Page 76 - BH-2-2

Brain & Heart A mini-review on heart-brain communication

established as a modulator of cardiac arrhythmias where BOLD functional magnetic resonance imaging signal is
arrhythmic signals can arise from the insular cortex associated with HF-HRV. Several other studies have also
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and intrinsic cardiac neurons. Myocardial ischemia shown reduced resting-state HF-HRV in depression 51,52 and
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leads to the formation of large inclusion bodies and schizophrenia, where severity of symptoms is correlated
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induces degenerative changes in the dendrites and axons with reduced HRV. Although psychiatric medication
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of these ICNS. While myocardial infarction induces could reduce HRV, medication-free schizophrenic
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hyperexcitability and altered synaptic efficacy, activation patients still exhibit altered HRV complexity. Apart from
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of inflammatory pathways leads to the production of HF-HRV, time domain measures like R-R intervals show a
nerve growth factor, which promotes neuronal sprouting declining pattern in depression while SDNN and RMSSD
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leading to sympathetic hyperinnervation. In response demonstrate a negative correlation with the severity of
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to the ischemic insult, an intrinsic adrenal system of the psychotic symptoms in schizophrenia. Both time domain
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heart increases the production of catecholamines, leading and frequency domain measures suggest altered vagal
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to increased sympathetic tone. It has been hypothesized activity, but the loss of efferent vagal activity has been
that both catecholamine release and activation through established in schizophrenia. Apart from psychiatric
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a feedback system are implicated in ICNS-mediated disorders, altered HRV in patients of coronary artery disease
ischemia remodeling. While the feedback mechanism also leads to abnormal circadian rhythm, which in turn
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involves modulation of reflex control, it is likely that predict severity of symptoms seen in psychiatric disorders,
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activity of ICNS could be translated and observed through such as major depressive disorder, anxiety, bipolar disorder,
measurements of HRV, which has been proposed to predict and schizophrenia. 60,61 These symptoms, which have been
the lifetime risk of cardiac disorders. 41 postulated as the schema of associations between HRV,
HRV is a predictor of autonomic function and can quantitative encephalograms, and cognitive function where
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be obtained to assess short-term or long-term variability. altered brain signals are detected through quantitative
Short-term variability, which can be analyzed using electroencephalogram, cause cognitive dysfunction,
5-min-long recordings, is commonly utilized to study impacting autonomic activity and thereby altering HRV. 61
autonomic function and baroreceptor reflex activity, while 8. HEPs as the language of cardiac afferents
long-term variability analysis provides insights to a variety
of physiological processes such as circadian rhythms, to brain
metabolism, renin-angiotensin-aldosterone system, and HEPs, first described by Jones et al., are electrical potentials
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vagal activity. HRV is studied using various indices that in the brain influenced by cardiac activity, reflecting the
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are broadly categorized into time domain and frequency interoceptive ability. 62,63 HEPs are time-locked to the R wave
domain measures, where a higher range in the time and of the electrocardiogram signal. Changes in the electrical
frequency domain reflects increased parasympathetic activity of the brain in response to cardiac afferents are
activity while decreases in time or frequency domain observed during 50 – 550 ms in 1 s epoch. Differences
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measures are usually linked to disease states. 44,45 Non-linear in HEP are evident in high and low arousal states. Apart
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measures such as entropy represent an underexplored from interoceptive ability reflected by heartbeat perception
area of HRV analysis, but they have been shown to be through mental tracking task, where good heartbeat
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decreased in patients with Takotsubo cardiomyopathy or perceivers reflect positive HEP peaks during attention as
broken-heart syndrome. 46,47 Non-linear measures have compared to poor heartbeat perceivers. The interoceptive-
shown 90% accuracy, 86% specificity, and 95% sensitivity exteroceptive integration is also reflected by HEP. HEPs
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for predicting real-life stress situations, which make them emerge as a new index of heart-brain communication. A case
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potential diagnostic tools of other psychological disorders. report has shown reduced HEP in near-death experience
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in the absence of a bidirectional control in a patient with
7. HRV trends in psychiatric disorders ventricular fibrillation. The molecular mechanism behind
Among the frequency domain measures of HRV, high- the HEP is largely unknown but can be reflected through
frequency (HF) HRV reflects breathing patterns and has its interaction with HRV, where HRV modulates the HEP
shown the most association with altered breathing patterns amplitude at N250 during emotional states and resonant
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in psychiatric illnesses as compared to vagal tone. breathing. Resonant breathing exerts its effects through
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Association of HF domain of HRV has been rigorously biofeedback mechanism, where it improves interoceptive
studied in many psychiatric illnesses; for instance, capacity and reduces sympathetic overload Similarly,
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impaired parasympathetic autonomic modulation leads exercise, which serves as another biofeedback mechanism,
to altered cortical and subcortical processing in post- improves interoceptive capacity in physically fit individuals;
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traumatic stress disorder (PTSD) where a decrease in however, since exercise training induces sinus bradycardia
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Volume 2 Issue 2 (2024) 4 doi: 10.36922/bh.2901