (Pellow et al., 2025) and PTSD (Kenemore et al., 2024). For tinnitus, only one publication, including three cases, reported a potential effect of HRVB on tinnitus handicap in only one tested patient (Meckley Kutyana, 2015). However, empirical evidence supporting HRVB for these conditions remains limited, highlighting the need for further research to clarify its potential clinical efficacy.

Despite the high prevalence of tinnitus, PTSD, and chronic pain among veterans; common precipitating events (e.g., trauma, brain injury); shared physiological and psychological processes; and the growing interest in HRVB, the extent to which this intervention has been studied in veteran populations across these conditions remains unclear, particularly for chronic pain and tinnitus. This approach is of particular interest, as the Veterans Affairs/Department of Defense (VA/DOD) guidelines for managing chronic conditions emphasize the underutilization of non-pharmacological interventions, which may provide symptom relief while reducing the risks associated with polypharmacy (Kligler et al., 2018). Accordingly, the objectives of this scoping review were to (1) identify studies examining HRVB interventions for tinnitus, PTSD, and/or chronic pain in veterans and (2) map the characteristics and reported outcomes of HRVB studies conducted in these populations.

Methods

As a guideline to conduct this scoping review, the Preferred Reporting Items for Systematic Reviews and Meta-Analyses extension for Scoping Reviews (PRISMA-ScR) was used (Tricco et al., 2018). Studies were included if they (1) involved veteran-only populations (including control groups, when applicable); (2) evaluated the effects of HRVB, either as a standalone intervention or in combination with standard care or another non-pharmacological intervention; (3) reported quantitative outcomes related to chronic symptoms (e.g., PTSD, chronic pain, or tinnitus); and (4) were published in English or French. Studies were excluded if they included active-duty military personnel or non-veteran populations, were conference abstracts, or did not report quantitative outcome measures, as these were considered insufficient for meaningful comparison across studies.

The search strategy was intentionally designed to identify all studies examining HRVB interventions in veteran populations rather than restricting the search to specific health conditions. Consequently, the search included terms related only to veterans and HRVB and did not include condition-specific terms such as PTSD, chronic pain, tinnitus, migraine, headache, or post-traumatic headache. All retrieved studies were subsequently screened to identify the chronic conditions investigated. This broad approach maximized search sensitivity and allowed the full spectrum of chronic conditions studied using HRVB in veterans to be identified.

The PubMed, Embase, and CINAHL databases were searched in August 2024 to identify relevant articles. An updated search was performed in February 2025. The search strategy for each database is presented in Table 1. The first screening involved reading the studies' titles and abstracts. The second screening consisted of reading the full text of potentially eligible studies. Screening was performed independently by two members of the research team (ELR and MOM) to ensure objectivity and minimize bias. The quality control checks were conducted by members of the research team with complementary expertise in audiology, psychology and psychophysiology, including P.F. and G.T.V. Study characteristics extracted included the authors, the year of publication, the study design, the samples, the device used for assessing and analyzing heart rate data, the duration of heart rate recording, assessment times, and a description of the HRVB intervention and HRVB effects on HRV and chronic symptoms (outcome measures). Ethics approval was not required for this scoping review because it involved analyzing and synthesizing data from previously published studies and did not involve collecting primary data from human participants or accessing identifiable personal information.

Table 1. Search strategy.

#1, #2, etc., and S1, S2, etc. denote intermediate search sets generated by the database search engines and combined using Boolean operators.

Results

In all, 25 studies were identified in PubMed, 34 in Embase, and 10 in CINAHL for a total of 69 studies. After excluding 28 duplicates, the titles and abstracts of 41 studies were screened. At this level, 35 articles were excluded based on the eligibility criteria. The full text of six articles was read, and one article was excluded based on the eligibility criteria. Five articles were then included in this scoping review. Figure 1 presents the article selection process flowchart.

Methodologies

The methodology used for each study included in the review is presented in Table 2. The number of participants included in the studies ranges from 12 to 35. In all, 108 participants were included in this review. Four studies are pilot studies with a control group. Analysis populations varied across studies. Reyes (2014) analyzed only participants who completed the intervention according to the study protocol, which is compatible with a per-protocol approach, although not explicitly stated by the author. Schuman and Killian (2019) identified an intention-to-treat (ITT) sample, which includes participants according to their original group assignment regardless of intervention completion; however, those with missing data were excluded from the corresponding analyses. Schuman et al. (2023) included the full randomized sample in the analyses and used the expectation-maximization algorithm to replace missing values, without explicitly characterizing their analysis as ITT. In Berry et al. (2014), all participants completed their assigned condition, with no reported withdrawals. For Tan et al. (2011), the analysis approach was unclear: one participant withdrew prematurely, but all randomized participants appear to have been included in the reported analyses. Furthermore, one article did not report its study design (Reyes, 2014).

Table 2. Methodology of studies included in the scoping review.

HRVB, heart rate variability biofeedback; HRV, heart rate variability; HR, heart rate; PPG, photoplethysmography; ECG, electrocardiogram; BVP, blood volume pulse; FUP, follow-up; BMI, body mass index; PTSD, post-traumatic stress disorder; PCL-5, PTSD Checklist for DSM-5; DB, diaphragmatic breathing; LF, low frequency; HF, high frequency; RMSSD, root mean square of successive differences between normal heart- beats; SDNN, standard deviation of the inter-beat interval of normal sinus beat.

^aOEF/OIF/OND veterans refers to U.S. military veterans who served in specific military operations (in Afghanistan or Iraq).

^bA federal agency of the U.S. government who provides health care, benefits, and support services to military veterans.

^cNot explicitly mentioned for the 4 and 16-week FUP. The safest assumption is that the same protocol was used. After a stabilization period of 5 min, HR was measured.

^dThe collected data before the intervention is reported, but the collected data after the intervention is no reported.

The inclusion and exclusion criteria to select participants vary from one study to another, but previous participation in an HRV study, psychosis, substance abuse, and cognitive deficits are the most used exclusion criteria. Concomitant medication use was variably addressed across studies. Berry et al. (2014) excluded participants who regularly used medications known to affect autonomic nervous system function or pain perception (antidepressants, benzodiazepines, anti-inflammatory medications, and beta-blockers). Schuman and Killian (2019) excluded participants with current prescriptions for beta-blockers, antipsychotics, acetylcholinesterase inhibitors, or serotonin–norepinephrine reuptake inhibitors. It should be noted that the authors reported one participant who required opioid medication following an injury during the follow-up period; HRV measures decreased markedly for this participant (Schuman and Killian, 2019). Schuman et al. (2023) excluded participants prescribed cardio-active medications. However, psychotropic medication use was reported in 68.6% of the sample, with no significant difference between groups. Reyes (2014) and Tan et al. (2011) did not clearly report concomitant medication use or medication-related eligibility criteria relevant to autonomic function.

Moreover, somatic comorbidities that may influence HRV measurements were addressed to varying degrees across studies. Berry et al. (2014) excluded participants with diabetes, hypertension, coronary artery disease, and other specified medical conditions. Schuman and Killian (2019) excluded those with a history or diagnosis of diabetes or cardiovascular disease, while Schuman et al. (2023) excluded those with uncontrolled diabetes, a history of stroke, heart disease, or pulmonary disease. Therefore, somatic comorbidities' potential contribution to HRV measures could not be determined for Reyes (2014) and Tan et al. (2011), as no specific exclusion criteria related to these conditions were reported.

Finally, the consideration of psychiatric comorbidities potentially affecting HRV also varied across studies. Berry et al. (2014) excluded participants reporting substance abuse, while Schuman and Killian (2019) excluded those with current or prior psychosis, bipolar disorder, or excessive substance use within the previous 6 months. Schuman et al. (2023) excluded participants with active psychosis or mania and required abstinence from alcohol and non-prescribed drugs before HRV measurements. Tan et al. (2011) excluded severe psychopathology, including active psychosis or substance abuse. Reyes (2014) excluded participants with psychotic symptoms; however, 33% had a history of depression, and 48% had a history of substance abuse, both reported as pre-enlistment conditions.

Intervention characteristics

The HRVB intervention duration is 4 weeks in two studies (Berry et al., 2014; Schuman and Killian, 2019) and 8 weeks in three studies (Tan et al., 2011; Reyes, 2014; Schuman et al., 2023). One study (Schuman and Killian, 2019) evaluated the persistence of the intervention 8 weeks after the end of the last HRVB session. Most studies included a daily HRVB practice component performed independently by participants, in addition to supervised sessions, because all studies included at least one clinician-supervised HRVB session. One study did not include a self-directed daily practice component, and the practice duration was not reported (Berry et al., 2014). Regarding the other studies, the duration of the HRVB practice at home ranged from 10 to 40 min per day. In one study (Schuman et al., 2023), adherence to home practice was measured using data collected from the smartphones of the participants.

HRV assessments

HRV recordings, using either photoplethysmography, electrocardiogram, or blood volume pulse captor placed on the finger, wrist, or forearm, were all performed at rest and varied from 7 to 15 min. Studies used different HRV metrics, such as coherence ratio, root mean square of successive differences between normal heartbeats, standard deviation of the inter-beat interval of normal sinus beat, high-frequency power, and low-frequency power, to assess HRVB effects on HRV. For most studies, the measurements were performed before and after treatment sessions, sometimes at time points in between, and always at rest, not during exercise. The effects of the HRVB intervention on chronic symptoms were measured using different standardized questionnaires.

Clinical outcomes

The clinical outcomes of each study are presented in Table 3. Only significant results are reported in this table. Four studies (Berry et al., 2014; Reyes, 2014; Schuman and Killian, 2019; Schuman et al., 2023) reported the HRVB effects on HRV. All these studies reported that HRVB intervention resulted in increased HRV measured at rest. Four studies (Tan et al., 2011; Reyes, 2014; Schuman and Killian, 2019; Schuman et al., 2023) investigated the effects of HRVB on PTSD symptoms. Among these studies, two (Reyes, 2014; Schuman and Killian, 2019) reported a significant effect of HRVB intervention on PTSD symptoms. One of these studies (Schuman and Killian, 2019) even reported that HRVB effects on PTSD symptoms persist 16 weeks after the end of the HRVB training. HRVB was not significantly more effective than standard care in a study (Tan et al., 2011) and was only associated with a decrease in PTSD intrusion symptoms in Schuman et al.'s (2023) study. This study is also the only one among the studies on PTSD symptoms that measured and reported a decrease in depressive symptoms. One article (Berry et al., 2014) focuses on the effects of HRVB on chronic pain. In this article, a significant decrease in perceived pain was observed following the HRVB intervention. No studies on HRVB and tinnitus were found.

Table 3. Clinical outcomes.

HRVB, heart rate variability biofeedback; HRV, heart rate variability; RMSSD, root mean square of successive differences between normal heart beats; SDNN, standard deviation of the inter-beat interval of normal sinus beat; PTSD, post-traumatic stress disorder; BPI, Brief Pain Inventory; PSS, Perceived Stress Scale; PCL-S, Post-traumatic Stress Disorder Checklist Scale; PCL-5, PTSD Checklist for DSM-5; BDI-II, Beck Depression Inventory; CAPS, Clinician-Administered PTSD Scale; D, diaphragmatic breathing; FUP, follow-up; MD, mean difference; Md, difference in the median.

^aThe coherence ratio is formulated as coherence = peak power/(total power – peak power). Based on power ms², frequency-domain metric.

^bIt is unclear whether changes in treatment group are significantly different from changes in control group.

^cFor these analyses, data of DB + HRVB and DB (control) groups have been combined. A non-parametric test has been used (based on the median).

^dIt is unclear whether the changes in HRVB + DB group are significantly different from changes in DB group 4 weeks after the DB intervention.

^eSDNN ln: natural log-transformed. The Bold formatting was applied to significant outcomes for both, HRVB and Chronic symptoms.

Discussion

This review found only five articles investigating the HRVB effects on chronic symptoms of either PTSD, chronic pain, or tinnitus in veterans. We found that HRVB has been used to manage PTSD, depression, and chronic pain in veterans. None was found for tinnitus. Most of the studies have focused on the HRVB effects on PTSD symptoms. Overall, the included studies suggest that HRVB may influence symptoms of depression, PTSD, and chronic pain in veterans; however, the evidence remains preliminary and heterogeneous, with small sample sizes, variable methodologies, and inconsistent use of HRV metrics. Accordingly, these findings do not allow firm conclusions regarding treatment efficacy. Rather, they highlight a potential association that warrants further investigation. Our findings are broadly consistent with those of a systematic review by Fournié et al. (2021), which also identified promising but inconclusive evidence for the role of HRVB in managing chronic conditions in the general population. Collectively, the current literature supports the need for more rigorous and adequately powered studies to clarify the potential role of HRVB as an adjunctive intervention.

Tinnitus, PTSD, and chronic pain

Surprisingly, in this scoping review, no studies specifically examining HRVB's potential benefits on tinnitus were found, and only one study on chronic pain in veterans was identified. Fournié et al. (2021) similarly reported no studies on the impact of HRVB on tinnitus in the general population but did identify several studies on chronic pain. The absence of tinnitus-focused studies is particularly unexpected given that tinnitus is the most prevalent service-connected disability among U.S. veterans (U. S. Department of Veterans Affairs, 2025). In contrast, although evidence remains limited in veteran populations, chronic pain has been more studied in relation to HRVB in the general population, with some studies suggesting that some potential improvements in pain perception may be related to changes in autonomic regulation (Fournié et al., 2021). However, this interpretation should be nuanced as another study did not show significant pain reduction in patients with various profiles of chronic pain (Weeks et al., 2015).

Evidence linking ANS dysfunction to PTSD and chronic pain is well-established, and although more limited, some studies have also reported associations between ANS dysfunction and tinnitus-related distress (Datzov et al., 1999; Ylikoski et al., 2017). Stress, particularly referring here to heightened physiological arousal and maladaptive autonomic responses, is known to exacerbate both tinnitus and chronic pain symptoms (Abdallah and Geha, 2017; Patil et al., 2023). However, both tinnitus and chronic pain represent heterogeneous conditions with multiple underlying mechanisms (Courtney et al., 2017; Genitsaridi et al., 2020). In this context, HRVB may be particularly relevant for subgroups in which autonomic and cardiovascular regulation contribute to symptom expression. For chronic pain, for example, autonomic dysregulation has been implicated in altered pain modulation, suggesting a plausible mechanism through which HRVB may be associated with changes in symptoms, although current evidence remains insufficient to confirm this effect. Similarly, for tinnitus, certain subtypes, such as pulsatile tinnitus characterized by heartbeat-synchronous sounds, may be more directly influenced by short-term changes in blood pressure and autonomic control, which HRVB can transiently modulate (Alvear et al., 2025). Nevertheless, given the heterogeneity of tinnitus and chronic pain presentations and the limited and methodologically variable evidence base, these potential effects should be interpreted cautiously and not generalized across all subtypes.

There is a clear need to better characterize the co-occurrence of tinnitus with conditions such as PTSD and chronic pain, where autonomic alterations have been reported more consistently. These considerations underscore the importance of adopting more precise phenotyping approaches and standardized methodologies in future studies. Such efforts may help clarify HRVB's relevance for specific subgroups and support more targeted investigations in both the general population and veteran populations. Conversely, the effects of HRVB on PTSD symptomatology in veterans have been the most studied; however, the evidence remains limited and does not support its use as a stand-alone intervention. This conclusion is consistent with a recent meta-analysis examining HRVB for PTSD in active military personnel and veterans (Kenemore et al., 2024).

With the sole exception of one study that assessed both depression and PTSD symptoms, no other studies have examined the effects of HRVB across multiple chronic conditions simultaneously. Although comorbidity is common in veteran populations, the existing evidence does not allow conclusions regarding whether the effects of HRVB extend across co-occurring conditions, as observed outcomes may reflect condition-specific responses or be influenced by small sample sizes and convenience sampling. Furthermore, while several chronic conditions in veterans have been associated with autonomic dysregulation, this shared feature does not imply that HRVB will produce similar therapeutic effects across conditions. Therefore, rather than assuming generalizability, future studies should explicitly investigate the effects of HRVB across multiple co-occurring conditions using adequately powered and methodologically rigorous designs. Such approaches would help determine whether HRVB has transdiagnostic utility or condition-specific effects in veteran populations.

A need to standardize HRV assessment

The current review also highlights several methodological limitations in HRV assessment across studies. Importantly, these limitations directly affect the interpretability and comparability of findings (Laborde et al., 2017). For example, the wide variability in HRV metrics and signal-processing approaches, including the use of indices with limited physiological validity, introduces uncertainty regarding what aspect of autonomic function is being measured. This issue is compounded by differences in data acquisition methods, as HRV derived from electrocardiography (ECG) vs. photoplethysmography or pulse-wave-based techniques may not capture beat-to-beat variability with equivalent temporal precision. In particular, non-ECG methods are more susceptible to peak-detection errors and may artificially inflate or attenuate variability estimates, thereby introducing measurement artifacts rather than true physiological differences. Additionally, inconsistencies in sampling frequency across devices further impact the resolution of inter-beat interval detection, with lower sampling rates potentially obscuring high-frequency (vagally mediated) components of HRV. Together, these methodological inconsistencies limit the ability to draw reliable conclusions regarding HRVB efficacy, as observed changes in HRV may reflect differences in measurement techniques rather than true physiological or clinical effects. By highlighting these issues, our review underscores the need for standardized HRV acquisition and analysis protocols to improve internal validity and enable meaningful cross-study comparisons. Given the methodological variability and limited quality of existing studies, the current evidence should be interpreted as preliminary and hypothesis-generating. Collectively, these findings support the need for rigorously designed, adequately powered studies to clarify the potential role of HRVB as an adjunctive intervention and to establish reliable links between autonomic regulation and clinical outcomes in veteran populations.

The rationale for selecting some HRV parameters over others is not clearly presented in the studies. This raises the question of whether some HRV parameters (coherence ratio, time-domain parameters, and frequency-domain parameters) are more appropriate than others for measuring HRVB effects on HRV. Standardizing reported measures is essential to allow meaningful comparisons across studies. At present, determining whether the positive or negative outcomes in HRV measurements are attributable to the intervention itself or simply to differences in the metrics used is difficult. Established guidelines, such as those from the 1996/1999 European Society of Cardiology and the North American Society of Pacing and Electrophysiology Task Force on HRV (Task Force of the European Society of Cardiology and the North American Society of Pacing and Electrophysiology, 1996), emphasize the importance of standardized acquisition, processing, and reporting of HRV data. Adhering to these recommendations in future HRVB studies would improve methodological rigor, enhance comparability across studies, and strengthen the interpretation of autonomic outcomes.

Caveats in study design

Several methodological limitations were identified across the reviewed studies. First, although reductions in chronic symptoms were often reported following HRVB interventions, the interpretation of concurrent changes in HRV remains unclear. In many cases, reported increases in HRV may reflect transient physiological responses, such as reductions in heart rate and short-term shifts in parasympathetic activity during paced breathing, rather than sustained changes in autonomic regulation. Given that vagal modulation of the sinoatrial node operates rapidly, increases in beat-to-beat variability are more pronounced at lower heart rates and may occur as individuals enter a more relaxed physiological state, rather than as a direct effect of the intervention itself. Consequently, observed HRV changes may reflect a return toward baseline autonomic balance rather than a specific therapeutic mechanism.

Importantly, none of the studies examined the relationship between changes in HRV metrics and improvements in clinical symptoms. Future studies should therefore incorporate appropriate statistical approaches, such as correlational or regression analyses, to determine whether HRV changes are meaningfully associated with symptom improvement, thereby clarifying whether HRV represents a mechanism of action or a non-specific physiological correlate. Interestingly, such associations have been reported for evidence-based psychotherapies for PTSD, which have been linked to changes in psychophysiological responses among veterans. Maples-Keller et al. (2019) examined objective indices of physiological reactivity and arousal in response to trauma-related stimuli, including trauma-potentiated startle, heart rate reactivity, and skin conductance, before and after a 2-week intensive prolonged exposure treatment. Nearly half of participants (48.6%) were classified as high treatment responders, based on a ≥50% reduction in PTSD Checklist for DSM-5 scores. Following treatment, trauma-related stimuli no longer elicited an increased startle response among these veterans, reflecting reduced physiological reactivity to trauma-related cues. Heart rate reactivity also changed significantly over treatment, whereas skin conductance reactivity remained unchanged. More recently, Maples-Keller et al. (2022) further examined psychophysiological changes following intensive prolonged exposure in veterans with PTSD using a fear-potentiated startle paradigm, which measures increased startle responses to stimuli associated with fear or threat. Fear-potentiated startle decreased following treatment, reflecting reduced fear reactivity. Moreover, high treatment responders showed reduced fear-potentiated startle and maintained fear extinction, defined as a reduction in a learned fear response when a previously threatening stimulus is repeatedly presented without the expected threat, whereas low responders demonstrated a return of fear. Altogether, these findings indicate that evidence-based PTSD treatment may be accompanied by measurable changes in specific psychophysiological responses, providing additional rationale for investigating interventions targeting physiological processes, such as HRVB. Current VA/DoD Clinical Practice Guidelines outline evidence-based recommendations for PTSD, tinnitus, and chronic pain. Although treatment recommendations differ across these conditions, cognitive and behavioral interventions are included among their therapeutic approaches (Department of Veterans Affairs and Department of Defense, 2023, 2024).

Second, some studies reported greater improvements when HRVB was combined with standard treatment; however, these findings remain limited and inconclusive. Comparative studies evaluating HRVB as a standalone intervention vs. standard care are needed to better determine its independent contribution to clinical outcomes. Third, only one study investigated the long-term effects of HRVB, limiting conclusions regarding the durability of treatment effects. It remains unclear whether reported improvements are sustained over time or attenuated following discontinuation of regular practice. Additionally, substantial variability in outcomes across studies may reflect differences in individual responsiveness to HRVB rather than consistent intervention effects. While HRVB is generally expected to produce acute increases in HRV and promote relaxation, these responses are not universal. In some cases, improvements may be driven by short-term physiological or psychological changes that do not translate into sustained clinical benefits, whereas in others, longer-term effects may result from cumulative engagement with the intervention. The extent to which intervention characteristics, such as session duration, frequency, and overall dose, contribute to these outcomes remains unclear.

Future studies should therefore account for both intervention-related factors and individual-level characteristics (e.g., baseline autonomic function, comorbidities, and symptom profiles) to better understand variability in treatment response and to identify optimal HRVB protocols.

Feasibility of HRVB intervention for veterans

Despite these various limitations and caveats, the current review highlights the feasibility of studying HRVB interventions in veterans with chronic conditions. HRVB is a non-invasive intervention, requires minimal equipment, and has not been associated with any adverse effects. In fact, none of the reviewed studies reported any negative side effects. Several studies reported some level of improvement in the targeted chronic condition, and it is plausible that additional, unmeasured benefits may have occurred, although this remains to be validated in future research. These positive and encouraging findings support the need for more rigorous research on the effects of HRVB on chronic conditions, including well-designed randomized controlled trials.

Limitations

This review has several limitations. The most significant is the limited number of studies included, along with the small sample sizes within those studies. While the findings are encouraging, there is a clear need for further research to better understand the effects of HRVB on chronic conditions in veterans. The unique characteristics of the veteran population warrant targeted investigation, as interventions effective in the general population may be less effective or even ineffective in this group. This may reflect the higher prevalence of complex comorbidities (e.g., tinnitus, PTSD, and chronic pain), which are known to interact, exacerbate symptom severity, and complicate treatment response, often resulting in poorer clinical outcomes and increased health care utilization (Saadoun et al., 2021). Additionally, high rates of polypharmacy and multimorbidity may further influence treatment responsiveness and introduce variability in intervention effects (Hadlandsmyth and Lund, 2026). Second, the lack of standardized protocols and outcome measures across studies makes comparing results difficult. Notable differences were observed in the devices used to collect HRV data, assessment time points, duration of HRVB interventions, and the questionnaires used to assess chronic symptoms, particularly PTSD. These inconsistencies may partly explain the variation in outcomes across studies. Third, the methodological quality of some studies is questionable. In several cases, it is unclear whether changes observed in the treatment groups significantly differed from those in control groups because the statistical analysis is not presented. Demonstrating a significant difference between treatment and control groups is essential to establishing the efficacy of an intervention. Together, these limitations may affect the validity of the conclusions drawn from this scoping review. Therefore, future research should include larger sample sizes and rigorous study designs to more reliably assess the effects of HRVB on chronic symptoms in veterans.

Conclusion

This scoping review aimed to identify HRVB interventions used for tinnitus, PTSD, and chronic pain in veterans, and to evaluate the extent of existing evidence in this population. HRVB has been explored in veterans primarily in relation to PTSD, depression, and chronic pain symptoms; however, the current evidence base is limited by small sample sizes, methodological heterogeneity, and substantial individual variability in physiological responses. As such, no definitive conclusions can be drawn regarding its efficacy as a standalone treatment, although it may have a role as an adjunctive or supportive intervention in certain contexts. Importantly, this review highlights a notable absence of studies investigating HRVB specifically for tinnitus in veterans. Given the prevalence of tinnitus and its frequent co-occurrence with other chronic conditions in this population, further research is warranted to explore the feasibility, safety, and potential relevance of HRVB in well-characterized veteran cohorts. Future studies should avoid assuming shared underlying mechanisms across conditions and instead adopt rigorous designs that account for clinical heterogeneity, individual differences, and tinnitus subtypes.

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The author(s) declared that Generative AI was used in the creation of this manuscript. Microsoft Copilot was used solely for English language editing, grammar correction, and stylistic improvements. The author(s) reviewed all suggested changes and assume full responsibility for the manuscript's content.

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Footnotes

Edited by: Marc Fagelson, East Tennessee State University, United States

Reviewed by: Martin Siepmann, Technical University Dresden, Germany

John Moring, University of Texas at San Antonio, TX, United States