Human autonomic rhythms: vagal cardiac mechanisms in tetraplegic subjects

Publication guidelines for human heart rate and heart rate variability studies in psychophysiology-Part 1: Physiological underpinnings and foundations of measurement

This Committee Report provides methodological, interpretive, and reporting guidance for researchers who use measures of heart rate (HR) and heart rate variability (HRV) in psychophysiological research. We provide brief summaries of best practices in measuring HR and HRV via electrocardiographic and photoplethysmographic signals in laboratory, field (ambulatory), and brain-imaging contexts to address research questions incorporating measures of HR and HRV. The Report emphasizes evidence for the strengths and weaknesses of different recording and derivation methods for measures of HR and HRV. Along with this guidance, the Report reviews what is known about the origin of the heartbeat and its neural control, including factors that produce and influence HRV metrics. The Report concludes with checklists to guide authors in study design and analysis considerations, as well as guidance on the reporting of key methodological details and characteristics of the samples under study. It is expected that rigorous and transparent recording and reporting of HR and HRV measures will strengthen inferences across the many applications of these metrics in psychophysiology. The prior Committee Reports on HR and HRV are several decades old. Since their appearance, technologies for human cardiac and vascular monitoring in laboratory and daily life (i.e., ambulatory) contexts have greatly expanded. This Committee Report was prepared for the Society for Psychophysiological Research to provide updated methodological and interpretive guidance, as well as to summarize best practices for reporting HR and HRV studies in humans.

Temporal relationships among changes in the RR-interval and the powers of the low- and high-frequency components of heart rate variability in normal subjects

Spectral analysis of heart rate variability (HRV) is widely used as a non-invasive method to assess the cardiovascular autonomic function. Of the two main frequency components of HRV, namely low-frequency (LF, 0.04-0.15 Hz) and high-frequency (HF, 0.15-0.4 Hz) components, it is generally accepted that the HF power reflects modulation of heart rate which is mediated by cardiac parasympathetic (vagal) nerve activity. In contrast, the origin and functional correlates of the LF component are still controversial. Although several lines of evidence have indicated a close correlation between LF power and the baroreflex modulation of autonomic outflows, the detailed mechanisms underlying the genesis of the LF component remain unclarified. In this study, we conducted an ultra-short-term (UST) spectral analysis of R-R interval (RRI) time series using Fast Fourier Transform (FFT) with 5- and 25-s windows to clarify the temporal relationships among transient changes in the RRI and, LF and HF powers in healthy subjects. We found that during active standing, transient RRI increases occurred sporadically. The UST spectral analysis revealed that this RRI increase was associated with a simultaneous increase in HF power which was closely linked to the prominent LF power increase. These results indicate that during active standing, increases in LF and HF powers occur simultaneously, and they may reflect enhanced cardiac vagal activity which generates transient bradycardia.

Reliability of Heart Rate Variability During Stable and Disrupted Polysomnographic Sleep

Heart rate variability (HRV) is commonly used within sleep and cardiovascular research, yet HRV reliability across various sleep stages remains equivocal. The present study examined the reliability of frequency- and time-domain HRV within stage II (N2), slow wave (SWS), and rapid eye movement (REM) sleep during both stable and disrupted sleep. We hypothesized that high-frequency (HF) HRV would be reliable in all three sleep stages, low-frequency (LF) HRV would be reliable during N2 and SWS, and that disrupted sleep via spontaneous cortical arousals would decrease HRV reliability. Twenty-seven participants (11 male, 16 female, 26±1 years) were equipped with laboratory polysomnography for one night. Both frequency- and time-domain HRV were analyzed in two 5-10 minute blocks during multiple stable and disrupted sleep cycles across N2, SWS and REM sleep. HF HRV was highly correlated across stable N2 (r=0.839, p<0.001), SWS (r=0.765, p<0.001) and REM (r=0.881, p<0.001). LF HRV was moderate-to-highly correlated during stable cycles of N2 sleep (r=0.694, p < 0.001), SWS, (r=0.765, p < 0.001), and REM (r=0.699, p<0.001) sleep. When stable sleep was compared with disrupted sleep, both time- and frequency-domain HRV were reliable (α>0.90, p<0.05) in N2, SWS, and REM, with the exception of LF HRV during SWS (α=0.62, p=0.089). In conclusion, time- and frequency-domain HRV demonstrated reliability across stable N2, SWS and REM sleep, and remained reliable during disrupted sleep. These findings support the use of HRV during sleep as a tool for assessing cardiovascular health and risk stratification.

Unexpected Cardiovascular Oscillations at 0.1 Hz During Slow Speech Guided Breathing (OM Chanting) at 0.05 Hz

Slow breathing at 0.1 Hz (i.e., 6 cycles per minute, cpm) leads to strong cardiovascular oscillations. However, the impact of breathing below 6 cpm is rarely addressed. We investigated the influence of OM chanting, an ancient Indian mantra, with approx. 3 respiratory cpm (0.05 Hz) on the synchronisation of heart period (RR), respiration (RESP) and systolic blood pressure (SBP). Nine healthy, trained speech practitioners chanted three sequences of five subsequent OM with 2 min pauses in between. Each single OM chanting consisted of taking a deep breath and a long “OM” during expiration and lasted approx. 20 s. ECG, respiration and blood pressure were recorded continuously, of which the RR tachogram, RESP and SBP were derived. Synchronisation between the signals was computed using the phase difference between two signals. During OM chanting synchronisation among the oscillations of RR, SBP and RESP was significantly increased compared to rest. Furthermore, OM chanting at breathing frequencies between 0.046 and 0.057 Hz resulted in 0.1 Hz oscillations in RR and SBP. In conclusion, OM chanting strongly synchronized cardiorespiratory and blood pressure oscillations. Unexpected oscillations at 0.1 Hz in SBP and RR appear at breathing frequencies of approx. 0.05 Hz. Such frequency doubling may originate from an interaction of breathing frequency with endogenous Mayer waves.

Nesfatin-1-like peptide is a negative regulator of cardiovascular functions in zebrafish and goldfish

Nucleobindins (NUCB1 and NUCB2) were originally identified as calcium and DNA binding proteins. Nesfatin-1 (NEFA/nucleobindin-2-Encoded Satiety and Fat-Influencing proteiN-1) is an 82 amino acid anorexigenic peptide encoded in the N-terminal region of NUCB2. We have shown that nesfatin-1 is a cardiosuppressor in zebrafish. Both NUCB1 and NUCB2 possess a -very highly conserved bioactive core. It was found that a nesfatin-1-like peptide (NLP) encoded in NUCB1 suppresses food intake in fish. In this research, we investigated whether NLP has nesfatin-1-like effects on cardiovascular functions. NUCB1/NLP-like immunoreactivity was found in the atrium and ventricle of the heart and skeletal muscle of zebrafish. Intraperitoneal injection (IP) of either zebrafish NLP or rat NLP suppressed cardiac functions in both zebrafish and goldfish. Irisin and RyR1b mRNA expression was downregulated by NLP in zebrafish cardiac and skeletal muscles. However, cardiac ATP2a2 mRNA expression was elevated after NLP injection. Administration of scrambled NLP did not affect irisin, RyR1b or ATP2a2 mRNA expression in zebrafish. Together, these results implicate NLP as a suppressor of cardiovascular physiology in zebrafish and goldfish.

Use of Mayer wave activity to demonstrate aberrant cardiovascular autonomic control following sports concussion injury

Dysregulation of cardiovascular autonomic control is gaining recognition as a prevailing consequence of concussion injury. Characterizing the presence of autonomic dysfunction in concussed persons is inconsistent and conventional metrics of autonomic function cannot differentiate the presence/absence of injury. Mayer wave (MW) activity originates through baroreflex adjustments to blood pressure (BP) oscillations that appear in the low‐frequency (LF: 0.04–0.15 Hz) band of the BP and heart rate (HR) power spectrum after a fast Fourier transform. We prospectively explored MW activity (∼0.1 Hz) in 19 concussed and 19 noninjured athletes for 5 min while seated at rest within 48 h and 1 week of injury. MW activity was derived from the LF band of continuous digital electrocardiogram and beat‐to‐beat BP signals (LFHR, LF‐SBP, MWHR, and MW‐SBP, respectively); a proportion between MWBP and MWHR was computed (cMW). At 48 h, the concussion group had a significantly lower MWBP and cMW than controls; these differences were gone by 1 week. MWHR, LFHR, and LF‐SBP were not different between groups at either visit. Attenuated sympathetic vasomotor tone was present and the central autonomic mechanisms regulating MW activity to the heart and peripheral vasculature became transiently discordant early after concussion with apparent resolution by 1 week.

Non-contact heart rate variability monitoring using Doppler radars located beneath bed mattress: a case report

Background

Heart rate variability (HRV) has been investigated previously in autonomic nervous system-related clinical settings. In these settings, HRV is determined by the time-series heartbeat peak-to-peak intervals using electrocardiography (ECG). To reduce patient discomfort, we designed a Doppler radar-based autonomic nervous activity monitoring system (ANMS) that allows cardiopulmonary monitoring without using ECG electrodes or spirometry monitoring.

Case summary

Using our non-contact ANMS, we observed a bedridden 80-year-old female patient with terminal phase sepsis developed the daytime Cheyne-Stokes respiration (CSR) associated with the attenuation of the low frequency (LF) and high frequency (HF) of HRV components 20 days prior to her death. The patient developed a marked linear decrease in the LF and the HF of HRV components for over 3 days in a row. Furthermore, after the decrease both the LF and the HF showed low and linear values. Around the intersection of the two lines, the decreasing LF and HF lines and the constant LF and HF lines, the ANMS automatically detected the daytime CSR pathogenesis. The attenuation rate of HF (1340 ms²/day) was higher than that of LF (956 ms²/day). Heart rate increased by ∼10 b.p.m. during these 3 days.

Discussion

We detected CSR-associated LF and HF attenuation in a patient with terminal phase sepsis using our ANMS. The proposed system without lead appears promising for future applications in clinical settings, such as remote cardiac monitoring of patients with heart failure at home or in long-term acute care facilities.

Evening Binge Alcohol Disrupts Cardiovagal Tone and Baroreflex Function During Polysomnographic Sleep

STUDY OBJECTIVES

Binge alcohol consumption is associated with increased cardiovascular risk. The effects of evening binge alcohol consumption (i.e., 4-5 beverages within two hours) on the vagal components of HRV and cardiovagal baroreflex sensitivity (cvBRS) during sleep remain largely equivocal. The present study examined the effects of evening binge alcohol consumption on nocturnal cardiac vagal tone and baroreflex sensitivity during stage N2, slow wave (SWS), and rapid eye movement (REM) sleep. We hypothesized that evening binge drinking would reduce HRV and cvBRS in each sleep stage.

METHODS

Following a familiarization night within the laboratory, twenty-three participants were examined following a night of binge alcohol consumption and a fluid control (randomized, crossover design). A quality nocturnal beat-to-beat blood pressure signal was obtained in both conditions in 16 participants (7 men, 9 women; 25±1 years).

RESULTS

Binge drinking reduced both the high frequency (HF) and time-domain components (i.e., pNN50 and RMSSD) of HRV in stage N2 sleep, SWS, and REM. In addition, cvBRS up-up (vagal activation) was reduced following binge alcohol consumption in stage N2 (21±3 vs. 15±3 ms/mmHg, P=0.035) and REM (15[11-28] vs. 11[9-18] ms/mmHg, P=0.009). Binge alcohol consumption reduced cvBRS down-down (vagal withdrawal) in stage N2 (23±2 vs. 14±2 ms/mmHg, P<0.001), SWS (20[14-30] vs. 14[9-17] ms/mmHg, P=0.022), and REM (14[11-24] vs. 10[7-15] ms/mmHg, P=0.006).

CONCLUSIONS

Evening binge alcohol consumption disrupts cardiac vagal tone and baroreflex function during nearly all sleep stages. These findings provide mechanistic insight into the potential role of binge drinking and alcohol abuse on cardiovascular risk.

High-intensity, whole-body exercise improves blood pressure control in individuals with spinal cord injury: A prospective randomized controlled trial

Blood pressure regulation following spinal cord injury (SCI) is often compromised due to impaired vascular sympathetic control, leading to increased reliance on cardiovagal baroreflex sensitivity to maintain pressure. Whole-body exercise improves cardiovagal baroreflex sensitivity in uninjured individuals, though has not been explored in those with SCI. Our objective was to determine changes in cardiovagal baroreflex sensitivity following 6 months of high-intensity, whole-body exercise in individuals with SCI compared to lower-intensity, arms only exercise, or waitlist. This randomized controlled trial recruited individuals with SCI aged 18-40 years old. Sixty-one individuals were randomized, with 38 completing at least one cardiovagal baroreflex sensitivity assessment. Whole-body exercise was performed with hybrid functional electrical stimulation rowing prescribed as two to three times per week, for 30-60 minutes with a target heart rate of >75% of maximum. The arms only exercise group performed upper body rowing exercise with the same prescription as whole-body exercise. Waitlist controls were not enrolled in any explicit training regimen. After 6 months, those in arms only exercise or waitlist crossed over to whole-body exercise. Cardiovagal baroreflex sensitivity was assessed via the neck suction technique at baseline and at three-month intervals thereafter. Intention to treat analysis with a structured equation model demonstrated no significant effect of waitlist control or arms only exercise on cardiovagal baroreflex sensitivity. Whole-body exercise significantly improved cardiovagal baroreflex sensitivity at 6 months for those initially randomized (p = 0.03), as well as those who crossed over from arms only exercise or waitlist control (p = 0.03 for each). However, amount of exercise performed and aerobic gains (VO2max) each poorly correlated with increases in cardiovagal baroreflex sensitivity (R2<0.15). In post-hoc analyses, individuals with paraplegia made significantly greater gains in baroreflex sensitivity compared to those with tetraplegia (p = 0.02), though gains within this group were again poorly correlated to gains in aerobic capacity. Clinicaltrials.gov number NCT02139436.

Cardiovagal baroreflex gain relates to sensory loss after spinal cord injury

Spinal cord injury (SCI) leads to autonomic nervous system damage, resulting in loss of sympathetic control to the vasculature and the heart proportional to injury level. Given maintained cardiac parasympathetic control, we hypothesized that SCI demonstrates a compensatory, higher baroreflex gain compared to able-bodied that relates to injury level (neurological and/or sensory). We compared baroreflex gain (average and across 10-20, 20-30, and 30-40 mmHg input stimuli) derived from neck chamber technique in SCI (N = 29; neurological level C1-T10, sensory zone of partial preservation C4-S4/5; ≤2 yrs since injury) and able-bodied (N = 14). Average gain tended to be higher in able-bodied compared to SCI (p = 0.06), primarily due to higher gains at 10-20 and 20-30 mmHg (p = 0.03, p = 0.06). In SCI, although gain was not related to neurological level, average gain and gain at 10-20 mmHg was related to sensory zone of partial preservation and resting RR-interval (all p < 0.02). Multiple regression showed that both sensory level and RR-interval were strongly predictive of average baroreflex gain (r² = 0.41, p < 0.01) and gain at 10-20 mmHg (r² = 0.51, p < 0.01); gain decreased with higher sensory zone of partial preservation and lower resting RR-interval. Moreover, gain was significantly lower in those with high sensory level compared to both able-bodied (average gain and gain at 10-20 and 20-30 mmHg p < 0.01) and those with low level injury (all p < 0.05). In SCI, sensory zone of partial preservation is more predictive of gain than neurological level. This might reflect that those with high level sensory injuries may have the lowest likelihood of intact cardiac sympathetic innervation and therefore lesser cardiac vagal responsiveness due to vagal-sympathetic interactions.

Haemodynamic effects of hyperventilation on healthy men with different levels of autonomic tone

The topicality of the research is stipulated by insufficient study of the correlation between the functional state of the cardiorespiratory system and autonomic tone. The goal of the research was to analyze the changes of central haemodynamics with 10-minute regulated breathing at the rate of 30 cycles per minute and within 40 minutes of recovery after the test in healthy young men with different levels of autonomic tone. Records of the chest rheoplethysmogram were recorded on a rheograph KhAI-medica standard (KhAI-medica, Kharkiv, Ukraine), a capnogram - in a lateral flow on a infrared capnograph (Datex, Finland), and the duration of R-R intervals was determined by a Polar WIND Link in the program of Polar Protrainer 5.0 (Polar Electro OY, Finland). Systolic and diastolic blood pressure were measured by Korotkov’s auscultatory method by mercury tonometer (Riester, Germany). The indicator of the normalized power of the spectrum in the range of 0.15–0.40 Hz was evaluated by 5-minute records; three groups of persons were distinguished according to its distribution at rest by the method of signal deviation, namely, sympathicotonic, normotonic and parasympathicotonic. The initial level of autonomic tone was found to impact the dynamics of СО2 level in alveolar air during hyperventilation and during recovery thereafter. Thus, PetCО2 was higher (41.3 mm Hg) in parasympathicotonic than in sympathicotonic (39.3 mm Hg) and normotonic (39.5 mm Hg) persons. During the test, R-R interval duration decreased being more expressed in normotonic persons. At the same time, the heart index was found to increase in three groups, and general peripheral resistance – to decrease mostly in normo- and parasympathicotonic persons. In addition, the reliable increase of stroke index and heart index was found in these groups. In the recovery period after hyperventilation, the decrease of tension index and ejection speed was found in normo- and, particularly, parasympathicotonic compared with sympathicotonic men and the increase of tension phase and ejection phase duration.

Grafting Embryonic Raphe Neurons Reestablishes Serotonergic Regulation of Sympathetic Activity to Improve Cardiovascular Function after Spinal Cord Injury

Cardiovascular dysfunction often occurs after high-level spinal cord injury. Disrupting supraspinal vasomotor pathways affects basal hemodynamics and contributes to the development of autonomic dysreflexia (AD). Transplantation of early-stage neurons to the injured cord may reconstruct the descending projections to enhance cardiovascular performance. To determine the specific role of reestablishing serotonergic regulation of hemodynamics, we implanted serotonergic (5-HT⁺) neuron-enriched embryonic raphe nucleus-derived neural stem cells/progenitors (RN-NSCs) into a complete spinal cord transection lesion site in adult female rats. Grafting embryonic spinal cord-derived NSCs or injury alone served as 2 controls. Ten weeks after injury/grafting, histological analysis revealed well-survived grafts and partial integration with host tissues in the lesion site. Numerous graft-derived serotonergic axons topographically projected to the caudal autonomic regions. Neuronal tracing showed that host supraspinal vasomotor pathways regenerated into the graft, and 5-HT⁺ neurons within graft and host brainstem neurons were transsynaptically labeled by injecting pseudorabies virus (PRV-614) into the kidney, indicating reconnected serotonergic circuits regulating autonomic activity. Using an implanted telemeter to record cardiovascular parameters, grafting RN-NSCs restored resting mean arterial pressure to normal levels and remarkably alleviated naturally occurring and colorectal distension-induced AD. Subsequent pharmacological blockade of 5-HT_2A receptors with ketanserin in RN-NSC-grafted rats reduced resting mean arterial pressure and increased heart rate in all but 2 controls. Furthermore, spinal cord retransection below RN-NSC grafts partially eliminated the recovery in AD. Collectively, these data indicate that RN-NSCs grafted into a spinal cord injury site relay supraspinal control of serotonergic regulation for sympathetic activity to improve cardiovascular function.SIGNIFICANCE STATEMENT Disruption of supraspinal vasomotor pathways results in cardiovascular dysfunction following high-level spinal cord injury. To reestablish the descending regulation of autonomic function, we transplanted serotonergic neuron enriched embryonic raphe nucleus-derived neural stem cells/progenitors into the lesion site of completely transected rat spinal cord. Consequently, grafted raphe nucleus-derived neural stem cells/progenitors acted as a neuronal relay to reconnect supraspinal center and spinal sympathetic neurons below the injury. The reconstituted serotonergic regulation of sympathetic activity led to the improvement of hemodynamic parameters and mitigated autonomic dysreflexia. Based on morphological and physiological results, this study validates the effectiveness of transplanting early-stage serotonergic neurons into the spinal cord for cardiovascular functional recovery after spinal cord injury.

Spinal genesis of Mayer waves

Variability in cardiovascular spectra was first described by Stephan Hales in 1733. Traube and Hering initially noted respirophasic variation of the arterial pressure waveform in 1865 and Sigmund Mayer noted a lower frequency oscillation of the same in anesthetized rabbits in 1876. Very low frequency oscillations were noted by Barcroft and Nisimaru in 1932, likely representing vasogenic autorhythmicity. While the origins of Traube Hering and very low frequency oscillatory variability in cardiovascular spectra are well described, genesis mechanisms and functional significance of Mayer waves remain in controversy. Various theories have posited baroreflex and central supraspinal mechanisms for genesis of Mayer waves. Several studies have demonstrated the persistence of Mayer waves following high cervical transection, indicating a spinal capacity for genesis of these oscillations. We suggest a general tendency for central sympathetic neurons to oscillate at the Mayer wave frequency, the presence of multiple Mayer wave oscillators throughout the brainstem and spinal cord, and possible contemporaneous genesis by baroreflex and vasomotor mechanisms.

[Orthostatic hypotension in patients with posttraumatic cervical myelopathy]

AIM

To define the mechanism of formation of orthostatic hypotension in patients with traumatic tetraplegia.

MATERIAL AND METHODS

The study included 64 patients with chronic (more 6 month) and subacute (less 6 month) tetraplegia; the period of observation was about 30 days after admission to the rehabilitation center. Changes in the state of the autonomic nervous systems (heart rate variability at rest and head-up tilt test, sinus arrhythmia) were studied.

RESULTS

In the first 6 months, all patients, and after 6-12 months, one third of the patients experienced orthostatic hypotension, which was accompanied by lower values of the normalized sympatovagal index (LF_n/HF_n) 0.35 (0.260; 0.650) and expiratory-inspiratory coefficient (RR_max/RR_min) 1.09 (1.040; 1200). An increase in expiratory-inspiratory coefficient as a result of physical rehabilitation up to 1.16 (1.120; 1.24) was noted only in patients less than 6 months after injury.

CONCLUSION

Both sympathetic and parasympathetic parts of the autonomic nervous system are involved in the formation of orthostatic hypotension syndrome in patients with spinal tetraplegia.

The effect of level of injury and physical activity on heart rate variability following spinal cord injury

OBJECTIVE/BACKGROUND

To assess frequency domain heart rate variability (HRV) parameters at rest and in response to postural autonomic provocations in individuals with spinal cord injury (SCI) and investigate the autonomic influences on the heart of different physical activities.

DESIGN

Cross-sectional study.

METHODS

Ten subjects with complete cervical SCI and fourteen subjects with complete low thoracic SCI were prospectively recruited from the community and further divided in sedentary and physically active groups, the latter defined as regular weekly 4 hour physical activity for the preceding 3 months. Sixteen healthy individuals matched for sex and age were recruited to participate in the control group. The Low Frequency (LF), High Frequency (HF) powers and the LF/HF ratio of HRV were measured from continuous electrocardiogram (ECG) recordings at rest and after sitting using a fast Fourier transformation.

OUTCOME MEASURES

The LF,HF, and the LF/HF ratio at rest and after sitting.

RESULTS

A significant decrease in all HRV parameters in patients with SCI was found compared to controls. The change in HF, LF and LF/HF following sitting maneuver was significantly greater in controls as compared with the SCI group and greater in subjects with paraplegia as compared to subjects with tetraplegia. Better HRV values and enhanced vagal activity appears to be related to the type of physical activity in active subjects with paraplegia.

CONCLUSION

In this cohort of subjects spectral parameters of HRV were associated with the level of the injury. Passive standing was associated with higher HRV values in subjects with paraplegia.

Assessing low-frequency oscillations in cerebrovascular diseases and related conditions with near-infrared spectroscopy: a plausible method for evaluating cerebral autoregulation?

BACKGROUND

Cerebral autoregulation (CA) is the brain's ability to always maintain an adequate and relatively constant blood supply, which is often impaired in cerebrovascular diseases. Near-infrared spectroscopy (NIRS) examines oxygenated hemoglobin (OxyHb) in the cerebral cortex. Low- and very low-frequency oscillations ( LFOs ≈ 0.1    Hz and VLFOs ≈ 0.05 to 0.01 Hz) in OxyHb have been proposed to reflect CA.

AIM

To systematically review published results on OxyHb LFOs and VLFOs in cerebrovascular diseases and related conditions measured with NIRS.

APPROACH

A systematic search was performed in the MEDLINE database, which generated 36 studies relevant for inclusion.

RESULTS

Healthy people have relatively stable LFOs. LFO amplitude seems to reflect myogenic CA being decreased by vasomotor paralysis in stroke, by smooth muscle damage or as compensatory action in other conditions but can also be influenced by the sympathetic tone. VLFO amplitude is believed to reflect neurogenic and metabolic CA and is lower in stroke, atherosclerosis, and with aging. Both LFO and VLFO synchronizations appear disturbed in stroke, while the former is also altered in internal carotid stenosis and hypertension.

CONCLUSION

We conclude that amplitudes of LFOs and VLFOs are relatively robust measures for evaluating mechanisms of CA and synchronization analyses can show temporal disruption of CA. Further research and more coherent methodologies are needed.

Oscillatory behavior of P wave duration and PR interval in experimental congestive heart failure: a preliminary study

OBJECTIVE

The relationship between the autonomic nervous system (ANS) modulation of the sinus node and heart rate variability has been extensively investigated. The current study sought to evaluate, in an animal experimental model of pacing-induced tachycardia congestive heart failure (CHF), a possible ANS influence on the P wave duration and PR interval oscillations.

APPROACH

Short-term (5 min) time and frequency domain analysis has been obtained in six dogs for the following electrocardiographic intervals: P wave duration (P), from the onset to peak of P wave (P _p), from the onset of P wave to the q onset (PR) and from the end of P wave to the onset of q wave (P _e R). Direct vagal nerve activity (VNA), stellate ganglion nerve activity (SGNA) and electrocardiogram (ECG) intervals have been evaluated contextually by implantation of three bipolar recording leads.

MAIN RESULTS

At the baseline, multiple regression analysis pointed out that VNA was strongly positively associated with the standard deviation of PP and P _e R intervals (r ²:0.997, p  <  0.05). The same variable was also positively associated with high-frequency (HF) of P expressed in normalized units, of P _p, and of P _e R (b: 0.001) (r ²: 0.993; p  <  0.05). During CHF, most of the time and frequency domain variability significantly decreased from 20% to 50% in comparison to the baseline values (p  <  0.05) and SGNA correlated inversely with the low frequency (LF) obtained from P _e R (p  <  0.05) and PR (p  <  0.05) (r ²:0.899, p  <  0.05). LF components, expressed in absolute and normalized power, obtained from all studied intervals, were reduced significantly during CHF. Any difference between the RR and PP spectral components was observed.

SIGNIFICANCE

The data showed a significant relationship between ANS and atrial ECG variables, independent of the cycle duration. In particular, the oscillations were vagal mediated at the baseline, while sympathetic mediated during CHF. Whereas P wave variability might have a clinical utility in CHF management, it needs to be addressed in specific studies.

Heart-Rate Variability-More than Heart Beats?

Heart-rate variability (HRV) is frequently introduced as mirroring imbalances within the autonomous nerve system. Many investigations are based on the paradigm that increased sympathetic tone is associated with decreased parasympathetic tone and vice versa. But HRV is probably more than an indicator for probable disturbances in the autonomous system. Some perturbations trigger not reciprocal, but parallel changes of vagal and sympathetic nerve activity. HRV has also been considered as a surrogate parameter of the complex interaction between brain and cardiovascular system. Systems biology is an inter-disciplinary field of study focusing on complex interactions within biological systems like the cardiovascular system, with the help of computational models and time series analysis, beyond others. Time series are considered surrogates of the particular system, reflecting robustness or fragility. Increased variability is usually seen as associated with a good health condition, whereas lowered variability might signify pathological changes. This might explain why lower HRV parameters were related to decreased life expectancy in several studies. Newer integrating theories have been proposed. According to them, HRV reflects as much the state of the heart as the state of the brain. The polyvagal theory suggests that the physiological state dictates the range of behavior and psychological experience. Stressful events perpetuate the rhythms of autonomic states, and subsequently, behaviors. Reduced variability will according to this theory not only be a surrogate but represent a fundamental homeostasis mechanism in a pathological state. The neurovisceral integration model proposes that cardiac vagal tone, described in HRV beyond others as HF-index, can mirror the functional balance of the neural networks implicated in emotion-cognition interactions. Both recent models represent a more holistic approach to understanding the significance of HRV.

Baroreflex autonomic control in human spinal cord injury: Physiology, measurement, and potential alterations

The arterial baroreflex is a primary regulator of autonomic outflow to effectively regulate acute changes in blood pressure. After a spinal cord injury (SCI), regulation of autonomic function is disrupted, although the damage of the autonomic pathways may not necessarily be related to the severity of injury (i.e. level and completeness). Nonetheless, it can be assumed that there would be greater loss of sympathetic innervation with higher level of injury and that cardiac parasympathetic control would remain intact regardless of injury level. In those with SCI, impaired baroreflex regulation has implications not only for adequate pressure regulation, but also for long term cardiovascular health. In this review, we discuss the expected impact ofan SCI on baroreflex control and the studies that have investigated baroreflex sensitivity in this population. The data generally indicates that baroreflex sensitivity is lesser in those with chronic injuries. However, these findings are counter to the expected effect of an SCI and hence may indicate that the effect of an SCI on baroreflex control might be secondary to long term deconditioning and/or vascular stiffening of baroreceptive arteries. Furthermore, the alterations in the ability to regulate pressure do not impact the relationship between spontaneous heart rate and blood pressure variabilities. In addition, those with SCI are not adequately able to control blood pressure changes in response to orthostasis, resulting in frank hypotension in a significant proportion of those with high level injuries.

Impaired Baroreflex Function during Orthostatic Challenge in Patients after Spinal Cord Injury

The level of spinal cord injury (SCI) affects baroreflex regulation of blood pressure. While a parasympathetic cardiac chronotropic effect is preserved, baroreflex response could be impaired by sympathetic dysfunction under the SCI level. This study was aimed to evaluate the baroreflex function in SCI patients by the analysis of causal interaction between systolic blood pressure (SBP) and inter-beat intervals (IBI). Blood pressure was continuously recorded in 13 cervical SCI patients (CSCI), nine thoracic SCI (ThSCI) and 13 able-bodied controls (Con) during two phases: sitting (PS) and orthostatic challenge (PO). Beat-to-beat SBP and IBI sequences were obtained from continuous blood pressure recording. Closed loop of SBP-IBI interaction was mathematically opened by bivariate autoregressive model; causal coherence and baroreflex sensitivity (BRS) were calculated in baroreflex direction. Coherence quantifies causal synchronicity between SBP and IBI. The gain of transfer function from SBP to IBI represents BRS. PS (medians of CSCI/ThSCI/Con) coherence was 0.28/0.33/0.25 (no significant difference) and PS BRS was 6.98/7.54/6.66 (no difference). PO coherence was 0.18/0.58/0.45 (CSCI < ThCSI and Con; p < 0.01) and PO BRS was 2.38/5.87/6.22 (CSCI < ThCSI and Con; p < 0.01). For position change effect, there was no change in CSCI coherence; for ThSCI and Con, PS < PO (p < 0.05). For BRS in the CSCI group, PS < PO (p < 0.01); for ThSCI and Con, there was no change. BRS and coherence correlated negatively with SCI level (p < 0.01). In conclusion, baroreflex dysfunction in SCI patients was detected using causal analysis methods during orthostatic challenge only. Baroreflex dysfunction is probably an important mechanism of the more expressed blood pressure decrease associated with CSCI. The severity of autonomic dysfunction was related to SCI level.

Acute Effects of Caffeine on Heart Rate Variability, Blood Pressure and Tidal Volume in Paraplegic and Tetraplegic Compared to Able-Bodied Individuals: A Randomized, Blinded Trial

Caffeine increases sympathetic nerve activity in healthy individuals. Such modulation of nervous system activity can be tracked by assessing the heart rate variability. This study aimed to investigate the influence of caffeine on time- and frequency-domain heart rate variability parameters, blood pressure and tidal volume in paraplegic and tetraplegic compared to able-bodied participants. Heart rate variability was measured in supine and sitting position pre and post ingestion of either placebo or 6 mg caffeine in 12 able-bodied, 9 paraplegic and 7 tetraplegic participants in a placebo-controlled, randomized and double-blind study design. Metronomic breathing was applied (0.25 Hz) and tidal volume was recorded during heart rate variability assessment. Blood pressure, plasma caffeine and epinephrine concentrations were analyzed pre and post ingestion. Most parameters of heart rate variability did not significantly change post caffeine ingestion compared to placebo. Tidal volume significantly increased post caffeine ingestion in able-bodied (p = 0.021) and paraplegic (p = 0.036) but not in tetraplegic participants (p = 0.34). Systolic and diastolic blood pressure increased significantly post caffeine in able-bodied (systolic: p = 0.003; diastolic: p = 0.021) and tetraplegic (systolic: p = 0.043; diastolic: p = 0.042) but not in paraplegic participants (systolic: p = 0.09; diastolic: p = 0.33). Plasma caffeine concentrations were significantly increased post caffeine ingestion in all three groups of participants (p<0.05). Plasma epinephrine concentrations increased significantly in able-bodied (p = 0.002) and paraplegic (p = 0.032) but not in tetraplegic participants (p = 0.63). The influence of caffeine on the autonomic nervous system seems to depend on the level of lesion and the extent of the impairment. Therefore, tetraplegic participants may be less influenced by caffeine ingestion.

Cardiovascular response to peak voluntary exercise in males with cervical spinal cord injury

CONTEXT/OBJECTIVE

Traumatic damage to the cervical spinal cord is usually associated with a disruption of the autonomic nervous system (ANS) and impaired cardiovascular control both during and following exercise. The magnitude of the cardiovascular dysfunction remains unclear. The aim of the current study was to compare cardiovascular responses to peak voluntary exercise in individuals with tetraplegia and able-bodied participants.

DESIGN

A case-control study.

SUBJECTS

Twenty males with cervical spinal cord injury (SCI) as the Tetra group and 27 able-bodied males as the Control group were included in the study.

OUTCOME MEASURES

Blood pressure (BP) response one minute after the peak exercise, peak heart rate (HRpeak), and peak oxygen consumption (VO2peak) on an arm crank ergometer were measured. In the second part of the study, 17 individuals of the Control group completed the Tetra group's workload protocol with the same parameters recorded.

RESULTS

There was no increase in BP in response to the exercise in the Tetra group. Able-bodied individuals exhibited significantly increased post-exercise systolic BP after the maximal graded exercise test (123±16%) and after completion of the Tetra group's workload protocol (114±11%) as compared to pre-exercise. The Tetra group VO2peak was 59% and the HRpeak was 73% of the Control group VO2peak and HRpeak, respectively.

CONCLUSIONS

BP did not increase following maximal arm crank exercise in males with a cervical SCI unlike the increases observed in the Control group. Some males in the Tetra group appeared to be at risk of severe hypotension following high intensity exercise, which can limit the ability to progressive increase and maintain high intensity exercise.

The utility of low frequency heart rate variability as an index of sympathetic cardiac tone: a review with emphasis on a reanalysis of previous studies

This article evaluates the suitability of low frequency (LF) heart rate variability (HRV) as an index of sympathetic cardiac control and the LF/high frequency (HF) ratio as an index of autonomic balance. It includes a comprehensive literature review and a reanalysis of some previous studies on autonomic cardiovascular regulation. The following sources of evidence are addressed: effects of manipulations affecting sympathetic and vagal activity on HRV, predictions of group differences in cardiac autonomic regulation from HRV, relationships between HRV and other cardiac parameters, and the theoretical and mathematical bases of the concept of autonomic balance. Available data challenge the interpretation of the LF and LF/HF ratio as indices of sympathetic cardiac control and autonomic balance, respectively, and suggest that the HRV power spectrum, including its LF component, is mainly determined by the parasympathetic system.

Reduced metaboreflex control of blood pressure during exercise in individuals with intellectual disability: a possible contributor to exercise intolerance

The aim was to investigate the hemodynamic responses to isometric handgrip exercise (HG) and examine the role of the muscle metaboreflex in the exercise pressor response in individuals with intellectual disability (IID) and non-disabled control subjects. Eleven males with mild-moderate intellectual disabilities and eleven non-disabled males performed a testing protocol involving 3-min periods of baseline, HG exercise (at 30% MVC), circulatory occlusion, and recovery. The same protocol was repeated without occlusion. At baseline, no differences were detected between groups in beat-to-beat mean arterial pressure (MAP), heart rate (HR), stroke volume, and peripheral resistance. IID were able to sustain an exercise MAP response at comparable levels to the control group exerting similar peripheral resistance; however, IID exhibited a blunted chronotropic response to HG and a diminished exercise vagal withdrawal compared to controls. During occlusion, IID exhibited a lower pressor response than their control peers, associated with a lower increase in peripheral resistance during this task. In conclusion, although intellectual disabilities can be the consequence of many different genes, IID share common deficits in the chronotropic response to exercise and a blunted metaboreflex-induced pressor response.

Statistical analysis of RR series variability in spinal cord injured persons

Disabled persons with spinal cord injury are prone to cardiovascular dysfunction and an increased risk of cardiovascular disease. Rehabilitation of the disabled person is a critical task as it involves multiple therapies. Physical exercise is an important component of rehabilitation, and depends on cardiovascular health. Reduced RR variability is a marker of poor cardiac health. Time domain RR variability analysis of 38 normal healthy subjects and 20 spinal cord injured subjects has been carried out and compared. In this study, RR intervals were recorded in three different modes or positions: supine, sitting and five-second rhythm respiration. At a time of 150 s RR interval data were acquired in each mode and analysed. Statistical parameters (mean, HR, STD, NN50 and pNN50) were calculated. It was observed that most of the indices were significantly and substantially altered in spinal cord injured persons.

Heart rate variability - a historical perspective

Heart rate variability (HRV), the beat-to-beat variation in either heart rate or the duration of the R-R interval - the heart period, has become a popular clinical and investigational tool. The temporal fluctuations in heart rate exhibit a marked synchrony with respiration (increasing during inspiration and decreasing during expiration - the so called respiratory sinus arrhythmia, RSA) and are widely believed to reflect changes in cardiac autonomic regulation. Although the exact contributions of the parasympathetic and the sympathetic divisions of the autonomic nervous system to this variability are controversial and remain the subject of active investigation and debate, a number of time and frequency domain techniques have been developed to provide insight into cardiac autonomic regulation in both health and disease. It is the purpose of this essay to provide an historical overview of the evolution in the concept of HRV. Briefly, pulse rate was first measured by ancient Greek physicians and scientists. However, it was not until the invention of the "Physician's Pulse Watch" (a watch with a second hand that could be stopped) in 1707 that changes in pulse rate could be accurately assessed. The Rev. Stephen Hales (1733) was the first to note that pulse varied with respiration and in 1847 Carl Ludwig was the first to record RSA. With the measurement of the ECG (1895) and advent of digital signal processing techniques in the 1960s, investigation of HRV and its relationship to health and disease has exploded. This essay will conclude with a brief description of time domain, frequency domain, and non-linear dynamic analysis techniques (and their limitations) that are commonly used to measure HRV.

Comparison of 24-hour cardiovascular and autonomic function in paraplegia, tetraplegia, and control groups: implications for cardiovascular risk

BACKGROUND

Fluctuations in 24-hour cardiovascular hemodynamics, specifically heart rate (HR) and blood pressure (BP), are thought to reflect autonomic nervous system (ANS) activity. Persons with spinal cord injury (SCI) represent a model of ANS dysfunction, which may affect 24-hour hemodynamics and predispose these individuals to increased cardiovascular disease risk.

OBJECTIVE

To determine 24-hour cardiovascular and ANS function among individuals with tetraplegia (n=20; TETRA: C4-C8), high paraplegia (n=10; HP: T2-T5), low paraplegia (n=9; LP: T7-T12), and non-SCI controls (n=10). Twenty-four-hour ANS function was assessed by time domain parameters of heart rate variability (HRV); the standard deviation of the 5-minute average R-R intervals (SDANN; milliseconds/ms), and the root-mean square of the standard deviation of the R-R intervals (rMSSD; ms). Subjects wore 24-hour ambulatory monitors to record HR, HRV, and BP. Mixed analysis of variance (ANOVA) revealed significantly lower 24-hour BP in the tetraplegic group; however, BP did not differ between the HP, LP, and control groups. Mixed ANOVA suggested significantly elevated 24-hour HR in the HP and LP groups compared to the TETRA and control groups (P<0.05); daytime HR was higher in both paraplegic groups compared to the TETRA and control groups (P<0.01) and nighttime HR was significantly elevated in the LP group compared to the TETRA and control groups (P<0.01). Twenty-four-hour SDANN was significantly increased in the HP group compared to the LP and TETRA groups (P<0.05) and rMSSD was significantly lower in the LP compared to the other three groups (P<0.05). Elevated 24-hour HR in persons with paraplegia, in concert with altered HRV dynamics, may impart significant adverse cardiovascular consequences, which are currently unappreciated.

Heart rate variability - a historical perspective

Heart rate variability (HRV), the beat-to-beat variation in either heart rate or the duration of the R-R interval - the heart period, has become a popular clinical and investigational tool. The temporal fluctuations in heart rate exhibit a marked synchrony with respiration (increasing during inspiration and decreasing during expiration - the so called respiratory sinus arrhythmia, RSA) and are widely believed to reflect changes in cardiac autonomic regulation. Although the exact contributions of the parasympathetic and the sympathetic divisions of the autonomic nervous system to this variability are controversial and remain the subject of active investigation and debate, a number of time and frequency domain techniques have been developed to provide insight into cardiac autonomic regulation in both health and disease. It is the purpose of this essay to provide an historical overview of the evolution in the concept of HRV. Briefly, pulse rate was first measured by ancient Greek physicians and scientists. However, it was not until the invention of the "Physician's Pulse Watch" (a watch with a second hand that could be stopped) in 1707 that changes in pulse rate could be accurately assessed. The Rev. Stephen Hales (1733) was the first to note that pulse varied with respiration and in 1847 Carl Ludwig was the first to record RSA. With the measurement of the ECG (1895) and advent of digital signal processing techniques in the 1960s, investigation of HRV and its relationship to health and disease has exploded. This essay will conclude with a brief description of time domain, frequency domain, and non-linear dynamic analysis techniques (and their limitations) that are commonly used to measure HRV.

Altered cardiovascular responses to tracheal intubation in patients with complete spinal cord injury: relation to time course and affected level

BACKGROUND

We determined cardiovascular responses to tracheal intubation in relation to the time since injury in patients with different levels of spinal cord injury.

METHODS

Two hundred and fourteen patients with complete cord injury were studied. They were either quadriplegics (>C7, n=71) or paraplegics (<T5, n=143), and were subdivided into six groups each according to the time since injury: <4 week (acute), 4 week-1 yr, 1-5, 5-10, 10-20, and >20 yr. Twenty patients with no cord injury served as controls. Systolic arterial pressure (SAP), heart rate (HR), and plasma catecholamine concentrations were determined.

RESULTS

Intubation did not affect SAP in the quadriplegics regardless of the time post-injury, but it significantly increased SAP in all paraplegics. Moreover, the pressor response was enhanced in the paraplegics who were 10 yr or more since injury (P<0.05). HR increased significantly in all groups; the magnitude of the increase was less only in acute quadriplegics compared with controls. Plasma concentrations of norepinephrine increased in every group except for the quadriplegics within 4 weeks of injury. The maximum increases in SAP, HR, and norepinephrine from awake baseline values were smaller in the quadriplegics than in the paraplegics (P<0.01).

CONCLUSIONS

The cardiovascular and catecholamine responses to intubation change as a function of the time elapsed and the level of the cord injury. In this study, the pressor response to tracheal intubation was abolished in the quadriplegics but not in paraplegics; indeed, it was enhanced at 10 yr or more since injury in this group.

Modelling the respiratory chemoreflex control of Acid-base balance

A mathematical model is presented describing the respiratory chemoreflex relations between the partial pressures of carbon dioxide and oxygen at the central and peripheral chemoreceptors and the resulting pulmonary ventilation. These chemoreflex relations in terms of carbon dioxide partial pressures are transformed to relations in terms of hydrogen ion concentrations in brain tissue and arterial blood using the Stewart approach to acid-base balance. In this way, the chemoreflex relations can be expressed in terms of the actual stimulus to the respiratory chemoreceptors.

Heart rate variability explored in the frequency domain: a tool to investigate the link between heart and behavior

The neural regulation of circulatory function is mainly effected through the interplay of the sympathetic and vagal outflows. This interaction can be explored by assessing cardiovascular rhythmicity with appropriate spectral methodologies. Spectral analysis of cardiovascular signal variability, and in particular of RR period (heart rate variability, HRV), is a widely used procedure to investigate autonomic cardiovascular control and/or target function impairment. The oscillatory pattern which characterizes the spectral profile of heart rate and arterial pressure short-term variability consists of two major components, at low (LF, 0.04-0.15Hz) and high (HF, synchronous with respiratory rate) frequency, respectively, related to vasomotor and respiratory activity. With this procedure the state of sympathovagal balance modulating sinus node pacemaker activity can be quantified in a variety of physiological and pathophysiological conditions. Changes in sympathovagal balance can be often detected in basal conditions, however a reduced responsiveness to an excitatory stimulus is the most common feature that characterizes numerous pathophysiological states. Moreover the attenuation of an oscillatory pattern or its impaired responsiveness to a given stimulus can also reflect an altered target function and thus can furnish interesting prognostic markers. The dynamic assessment of these autonomic changes may provide crucial diagnostic, therapeutic and prognostic information, not only in relation to cardiovascular, but also non-cardiovascular disease. As linear methodologies fail to provide significant information in conditions of extremely reduced variability (e.g. strenuous exercise, heart failure) and in presence of rapid and transients changes or coactivation of the two branches of autonomic nervous system, the development of new non-linear approaches seems to provide a new perspective in investigating neural control of cardiovascular system.

Assessment of autonomic function in cardiovascular disease: physiological basis and prognostic implications

Certain abnormalities of autonomic function in the setting of structural cardiovascular disease have been associated with an adverse prognosis. Various markers of autonomic activity have received increased attention as methods for identifying patients at risk for sudden death. Both the sympathetic and the parasympathetic limbs can be characterized by tonic levels of activity, which are modulated by, and respond reflexively to, physiological changes. Heart rate provides an index of the net effects of autonomic tone on the sinus node, and carries prognostic significance. Heart rate variability, though related to heart rate, assesses modulation of autonomic control of heart rate and carries additional prognostic information, which in some cases is more powerful than heart rate alone. Heart rate recovery after exercise represents the changes in autonomic tone that occur immediately after cessation of exercise. This index has also been shown to have prognostic significance. Autonomic evaluation during exercise and recovery may be important prognostically, because these are high-risk periods for sudden death, and the autonomic changes that occur with exercise could modulate this high risk. These markers provide related, but not redundant information about different aspects of autonomic effects on the sinus node.