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Feelings of safety emerge from inside the body, and humans, as social mammals, are on an enduring lifelong quest to feel safe.¹ These feelings form the foundational neural platform for sociality. So theorizes neuroscientist Stephen W. Porges, who introduced the concept of polyvagal theory in 1994 and has been conducting research on the topic for more than five decades. Polyvagal theory emphasizes sociality as a core human process that helps to mitigate threat and support mental health.¹ More specifically, it highlights the importance that the parasympathetic nervous system and vagal circuits play in the neurophysiological mechanisms related to trauma and trauma responses.²

This theory, and subsequent research, is significant for trauma survivors in that it shifts an individual’s feelings of safety from a subjective to an objective science, acknowledging that these feelings have a measurable underlying neurophysiological substrate.¹ Thus, post-traumatic stress disorder (PTSD) symptoms may be viewed as the product of a reconditioned autonomic nervous system that developed during extreme and/or repeated exposures to threat.¹ Polyvagal theory helps clinicians understand how an individual’s sense of safety, or danger and threat, can impact behavior and mental health. What is the state of the research on polyvagal theory and the autonomic nervous system as it relates to trauma? How does this research inform new treatment strategies for trauma-related conditions like PTSD?

How Trauma Impacts the Body: A Focus on PTSD

Porges explains in “Polyvagal theory: a science of safety,” published by Frontiers in Integrative Neuroscience in 2022, that feelings of safety reflect a core fundamental process that has enabled humans to survive.¹ When humans feel safe, their nervous systems support homeostatic functions of health, growth, and restoration while they become accessible to others without feeling threatened and vulnerable.¹ When the nervous system detects danger during a traumatic experience, a withdrawal of the parasympathetic system is initiated in the fight-or-flight response as a protective mode. While the fight-or-flight response is adaptive in acutely dangerous and life-threatening situations, in the context of everyday life, it can become damaging. In environments that are considered safe, a chronic fight-or-flight state is maladaptive and may lead to significant hypervigilant psychosocial distress and poor health outcomes.²

Post-traumatic stress disorder (PTSD) is a maladaptive, debilitating neuropsychiatric condition that involves the dysregulation of normal fear processes.³ It has a prevalence of up to 25-35% among individuals who have endured severe trauma, including adverse childhood experiences, physical, mental, and sexual abuse, natural disasters, war, parental death, and more.^3,4 Patients with PTSD often experience abnormal fluctuations in the autonomic states that influence the fight-or-flight behaviors of withdrawal, immobilization, and dissociation without an intervening calm state.² The symptoms of PTSD may include persistently re-experiencing the traumatic event, intrusive thoughts, nightmares, flashbacks, dissociation, and an intense negative emotional and physiological reaction on being exposed to the traumatic reminder.⁵ Problems with sleep and concentration, irritability, increased reactivity, increased startle response, hypervigilance, and an avoidance of traumatic triggers may also occur.⁵

Patients diagnosed with PTSD process environmental stimuli differently than people without PTSD.² In a functional neuroimaging study that compared 23 patients with PTSD and 42 healthy controls, the patients with PTSD showed greater activation of the amygdala in response to photographs of emotional facial expressions of fear.^2,6 Patients with PTSD may have altered acoustic startle responses when exposed to brief but loud auditory stimuli.² There is also evidence that patients with PTSD have decreased heart rate variability (HRV), indicative of an autonomic state that would support the mobilization necessary for fight-or-flight behaviors and result in lower vagal tone to the heart.² This autonomic reactivity is conceptualized by researchers within the framework of polyvagal theory, which provides a neurophysiological basis for how autonomic states and behavior interact.²

The Autonomic Nervous System & Heart Rate Variability

Fluctuations in autonomic nervous system (ANS) function are observed in a host of psychological disorders, including PTSD, which affects both the sympathetic (SNS) and parasympathetic nervous system (PNS).^7,8 Interestingly, survivors of maltreatment who do not reach the diagnostic criteria for PTSD may also have psychiatric and physical health features that relate to an autonomic nervous system that has a lower threshold to react to cues of threat.⁸

The branches of the ANS—the SNS and PNS—are interconnected and primarily function unconsciously; when balanced, they enable an individual’s internal environment to adapt to a changing external environment.⁷ Dysregulation in the ANS can lead to negative cognitions and mood and alterations in arousal and reactivity, as well as symptoms of intrusion and avoidance.⁷ In patients with PTSD, scientists may also see an increase in the heart rate and respiratory sinus arrhythmia (RSA) as well as lower vagal efficiency, which can lead to either an increased or blunted responsiveness to challenging and stressful tasks.⁸

These maladaptive autonomic responses may contribute to the development of comorbid mental health issues such as depression, loneliness, and hostility.² Over time, changes in autonomic, endocrine, and immune function contribute to a patient’s deteriorating health, which may be expressed as mental health disorders, asthma, cancer, back pain, peripheral vascular disease, gastrointestinal problems, thyroid disorders, and cardiovascular disorders.²

An analysis of HRV allows clinicians and researchers to gain insight into the functioning of the ANS, which is associated with parasympathetic (vagal) activity.⁷ Meta-analyses, including one conducted in 2020, on HRV and PTSD suggest that HRV is lower in PTSD patients compared to controls, both while at rest and during times of stress.⁷ The 2020 results highlight that no single HRV parameter is particularly indicative for PTSD; rather, changes in HRV occurred in vagally mediated HRV parameters, as well as in more complex measures, thus indicating a general pattern of ANS dysregulation.⁷

Polyvagal Theory

A core component of the parasympathetic nervous system, the vagus nerve, is responsible for visceral sensation, stimulating the muscles of the digestive tract and decreasing heart rate.⁹ It helps to modulate the body’s response to stressors such as chronic stress, fear, and trauma.⁹ What is the role of this nerve in regulating the psychoemotional and physiological responses that may influence disease pathology? This is the question polyvagal theory seeks to understand.

Polyvagal theory proposes that the neural evaluation of risk and safety reflexively triggers shifts in autonomic states without requiring conscious awareness.¹ Specifically, the protective responses of the ANS emerge through neuroception, a reflexive adaptive process that triggers specific response patterns that help the individual cope with dangerous conditions.⁸ Distinct from perception, neuroception describes a neural process that is capable of distinguishing between environmental and visceral features that are either safe, dangerous, or life-threatening.¹ If neuroception detects safety, the nervous system promotes social communication and engagement.⁸ If neuroception detects danger, a withdrawal of the parasympathetic system may be initiated. The vagus nerve—the main component of the parasympathetic nervous system, which oversees a vast array of crucial bodily functions, including control of mood, immune response, digestion, and heart rate—may become dysregulated or dampened, which may lead to less effective autonomic regulation and difficulties returning the body to a calm baseline after experiencing a stressor.⁸

A 2022 study expands upon prior research suggesting that a history of maltreatment may be associated with inefficient or atypical vagal regulation of the heart in response to physical and emotional stressors.⁸ The study investigated whether these findings relate to differences in vagal efficiency (VE), a metric proposed as a measure of the dynamic regulation of cardiac vagal tone on cardiac output represented in a single measure of slope between sequential measures of heart periods (HP) and RSA. Consistent with their hypothesis, researchers documented that a history of maltreatment (including emotional neglect and abuse, physical neglect and abuse, and sexual abuse) was related to lower levels of VE and that VE was related to dampened HRT and recovery to the physical stressor. Furthermore, participants in the study with lower VE levels reported more depression and anxiety symptoms and that VE mediated the relationship between maltreatment history and depression and anxiety symptoms. This is consistent with polyvagal theory, and the results are also consistent with prior research suggesting that major depression, anxiety disorders, and PTSD are associated with lower HRV.⁸

Polyvagal theory emerged from Porges’ research studying heart rate patterns in human fetuses and newborns.¹⁰ In obstetrics and neonatology, bradycardia (the slowing of heart rate that can be lethal) is a clinical index of risk and assumed to be mediated by the vagus. However, with the same clinical populations, a different index of vagal function was assumed to be a measure of resilience. Porges asked the question: how could cardiac vagal tone be both a positive indicator of health when monitored with heart rate variability and a negative indicator of health when it manifests as bradycardia? The resolution to this paradox came from studying the way the autonomic nervous system changed during evolution. These changes resulted in two vagal pathways operating in a hierarchal manner, with one pathway being protective and supporting homeostasis while the other evolutionarily older pathway supports homeostasis only when the more evolutionarily modern vagus is functional. Evolution transformed the autonomic nervous system into an integrated social engagement system that incorporates a brainstem communication area (the ventral vagal complex). According to polyvagal theory, this transition in neuroanatomy and function provides the basis to understand that for humans, connectedness and trusting relationships are direct expressions of our biological imperative and are integrated into our biology.¹⁰

Porges defines polyvagal theory as a “perspective” to frame research questions rather than a static theory. He believes that as the knowledge of neurophysiology increases, testable hypotheses will shape and expand the theory.¹¹ Over the years, scientists have been investigating the validity of polyvagal theory; many studies suggest results consistent with Porges’ perspective while other researchers claim the theory is fraught with speculation.¹² To this end, Porges has compiled, in his own words, a clinically detailed response to criticisms of polyvagal theory. As research on polyvagal theory and PTSD continue to evolve, how can clinicians incorporate the concepts from Porges’ theory into their daily practice?

Clinical Applications & Interventions

Porges explains that strategies that are polyvagal-informed focus on enabling the patient to experience their feelings without linking those feelings to thoughts or behaviors.¹ Feelings, in and of themselves, are not intentional or under voluntary control but are part of an adaptive reflexive system that is wired into our nervous system.¹ Applications of polyvagal theory in the clinical world focus on the autonomic state as a mediator of mental and physical health problems.¹⁰

Stress Reduction & the Vagus Nerve

Mind-body therapies cultivate somatic awareness, including both interoception and proprioception, combined with the mindfulness-based qualities of nonjudgement, nonreactivity, and curiosity.¹³

Studies suggest that contemplative practices, characterized by the attentive regulation of breathing, may activate the vagus nerve and increase RSA that, according to the polyvagal theory, reflects the activation of the ventral vagal complex (VVC) and may promote PTSD recovery.^14,15 A recent meta-analysis in accordance with these observations investigating baseline RSA as a psychophysiological marker of stress vulnerability in individuals with PTSD revealed an association between PTSD and lower resting RSA.^14,16 Common forms of contemplative practices include mindfulness, compassion and self-compassion, and Tai Chi/Qigong or yoga often practiced 20 minutes or more, once or twice daily.^13,14 In a clinical setting, mindfulness-related and compassion-related meditations would be conceptualized as neural exercises, expanding the capacity of the ventral vagal complex to regulate the present state and to promote resilience.¹⁴

Vagal Nerve Stimulation

Vagal nerve stimulators (VNS) work through feedback mechanisms described in polyvagal theory.¹⁰ VNS can potentially benefit treatment of psychiatric disorders partly due to the nerve’s projections to brain areas such as the amygdala and hippocampus by downregulating activity in these areas, which are known to be related to stress responses and hyperarousal.¹⁷ Although studies in humans are limited, some research has found decreased inflammatory markers, decreased sympathetic tone, and increased medial prefrontal function with VNS.^17,18 A variety of VNS techniques, described below, show promising results for clinical application.¹⁹

• Electrical VNS: Stimulation of the vagus nerve with bioelectronic devices represents a therapeutic opportunity for several disorders implicating the autonomic nervous system and affecting different organs.²⁰ Studies suggest that in asthma patients, stimulating afferent branches of the vagus nerve during shortness of breath may produce longer exhalations and slow the respiration rate.^19,21 Electrical VNS (eVNS) has also been shown to affect cognitive functioning.^19,22 Gerritsen et al, writing in Frontiers in Human Neuroscience, propose that VNS may actually increase PNS activity, as indicated by neuroimaging studies.^19,23
• Transcutaneous VNS: Transcutaneous VNS (tVNS) devices noninvasively target vagal projections in the ear (auricular branch of the vagus) and neck (cervical branch in the carotid sheath).^24,25 In 2014, Shiozawa et al suggested from a review of neuropsychiatric studies that tVNS may reduce symptoms of depression.^19,26 Gurel et al’s study suggests that tVNS modulates autonomic, cardiovascular, and vascular measures in PTSD with or without exposure to traumatic and mental stress.²⁴ The changes observed due to tVNS in this study were similar to an earlier study by the same researchers involving non-PTSD controls.²⁴ In the study, Gurel et al’s findings suggest that tVNS may attenuate sympathetic arousal associated with stress related to traumatic memories as well as mental stress, with effects persisting throughout multiple traumatic stress and stimulation testing days.²⁴

• Behavioral VNS: Behavioral forms of VNS, often called vagal maneuvers, stimulate VN bilaterally by using the Valsalva technique—pinching the nose closed and then trying to exhale through the nose.¹⁹ It is initiated by flexing the abdominal muscles and extending exhalation. Extending, slowing, and holding respiration are all considered vagal maneuvers as they stimulate the VN.¹⁹ The styles of respiration providing respiratory VNS (rVNS) are controlled breathing techniques that slow and deepen respiration and extend expiration.^19,27 Studies suggest that slow and deep breathing increases HRV indices of vagal tone^19,28 and lowers stress markers.^19,29

Trauma-based interventions like vagal nerve stimulation and meditative practice require a strong clinician-patient partnership that is rooted in trust. In functional medicine, the therapeutic relationship is a crucial part of the healing process and can significantly impact patient health outcomes, particularly in patients with trauma histories. To learn more about the power of the therapeutic partnership and alternative therapies for treating PTSD, please follow the links below.

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References

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2. Williamson JB, Porges EC, Lamb DG, Porges SW. Maladaptive autonomic regulation in PTSD accelerates physiological aging. Front Psychol. 2015;5:1571. doi:3389/fpsyg.2014.01571
3. Ressler K, Berretta S, Bolshakov VY, et al. Post-traumatic stress disorder: clinical and translational neuroscience from cells to circuits. Nat Rev Neurol. 2022;18(5):273-288. doi:1038/s41582-022-00635-8
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25.  Kim AY, Marduy A, de Melo PS, et al. Safety of transcutaneous auricular vagus nerve stimulation (taVNS): a systematic review and meta-analysis. Sci Rep. 2022;12(1):22055. doi:1038/s41598-022-25864-1
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