Increased phase synchronization and decreased cerebral autoregulation during fainting in the young

Characteristics of corticomuscular coupling during wheelchair Tai Chi in patients with spinal cord injury

BACKGROUND

Wheelchair Tai Chi (WCTC) has been proved to have benefits for the brain and motor system of spinal cord injury (SCI) patients. However, the characteristics of corticomuscular coupling during WCTC are scarcely known. We aimed to investigate changes following SCI on corticomuscular coupling, and further compare the coupling characteristics of WCTC with aerobic exercise in SCI patients.

METHODS

A total of 15 SCI patients and 25 healthy controls were recruited. The patients had to perform aerobic exercise and WCTC, while healthy controls needed to complete a set of WCTC. The participants accomplished the test following the tutorial video in a sitting position. The upper limb muscle activation was measured from upper trapezius, medial deltoid, biceps brachii and triceps brachii with surface electromyography. Cortical activity in the prefrontal cortex, premotor cortex, supplementary motor area and primary motor cortex was simultaneously collected by functional near-infrared spectroscopy. The functional connectivity, phase synchronization index and coherence values were then calculated and statistically analyzed.

RESULTS

Compared to healthy controls, changes in functional connectivity and higher muscle activation were observed in the SCI group. There was no significant difference in phase synchronization between groups. Among patients, significantly higher coherence values between the left biceps brachii as well as the right triceps brachii and contralateral regions of interest were found during WCTC than during aerobic exercise.

CONCLUSION

The patients may compensate for the lack of corticomuscular coupling by enhancing muscle activation. This study demonstrated the potential and advantages of WCTC in eliciting corticomuscular coupling, which may optimize rehabilitation following SCI.

Increased respiratory modulation of cardiovascular control reflects improved blood pressure regulation in pregnancy

Hypertensive pregnancy disorders put the maternal-fetal dyad at risk and are one of the leading causes of morbidity and mortality during pregnancy. Multiple efforts have been made to understand the physiological mechanisms behind changes in blood pressure. Still, to date, no study has focused on analyzing the dynamics of the interactions between the systems involved in blood pressure control. In this work, we aim to address this question by evaluating the phase coherence between different signals using wavelet phase coherence. Electrocardiogram, continuous blood pressure, electrocardiogram-derived respiration, and muscle sympathetic nerve activity signals were obtained from ten normotensive pregnant women, ten normotensive non-pregnant women, and ten pregnant women with preeclampsia during rest and cold pressor test. At rest, normotensive pregnant women showed higher phase coherence in the high-frequency band (0.15-0.4 Hz) between muscle sympathetic nerve activity and the RR interval, blood pressure, and respiration compared to non-pregnant normotensive women. Although normotensive pregnant women showed no phase coherence differences with respect to hypertensive pregnant women at rest, higher phase coherence between the same pairs of variables was found during the cold pressor test. These results suggest that, in addition to the increased sympathetic tone of normotensive pregnant women widely described in the existing literature, there is an increase in cardiac parasympathetic modulation and respiratory-driven modulation of muscle sympathetic nerve activity and blood pressure that could compensate sympathetic increase and make blood pressure control more efficient to maintain it in normal ranges. Moreover, blunted modulation could prevent its buffer effect and produce an increase in blood pressure levels, as observed in the hypertensive women in this study. This initial exploration of cardiorespiratory coupling in pregnancy opens the opportunity to follow up on more in-depth analyses and determine causal influences.

Spectral decomposition of cerebrovascular and cardiovascular interactions in patients prone to postural syncope and healthy controls

We present a framework for the linear parametric analysis of pairwise interactions in bivariate time series in the time and frequency domains, which allows the evaluation of total, causal and instantaneous interactions and connects time- and frequency-domain measures. The framework is applied to physiological time series to investigate the cerebrovascular regulation from the variability of mean cerebral blood flow velocity (CBFV) and mean arterial pressure (MAP), and the cardiovascular regulation from the variability of heart period (HP) and systolic arterial pressure (SAP). We analyze time series acquired at rest and during the early and late phase of head-up tilt in subjects developing orthostatic syncope in response to prolonged postural stress, and in healthy controls. The spectral measures of total, causal and instantaneous coupling between HP and SAP, and between MAP and CBFV, are averaged in the low-frequency band of the spectrum to focus on specific rhythms, and over all frequencies to get time-domain measures. The analysis of cardiovascular interactions indicates that postural stress induces baroreflex involvement, and its prolongation induces baroreflex dysregulation in syncope subjects. The analysis of cerebrovascular interactions indicates that the postural stress enhances the total coupling between MAP and CBFV, and challenges cerebral autoregulation in syncope subjects, while the strong sympathetic activation elicited by prolonged postural stress in healthy controls may determine an increased coupling from CBFV to MAP during late tilt. These results document that the combination of time-domain and spectral measures allows us to obtain an integrated view of cardiovascular and cerebrovascular regulation in healthy and diseased subjects.

Cerebral blood flow self-regulation in depression

BACKGROUND

Depression is a common neuropsychiatric disease with a high prevalence rate. Sleep problems, memory decline, dizziness and headaches are the most common neurological symptoms in depressed patients. Abnormality of cerebral blood flow (CBF) has been observed in depressive patients, but those patients did not have intracranial structural damage. Both of those phenomena might be related to cerebral blood flow self-regulation (CBFSR: cerebral blood flow self-regulation). CBFSR can maintain CBF relatively stable in response to changes in neurological and metabolic factors. Therefore, this review aimed to discuss CBFSR in depression.

METHODS

We searched for keywords such as "depression", "cerebral blood flow", "cerebral autoregulation", "cerebrovascular reactivity" and the words related to depression. We analyzed whether there is a change in the CBFSR in depression, further explored whether there is a relationship between the pathogenesis of depression and the CBFSR, and discussed the possible mechanism of impaired CBFSR in patients with depression.

RESULTS

Discovered by the literature review, CBFSR is significantly impaired in depressed patients. The level of circulating markers of endothelial dysfunction, nitric oxide, inflammatory cytokines, glucocorticoid and monoamine neurotransmitters is mostly abnormal in depression, which affected the CBFSR to varying degrees.

LIMITATIONS

Limitations include the small number of direct studies about depression and CBFSR mechanisms.

CONCLUSION

CBFSR is impaired in depression. The underlying mechanisms include endothelial dysfunction, overactivation of microglia and changes of cytokines, hyperactivation of the HPA axis, increased oxidative stress, monoamine neurotransmitter disorders, etc. These deepened our understanding of the clinical symptoms of depressed patients.

When Sinus Tachycardia Becomes Too Much: Negative Effects of Excessive Upright Tachycardia on Cardiac Output in Vasovagal Syncope, Postural Tachycardia Syndrome, and Inappropriate Sinus Tachycardia

BACKGROUND

Upright posture reduces venous return, stroke volume, and cardiac output (CO) while causing reflex sinus rate (heart rate [HR]) increase. Yet, in inappropriate sinus tachycardia (IST), postural tachycardia syndrome (POTS), and vasovagal syncope (VVS), symptomatic excessive HR occurs. We hypothesized that CO reaches maximum as function of HR in all.

METHODS

We recruited 12 healthy controls, 9 IST, 30 VVS, and 30 POTS patients (13-23years) selected randomly by disorder not by HR, each fulfilled appropriate diagnostic criteria. Subjects were instrumented for electrocardiography, beat-to-beat blood pressure, respiratory rate, CO-Modelflow algorithm, and central blood volume from impedance cardiography; 10-minute data were collected supine; subjects were tilted head-up for ≤10 minutes. We computed phase differences, ΔΦ, between fluctuations of HR (ΔHR) and CO (ΔCO) tabulating data when phases were synchronized, determined by a squared nonlinear phase synchronization index >0.5, describing extent/validity of CO/HR coupling. We graphed results supine, 1-minute post-tilt-up, mid-tilt, and pre-tilt-down using polar coordinates (HR-radius, ΔΦ-angle) plotting cos(ΔΦ) versus HR to determine if transition HR exists at which in-phase shifts to antiphase above which CO decreases when HR further increases.

RESULTS

At baseline HR, diastolic and mean arterial pressures in IST and POTS were higher versus controls. Upright HR increased most in POTS then IST and VVS, with diverse changes in CO, SVR, and central blood volume. Each patient grouping was separately and collectively analyzed for HR change showing transition from in-phase to anti-phase (ΔΦ) as HR increased: HRtransition=115±6 (IST), 123±8 (POTS), 124±7 (VVS), P=ns. Controls never reached transitional HR.

CONCLUSIONS

Excessive HR independently and equivalently reduces upright CO, in IST, POTS, and VVS.

Physical fitness contributes to cardio-respiratory synchronization

Cardio-respiratory synchronization is a phenomenon of particular interest- especially at a 1:1 ratio- and may give greater insight into the underlying mechanisms of cardio-respiratory communication. Synchronization of this ratio is hypothesised to occur when breathing rate exceeds heart rate, which is the premise of this research. A novel experimental design focused on guiding elevated respiration to induce the entrainment of heart rate, and produce an equivalent rise in value. Application of instantaneous phase for identification and analysis of synchronization allowed for a reliable method of measuring the interaction between these stochastic processes. We have identified 1:1 phase synchronization in all volunteers measured. Longer synchronization episodes were observed reliably in athletic individuals, corroborating previous research for spontaneous breathing. This observation suggests that cardio-respiratory synchronization at all respiration rates is associated with a common underlying communication mechanism. Furthermore, it presents cardio-respiratory synchronization as a potential future measurement of fitness and autonomic health.

Multivariate Analysis of Joint Motion Data by Kinect: Application to Parkinson's Disease

Analysis of joint motion data (AJMD) by Kinect, such as velocity, has been widely used in many research fields, many of which focused on how one joint moves with another, namely bivariate AJMD. However, these studies might not accurately reflect the motor symptoms in patients. The human body can be divided into six widely accepted parts (head, trunk and four limbs), which are interrelated and interact with each other. Therefore, in this study we attempted to investigate how the major joints of one body part move with the ones in another body part, namely multivariate AJMD. For method illustration, the motion data of sit-to-stand-to-sit for healthy participants and people with Parkinson disease (PD) were employed. Four types of multivariate AJMD were investigated by eigenspace-maximal-information-canonical-correlation-analysis, which obtained maximal- information-eigen-coefficients (MIECes), the parameters for quantifying the correlation between two sets of joints located in two different body parts. The results show that healthy participants have significantly higher MIECes than the PD patients (p-value < 0.0001). Furthermore, the area under the receiver operating characteristic curve value for the classification between healthy participants and PD patients reaches up to 1.00. In conclusion, we demonstrated the possibility of using multivariate AJMD for motion feature extraction, which may be helpful for medical research and engineering.

Wavelet decomposition analysis is a clinically relevant strategy to evaluate cerebrovascular buffering of blood pressure after spinal cord injury

The capacity of the cerebrovasculature to buffer changes in blood pressure (BP) is crucial to prevent stroke, the incidence of which is three- to fourfold elevated after spinal cord injury (SCI). Disruption of descending sympathetic pathways within the spinal cord due to cervical SCI may result in impaired cerebrovascular buffering. Only linear analyses of cerebrovascular buffering of BP, such as transfer function, have been used in SCI research. This approach does not account for inherent nonlinearity and nonstationarity components of cerebrovascular regulation, often depends on perturbations of BP to increase the statistical power, and does not account for the influence of arterial CO₂ tension. Here, we used a nonlinear and nonstationary analysis approach termed wavelet decomposition analysis (WDA), which recently identified novel sympathetic influences on cerebrovascular buffering of BP occurring in the ultra-low-frequency range (ULF; 0.02-0.03Hz). WDA does not require BP perturbations and can account for influences of CO₂ tension. Supine resting beat-by-beat BP (Finometer), middle cerebral artery blood velocity (transcranial Doppler), and end-tidal CO₂ tension were recorded in cervical SCI ( n = 14) and uninjured ( n = 16) individuals. WDA revealed that cerebral blood flow more closely follows changes in BP in the ULF range ( P = 0.0021, Cohen's d = 0.89), which may be interpreted as an impairment in cerebrovascular buffering of BP. This persisted after accounting for CO₂. Transfer function metrics were not different in the ULF range, but phase was reduced at 0.07-0.2 Hz ( P = 0.03, Cohen's d = 0.31). Sympathetically mediated cerebrovascular buffering of BP is impaired after SCI, and WDA is a powerful strategy for evaluating cerebrovascular buffering in clinical populations.

Applying time-frequency analysis to assess cerebral autoregulation during hypercapnia

OBJECTIVE

Classic methods for assessing cerebral autoregulation involve a transfer function analysis performed using the Fourier transform to quantify relationship between fluctuations in arterial blood pressure (ABP) and cerebral blood flow velocity (CBFV). This approach usually assumes the signals and the system to be stationary. Such an presumption is restrictive and may lead to unreliable results. The aim of this study is to present an alternative method that accounts for intrinsic non-stationarity of cerebral autoregulation and the signals used for its assessment.

METHODS

Continuous recording of CBFV, ABP, ECG, and end-tidal CO2 were performed in 50 young volunteers during normocapnia and hypercapnia. Hypercapnia served as a surrogate of the cerebral autoregulation impairment. Fluctuations in ABP, CBFV, and phase shift between them were tested for stationarity using sphericity based test. The Zhao-Atlas-Marks distribution was utilized to estimate the time-frequency coherence (TFCoh) and phase shift (TFPS) between ABP and CBFV in three frequency ranges: 0.02-0.07 Hz (VLF), 0.07-0.20 Hz (LF), and 0.20-0.35 Hz (HF). TFPS was estimated in regions locally validated by statistically justified value of TFCoh. The comparison of TFPS with spectral phase shift determined using transfer function approach was performed.

RESULTS

The hypothesis of stationarity for ABP and CBFV fluctuations and the phase shift was rejected. Reduced TFPS was associated with hypercapnia in the VLF and the LF but not in the HF. Spectral phase shift was also decreased during hypercapnia in the VLF and the LF but increased in the HF. Time-frequency method led to lower dispersion of phase estimates than the spectral method, mainly during normocapnia in the VLF and the LF.

CONCLUSION

The time-frequency method performed no worse than the classic one and yet may offer benefits from lower dispersion of phase shift as well as a more in-depth insight into the dynamic nature of cerebral autoregulation.

Indexes of cerebral autoregulation do not reflect impairment in syncope: insights from head-up tilt test of vasovagal and autonomic failure subjects

PURPOSE

The study of dynamic cerebral autoregulation (CA), which adapts cerebral blood flow to arterial blood pressure (ABP) fluctuations, has been limited in orthostatic intolerance syndromes, mainly due to its stationary prerequisites hardly to meet during maneuvers to provoke syncope itself. New techniques of continuous estimates of CA could overcome this pitfall. We aimed to evaluate CA during head-up tilt test in common conditions causing syncope.

METHODS

We compared three groups: eight controls; eight patients with autonomic failure due to familial amyloidotic polyneuropathy; eight patients with vasovagal syncope (VVS). ABP and cerebral blood flow velocity (CBFV) were measured with Finometer^® and transcranial Doppler. We calculated cerebrovascular resistance index (CVRi), critical closing pressure (CrCP) and resistance area product (RAP), and derived CA continuously from autoregulation index [ARI(t)].

RESULTS

With HUTT, AF subjects showed a pronounced decrease in CBFV (-36 ± 17 versus -7 ± 6%, p < 0.0001), ABP (-29 ± 27 versus 7 ± 12%, p < 0.0001) and RAP (-17 ± 23 versus 3 ± 18%, p < 0.0001) but not CVRi (p = 0.110). VVS subjects showed progressive cerebral vasoconstriction prior to syncope, (reduced CBFV 19 ± 15 versus 1 ± 6, p < 0.000; increased RAP 12 ± 18 versus 2 ± 3%, p = 0.024 and CVRi 12 ± 18 versus 2 ± 3%, p = 0.005). ARI(t) increased significantly in AF patients (5.7 ± 1.2 versus 6.9 ± 1.2, p = 0.040) and VVS (5.8 ± 1.2 versus 7.3 ± 1.2, p = 0.015) in response to ABP fall during syncope.

CONCLUSIONS

Our data suggest that dynamic cerebral autoregulatory response to orthostatic challenge is neither affected by autonomic dysfunction nor in neutrally mediated syncope. This study also emphasizes that RAP + CrCP model is more informative than CVRi, mainly during cerebral vasodilatory response to orthostatic hypotension.

Mechanisms of Vasovagal Syncope in the Young: Reduced Systemic Vascular Resistance Versus Reduced Cardiac Output

BACKGROUND

Syncope is a sudden transient loss of consciousness and postural tone caused by cerebral hypoperfusion. The most common form is vasovagal syncope (VVS). Presyncopal progressive early hypotension in older VVS patients is caused by reduced cardiac output (CO); younger patients have reduced systemic vascular resistance (SVR). Using a priori criteria for reduced CO (↓CO) and SVR (↓SVR), we studied 48 recurrent young fainters comparing subgroups of VVS with VVS-↓CO, VVS-↓SVR, and both VVS-↓CO&↓SVR.

METHODS AND RESULTS

Subjects were studied supine and during 70-degrere upright tilt with a Finometer to continuously measure blood pressure, CO, and SVR and impedance plethysmography to estimate thoracic, splanchnic, pelvic, and calf blood volumes, blood flows, and vascular resistances and electrocardiogram to measure heart rate and rhythm. Central blood volume was decreased in all VVS compared to control. VVS-↓CO was associated with decreased splanchnic blood flow and increased splanchnic blood pooling compared to control. Seventy-five percent of VVS patients had reduced SVR, including 23% who also had reduced CO. Many VVS-↓SVR increased CO during tilt, with no difference in splanchnic pooling, caused by significant increases in splanchnic blood flow and reduced splanchnic resistance. VVS-↓CO&↓SVR patients had splanchnic pooling comparable to VVS-↓CO patients, but SVR comparable to VVS-↓SVR. Splanchnic vasodilation was reduced, compared to VVS-↓SVR, and venomotor properties were similar to control. Combined splanchnic pooling and reduced SVR produced the earliest faints among the VVS groups.

CONCLUSIONS

Both ↓CO and ↓SVR occur in young VVS patients. ↓SVR is predominant in VVS and is caused by impaired splanchnic vasoconstriction.

Physical Activity Level and Symptom Duration Are Not Associated After Concussion

BACKGROUND

Physical rest after a concussion has been described as a key component in the management of the injury. Evidence supporting this recommendation, however, is limited.

PURPOSE

To examine the association between physical activity and symptom duration in a cohort of patients after a concussion.

STUDY DESIGN

Cohort study; Level of evidence, 2.

METHODS

This study included 364 patients who were diagnosed with a concussion, were seen by a physician within 3 weeks of injury, and completed a questionnaire at the initial clinic visit. The questionnaire assessed the postconcussion symptom scale (PCSS) score, previous number of concussions, presence of the loss of consciousness or amnesia at the time of injury, and prior treatment for headaches. During each follow-up clinic visit, physical activity level was self-reported. A Cox proportional hazard model was constructed to determine the association between symptom duration, initial clinic visit responses, and self-reported physical activity level after the injury.

RESULTS

Study participants ranged in age from 8 to 27 years (mean age, 15.0 years) and had sustained a mean of 0.8 prior concussions; 222 patients (61%) were male. On initial examination, the mean PCSS score was 34.7. The mean symptom duration was 48.9 days after the injury. Among the variables included in the model, initial PCSS score and female sex were independently associated with symptom duration, while physical activity level after the injury was not. For participants aged between 13 and 18 years, however, higher levels of physical activity after the injury were associated with a shorter symptom duration.

CONCLUSION

Results from this study indicate that physical activity after the injury may not be universally detrimental to the recovery of concussion symptoms.

Cerebrovascular effects of the thigh cuff maneuver

Arterial hypotension can be induced by sudden release of inflated thigh cuffs (THC), but its effects on the cerebral circulation have not been fully described. In nine healthy subjects [aged 59 (9) yr], bilateral cerebral blood flow velocity (CBFV) was recorded in the middle cerebral artery (MCA), noninvasive arterial blood pressure (BP) in the finger, and end-tidal CO2 (ETCO2) with nasal capnography. Three THC maneuvers were performed in each subject with cuff inflation 20 mmHg above systolic BP for 3 min before release. Beat-to-beat values were extracted for mean CBFV, BP, ETCO2 , critical closing pressure (CrCP), resistance-area product (RAP), and heart rate (HR). Time-varying estimates of the autoregulation index [ARI(t)] were also obtained using an autoregressive-moving average model. Coherent averages synchronized by the instant of cuff release showed significant drops in mean BP, CBFV, and RAP with rapid return of CBFV to baseline. HR, ETCO2 , and ARI(t) were transiently increased, but CrCP remained relatively constant. Mean values of ARI(t) for the 30 s following cuff release were not significantly different from the classical ARI [right MCA 5.9 (1.1) vs. 5.1 (1.6); left MCA 5.5 (1.4) vs. 4.9 (1.7)]. HR was strongly correlated with the ARI(t) peak after THC release (in 17/22 and 21/24 recordings), and ETCO2 was correlated with the subsequent drop in ARI(t) (19/22 and 20/24 recordings). These results suggest a complex cerebral autoregulatory response to the THC maneuver, dominated by myogenic mechanisms and influenced by concurrent changes in ETCO2 and possible involvement of the autonomic nervous system and baroreflex.

Cerebrovascular regulation, exercise, and mild traumatic brain injury

A substantial number of people who sustain a mild traumatic brain injury report persistent symptoms. Most common among these symptoms are headache, dizziness, and cognitive difficulties. One possible contributor to sustained symptoms may be compromised cerebrovascular regulation. In addition to injury-related cerebrovascular dysfunction, it is possible that prolonged rest after mild traumatic brain injury leads to deconditioning that may induce physiologic changes in cerebral blood flow control that contributes to persistent symptoms in some people. There is some evidence that exercise training may reduce symptoms perhaps because it engages an array of cerebrovascular regulatory mechanisms. Unfortunately, there is very little work on the degree of impairment in cerebrovascular control that may exist in patients with mild traumatic brain injury, and there are no published studies on the subacute phase of recovery from this injury. This review aims to integrate the current knowledge of cerebrovascular mechanisms that might underlie persistent symptoms and seeks to synthesize these data in the context of exploring aerobic exercise as a feasible intervention to treat the underlying pathophysiology.

Blunted cerebral blood flow velocity in response to a nitric oxide donor in postural tachycardia syndrome

Cognitive deficits are characteristic of postural tachycardia syndrome (POTS). Intact nitrergic nitric oxide (NO) is important to cerebral blood flow (CBF) regulation, neurovascular coupling, and cognitive efficacy. POTS patients often experience defective NO-mediated vasodilation caused by oxidative stress. We have previously shown dilation of the middle cerebral artery in response to a bolus administration of the NO donor sodium nitroprusside (SNP) in healthy volunteers. In the present study, we hypothesized a blunted middle cerebral artery response to SNP in POTS. We used combined transcranial Doppler-ultrasound to measure CBF velocity and near-infrared spectroscopy to measure cerebral hemoglobin oxygenation while subjects were in the supine position. The responses of 17 POTS patients were compared with 12 healthy control subjects (age: 14-28 yr). CBF velocity in POTS patients and control subjects were not different at baseline (75 ± 3 vs. 71 ± 2 cm/s, P = 0.31) and decreased to a lesser degree with SNP in POTS patients (to 71 ± 3 vs. 62 ± 2 cm/s, P = 0.02). Changes in total and oxygenated hemoglobin (8.83 ± 0.45 and 8.13 ± 0.48 μmol/kg tissue) were markedly reduced in POTS patients compared with control subjects (14.2 ± 1.4 and 13.6 ± 1.6 μmol/kg tissue), primarily due to increased venous efflux. The data indicate reduced cerebral oxygenation, blunting of cerebral arterial vasodilation, and heightened cerebral venodilation. We conclude, based on the present study outcomes, that decreased bioavailability of NO is apparent in the vascular beds, resulting in a downregulation of NO receptor sites, ultimately leading to blunted responses to exogenous NO.

Fine temporal structure of cardiorespiratory synchronization

Cardiac and respiratory rhythms are known to exhibit a modest degree of phase synchronization, which is affected by age, diseases, and other factors. We study the fine temporal structure of this synchrony in healthy young, healthy elderly, and elderly subjects with coronary artery disease. We employ novel time-series analysis to explore how phases of oscillations go in and out of the phase-locked state at each cycle of oscillations. For the first time we show that cardiorespiratory system is engaged in weakly synchronized dynamics with a very specific temporal pattern of synchrony: the oscillations go out of synchrony frequently, but return to the synchronous state very quickly (usually within just 1 cycle of oscillations). Properties of synchrony depended on the age and disease status. Healthy subjects exhibited more synchrony at the higher (1:4) frequency-locking ratio between respiratory and cardiac rhythms, whereas subjects with coronary artery disease exhibited relatively more 1:2 synchrony. However, multiple short desynchronization episodes prevailed regardless of the age and disease status. The same average synchrony level could be alternatively achieved with few long desynchronizations, but this was not observed in the data. This implies functional importance of short desynchronization dynamics. These dynamics suggest that a synchronous state is easy to create if needed but is also easy to break. Short desynchronization dynamics may facilitate the mutual coordination of cardiac and respiratory rhythms by creating intermittent synchronous episodes. It may be an efficient background dynamics to promote adaptation of cardiorespiratory coordination to various external and internal factors.

Dynamic cerebral autoregulation is acutely impaired during maximal apnoea in trained divers

Aims

To examine whether dynamic cerebral autoregulation is acutely impaired during maximal voluntary apnoea in trained divers.

Methods

Mean arterial pressure (MAP), cerebral blood flow-velocity (CBFV) and end-tidal partial pressures of O₂ and CO₂ (PETO₂ and PETCO₂) were measured in eleven trained, male apnoea divers (28±2 yr; 182±2 cm, 76±7 kg) during maximal “dry” breath holding. Dynamic cerebral autoregulation was assessed by determining the strength of phase synchronisation between MAP and CBFV during maximal apnoea.

Results

The strength of phase synchronisation between MAP and CBFV increased from rest until the end of maximal voluntary apnoea (P<0.05), suggesting that dynamic cerebral autoregulation had weakened by the apnoea breakpoint. The magnitude of impairment in dynamic cerebral autoregulation was strongly, and positively related to the rise in PETCO₂ observed during maximal breath holding (R² = 0.67, P<0.05). Interestingly, the impairment in dynamic cerebral autoregulation was not related to the fall in PETO₂ induced by apnoea (R² = 0.01, P = 0.75).

Conclusions

This study is the first to report that dynamic cerebral autoregulation is acutely impaired in trained divers performing maximal voluntary apnoea. Furthermore, our data suggest that the impaired autoregulatory response is related to the change in PETCO₂, but not PETO₂, during maximal apnoea in trained divers.

Time-varying modeling of cerebral hemodynamics

The scientific and clinical importance of cerebral hemodynamics has generated considerable interest in their quantitative understanding via computational modeling. In particular, two aspects of cerebral hemodynamics, cerebral flow autoregulation (CFA) and CO2 vasomotor reactivity (CVR), have attracted much attention because they are implicated in many important clinical conditions and pathologies (orthostatic intolerance, syncope, hypertension, stroke, vascular dementia, mild cognitive impairment, Alzheimer's disease, and other neurodegenerative diseases with cerebrovascular components). Both CFA and CVR are dynamic physiological processes by which cerebral blood flow is regulated in response to fluctuations in cerebral perfusion pressure and blood CO2 tension. Several modeling studies to date have analyzed beat-to-beat hemodynamic data in order to advance our quantitative understanding of CFA-CVR dynamics. A confounding factor in these studies is the fact that the dynamics of the CFA-CVR processes appear to vary with time (i.e., changes in cerebrovascular characteristics) due to neural, endocrine, and metabolic effects. This paper seeks to address this issue by tracking the changes in linear time-invariant models obtained from short successive segments of data from ten healthy human subjects. The results suggest that systemic variations exist but have stationary statistics and, therefore, the use of time-invariant modeling yields "time-averaged models" of physiological and clinical utility.

Disruption of phase synchronization between blood pressure and muscle sympathetic nerve activity in postural vasovagal syncope

Withdrawal of muscle sympathetic nerve activity (MSNA) may not be necessary for the precipitous fall of peripheral arterial resistance and arterial pressure (AP) during vasovagal syncope (VVS). We tested the hypothesis that the MSNA-AP baroreflex entrainment is disrupted before VVS regardless of MSNA withdrawal using the phase synchronization between blood pressure and MSNA during head-up tilt (HUT) to measure reflex coupling. We studied eight VVS subjects and eight healthy control subjects. Heart rate, AP, and MSNA were measured during supine baseline and at early, mid, late, and syncope stages of HUT. Phase synchronization indexes, measuring time-dependent differences between MSNA and AP phases, were computed. Directionality indexes, indicating the influence of AP on MSNA (neural arc) and MSNA on AP (peripheral arc), were computed. Heart rate was greater in VVS compared with control subjects during early, mid, and late stages of HUT and significantly declined at syncope (P = 0.04). AP significantly decreased during mid, late, and syncope stages of tilt in VVS subjects only (P = 0.001). MSNA was not significantly different between groups during HUT (P = 0.700). However, the phase synchronization index significantly decreased during mid and late stages in VVS subjects but not in control subjects (P < .001). In addition, the neural arc was significantly affected more than the peripheral arc before syncope. In conclusion, VVS is accompanied by a loss of the synchronous AP-MSNA relationship with or without a loss in MSNA at faint. This provides insight into the mechanisms behind the loss of vasoconstriction and drop in AP independent of MSNA at the time of vasovagal faint.

Slow breathing as a means to improve orthostatic tolerance: a randomized sham-controlled trial

Endogenous oscillations in blood pressure (BP) and cerebral blood flow have been associated with improved orthostatic tolerance. Although slow breathing induces such responses, it has not been tested as a therapeutic strategy to improve orthostatic tolerance. With the use of a randomized, crossover sham-controlled design, we tested the hypothesis that breathing at six breaths/min (vs. spontaneous breathing) would improve orthostatic tolerance via inducing oscillations in mean arterial BP (MAP) and cerebral blood flow. Sixteen healthy participants (aged 25 ± 4 yr; mean ± SD) had continuous beat-to-beat measurements of middle cerebral artery blood velocity (MCAv), BP (finometer), heart rate (ECG), and end-tidal carbon dioxide partial pressure during an incremental orthostatic stress test to presyncope by combining head-up tilt with incremental lower-body negative pressure. Tolerance time to presyncope was improved (+15%) with slow breathing compared with spontaneous breathing (29.2 ± 5.4 vs. 33.7 ± 6.0 min; P < 0.01). The improved tolerance was reflected in elevations in low-frequency (LF; 0.07-0.2 Hz) oscillations of MAP and mean MCAv, improved metrics of dynamic cerebrovascular control (increased LF phase and reduced LF gain), and a reduced rate of decline for MCAv (−0.60 ± 0.27 vs. −0.99 ± 0.51 cm·s−1·min−1; P < 0.01) and MAP (−0.50 ± 0.37 vs. −1.03 ± 0.80 mmHg/min; P = 0.01 vs. spontaneous breathing) across time from baseline to presyncope. Our findings show that orthostatic tolerance can be improved within healthy individuals with a simple, nonpharmacological breathing strategy. The mechanisms underlying this improvement are likely mediated via the generation of negative intrathoracic pressure during slow and deep breathing and the related beneficial impact on cerebrovascular and autonomic function.

Update on the theory and management of orthostatic intolerance and related syndromes in adolescents and children

Orthostasis means standing upright. One speaks of orthostatic intolerance (OI) when signs, such as hypotension, and symptoms, such as lightheadedness, occur when upright and are relieved by recumbence. The experience of transient mild OI is part of daily life. 'Initial orthostatic hypotension' on rapid standing is a normal form of OI. However, other people experience OI that seriously interferes with quality of life. These include episodic acute OI, in the form of postural vasovagal syncope, and chronic OI, in the form of postural tachycardia syndrome. Less common is neurogenic orthostatic hypotension, which is an aspect of autonomic failure. Normal orthostatic physiology and potential mechanisms for OI are discussed, including forms of sympathetic hypofunction, forms of sympathetic hyperfunction and OI that results from regional blood volume redistribution. General and specific treatment options are proposed.

Common syndromes of orthostatic intolerance

The autonomic nervous system, adequate blood volume, and intact skeletal and respiratory muscle pumps are essential components for rapid cardiovascular adjustments to upright posture (orthostasis). Patients lacking sufficient blood volume or having defective sympathetic adrenergic vasoconstriction develop orthostatic hypotension (OH), prohibiting effective upright activities. OH is one form of orthostatic intolerance (OI) defined by signs, such as hypotension, and symptoms, such as lightheadedness, that occur when upright and are relieved by recumbence. Mild OI is commonly experienced during intercurrent illnesses and when standing up rapidly. The latter is denoted "initial OH" and represents a normal cardiovascular adjustment to the blood volume shifts during standing. Some people experience episodic acute OI, such as postural vasovagal syncope (fainting), or chronic OI, such as postural tachycardia syndrome, which can significantly reduce quality of life. The lifetime incidence of ≥1 fainting episodes is ∼40%. For the most part, these episodes are benign and self-limited, although frequent syncope episodes can be debilitating, and injury may occur from sudden falls. In this article, mechanisms for OI having components of adrenergic hypofunction, adrenergic hyperfunction, hyperpnea, and regional blood volume redistribution are discussed. Therapeutic strategies to cope with OI are proposed.

Enhanced phase synchronization of blood flow oscillations between heated and adjacent non-heated sacral skin

The study of skin microcirculation may be used to assess risk for pressure ulcers. It is observed that local heating causes an increase in blood flow of not only the heated skin, but also in the adjacent non-heated skin. The underlying physiological mechanism of this indirect vasodilation of the non-heated skin remains unclear. We hypothesized that blood flow oscillations (BFO) in the adjacent non-heated skin area synchronize with BFO in the heated skin, thus inducing a vasodilatory response. We investigated BFO in the heated and adjacent non-heated skin (12.1 ± 1.2-cm distance) on the sacrum in 12 healthy participants. The ensemble empirical mode decomposition (EEMD) was used to decompose blood flow signals into a set of intrinsic mode functions (IMFs), and the IMFs with power spectra over the frequency range of 0.0095-0.02, 0.02-0.05, and 0.05-0.15 Hz were chosen as the characteristic components corresponding to metabolic, neurogenic, and myogenic regulations, respectively. Then, the instantaneous phase of the characteristic components was calculated using the Hilbert transform. From the time series of phase difference between a pair of characteristic components, the epochs of phase synchronization were detected. The results showed that myogenic and neurogenic BFO exhibited self-phase synchronization during the slower vasodilation of the heated skin. In the non-heated skin, the degree of synchronization of BFO is associated with the changes in blood flow.

Vasomotor reactivity as a predictor for syncope in patients with orthostatism

OBJECTIVES

Syncope in patients with orthostatic hypotension (OH) may be the result of impaired cerebral autoregulation. Cerebral autoregulation status can be determined by assessing cerebral vasomotor reactivity (VMR). We assessed and compared VMR in patients with OH with and without syncope.

MATERIAL AND METHODS

Twenty-nine patients with OH underwent transcranial Doppler (TCD) and the Diamox test (1 g acetazolamide IV) for assessing VMR during elaboration of their OH syndrome. The percent difference between cerebral blood flow velocities (BFV) in the middle cerebral (MCA) and vertebral (VA) arteries before and after acetazolamide was defined as VMR%. We considered increases of BFV of ≥ 40% as being indicative of good VMR and classified our study patients as having good or impaired VMRs accordingly.

RESULTS

Mean VMR% values of the MCA and VA in patients with OH with syncope (n = 12) were significantly lower as compared with patients with OH without syncope (n = 17): 25.2 ± 20.5% and 42.5 ± 18.6%; 20.9 ± 15.5% and 40.8 ± 28.5%, respectively (P < 0.05).

CONCLUSIONS

Among patients with OH, we found an association between the presence of syncope and impaired VMR. Assessment of VMR among patients with OH may predict those who are at higher risk to faint and fall and to support more aggressive intervention.

Mechanisms of sympathetic regulation in orthostatic intolerance

Sympathetic circulatory control is key to the rapid cardiovascular adjustments that occur within seconds of standing upright (orthostasis) and which are required for bipedal stance. Indeed, patients with ineffective sympathetic adrenergic vasoconstriction rapidly develop orthostatic hypotension, prohibiting effective upright activities. One speaks of orthostatic intolerance (OI) when signs, such as hypotension, and symptoms, such as lightheadedness, occur when upright and are relieved by recumbence. The experience of transient mild OI is part of daily life. However, many people experience episodic acute OI as postural faint or chronic OI in the form of orthostatic tachycardia and orthostatic hypotension that significantly reduce the quality of life. Potential mechanisms for OI are discussed including forms of sympathetic hypofunction, forms of sympathetic hyperfunction, and OI that results from regional blood volume redistribution attributable to regional adrenergic hypofunction.

Increased pulsatile cerebral blood flow, cerebral vasodilation, and postsyncopal headache in adolescents

OBJECTIVE

We hypothesize that, after a sudden decrease in cerebral blood flow velocity (CBFV) in adolescents, a faint, rapid hyperemic pulsatile CBFV occurs upon the patient's return to the supine position and is associated with postsyncopal headache.

STUDY DESIGN

This case-control study involved 16 adolescent subjects with a history of fainting and headaches. We induced fainting during 70° tilt-table testing and measured mean arterial pressure, heart rate, end-tidal CO(2), and CBFV. Fifteen control subjects were similarly evaluated with a tilt but did not faint, and comparisons with fainters were made at equivalent defined time points.

RESULTS

Baseline values were similar between the groups. Upon fainting, mean arterial pressure decreased 49% in the patients who fainted vs 6% in controls (P < .001). The heart rate decreased 15% in fainters and increased 35% in controls (P < .001). In patients who fainted, cerebrovascular critical closing pressure increased markedly, which resulted in reduced diastolic (-66%) and mean CBFV (-46%) at faint; systolic CBFV was similar to controls. Pulsatile CBFV (systolic-diastolic CBFV) increased 38% in fainters, which caused flow-mediated dilatation of cerebral vessels. When the fainters returned to the supine position, CBFV exhibited increased systolic and decreased diastolic flows compared with controls (P < .02).

CONCLUSION

Increased pulsatile CBFV during and after faint may cause postsyncopal cerebral vasodilation and headache.

Spontaneous fluctuations in cerebral blood flow regulation: contribution of PaCO2

To investigate the temporal variability of dynamic cerebral autoregulation (CA), the transient response of cerebral blood flow to rapid changes in arterial blood pressure, a new approach was introduced to improve the temporal resolution of dynamic CA assessment. Continuous bilateral recordings of cerebral blood flow velocity (transcranial Doppler, middle cerebral artery), end-tidal Pco2 (PetCO2, infrared capnograph), and blood pressure (Finapres) were obtained at rest and during breath hold in 30 young subjects (25 ± 6 yr old) and 30 older subjects (64 ± 4 yr old). Time-varying estimates of the autoregulation index [ARI( t)] were obtained with an autoregressive-moving average model with coefficients expanded by orthogonal decomposition. The temporal pattern of ARI( t) varied inversely with PetCO2, decreasing with hypercapnia. At rest, ARI( t) showed spontaneous fluctuations that were significantly different from noise and significantly correlated with spontaneous fluctuations in PetCO2 in the majority of recordings (young: 72% and old: 65%). No significant differences were found in ARI( t) due to aging. This new approach to improve the temporal resolution of dynamic CA parameters allows the identification of physiologically meaningful fluctuations in dynamic CA efficiency at rest and in response to changes in arterial CO2.

Respiration drives phase synchronization between blood pressure and RR interval following loss of cardiovagal baroreflex during vasovagal syncope

Loss of the cardiovagal baroreflex (CVB), thoracic hypovolemia, and hyperpnea contribute to the nonlinear time-dependent hemodynamic instability of vasovagal syncope. We used a nonlinear phase synchronization index (PhSI) to describe the extent of coupling between cardiorespiratory parameters, systolic blood pressure (SBP) or arterial pressure (AP), RR interval (RR), and ventilation, and a directional index (DI) measuring the direction of coupling. We also examined phase differences directly. We hypothesized that AP-RR interval PhSI would be normal during early upright tilt, indicating intact CVB, but would progressively decrease as faint approached and CVB failed. Continuous measurements of AP, RR interval, respiratory plethysomography, and end-tidal CO2 were recorded supine and during 70-degree head-up tilt in 15 control subjects and 15 fainters. Data were evaluated during five distinct times: baseline, early tilt, late tilt, faint, and recovery. During late tilt to faint, fainters exhibited a biphasic change in SBP-RR interval PhSI. Initially in fainters during late tilt, SBP-RR interval PhSI decreased (fainters, from 0.65±0.04 to 0.24±0.03 vs. control subjects, from 0.51±0.03 to 0.48±0.03; P<0.01) but then increased at the time of faint (fainters=0.80±0.03 vs. control subjects=0.42±0.04; P<0.001) coinciding with a change in phase difference from positive to negative. Starting in late tilt and continuing through faint, fainters exhibited increasing phase coupling between respiration and AP PhSI (fainters=0.54±0.06 vs. control subjects=0.27±0.03; P<0.001) and between respiration and RR interval (fainters=0.54±0.05 vs. control subjects=0.37±0.04; P<0.01). DI indicated respiratory driven AP (fainters=0.84±0.04 vs. control subjects=0.39±0.09; P<0.01) and RR interval (fainters=0.73±0.10 vs. control subjects=0.23±0.11; P<0.001) in fainters. The initial drop in the SBP-RR interval PhSI and directional change of phase difference at late tilt indicates loss of cardiovagal baroreflex. The subsequent increase in SBP-RR interval PhSI is due to a respiratory synchronization and drive on both AP and RR interval. Cardiovagal baroreflex is lost before syncope and supplanted by respiratory reflexes, producing hypotension and bradycardia.

Near infrared spectroscopy: guided tilt table testing for syncope

Syncope is transient loss of consciousness. Neurocardiogenic syncope (NCS) is the most common cause of syncope. Head-up tilt-table test (HUTT) has been used to demonstrate physiologic events during graded orthostatic challenge in individuals with significant handicap from NCS. Near-infrared spectroscopy (NIRS) provides a noninvasive, continuous method to monitor trends of regional tissue oxygenation (rSO2). We hypothesize that multisite NIRS monitoring will show differential desaturation patterns in the brain and renal vascular beds during postural stresses. All patients age 7-21 years old scheduled to undergo HUTT were recruited. Two probes for NIRS monitoring were placed on the forehead and above the left paravertebral level at the T10 to L1 space. These leads were attached to the Somanetics monitor (Somanetics, Troy MI). Tissue saturations (rSO2) obtained at two sites were recorded at rest, during the test, and throughout a 5-min recovery period. All data routinely obtained in HUTT were included in the research study database. Thirteen patients were recruited. The average age was 12.9 years. Five patients had a positive tilt-table test. The patients with syncope had rSO2 trends distinctly different from the normal subjects. In these patients, cerebral rSO2 showed a sudden decreasing trend from hypoperfusion, soon followed by various clinical symptoms. The cerebral rSO2 trend, which showed a dramatic increase, was paralleled by renal rSO2. These rSO2 trends were progressive until the patient was brought back to the supine position, which resulted in the rSO2 in both beds returning to baseline. Multisite NIRS-guided HUTT shows differential trends in the different vascular beds during postural gravitational stresses, and these patterns underlie the systemic oxygen consumption to flow-coupling dynamics observed during syncope.