Changes In Cerebral Blood Flow Velocity Research Articles

Validation of the Openwater wearable optical system: cerebral hemodynamic monitoring during a breath-hold maneuver.

Bedside cerebral blood flow (CBF) monitoring has the potential to inform and improve care for acute neurologic diseases, but technical challenges limit the use of existing techniques in clinical practice. Here, we validate the Openwater optical system, a novel wearable headset that uses laser speckle contrast to monitor microvascular hemodynamics. We monitored 25 healthy adults with the Openwater system and concurrent transcranial Doppler (TCD) while performing a breath-hold maneuver to increase CBF. Relative blood flow (rBF) was derived from changes in speckle contrast, and relative blood volume (rBV) was derived from changes in speckle average intensity. A strong correlation was observed between beat-to-beat optical rBF and TCD-measured cerebral blood flow velocity (CBFv), ; the slope of the linear fit indicates good agreement, 0.87 (95% CI: 0.83 ). Beat-to-beat rBV and CBFv were also strongly correlated, , but as expected the two variables were not proportional; changes in rBV were smaller than CBFv changes, with linear fit slope of 0.18 (95% CI: 0.17 to 0.19). Further, strong agreement was found between rBF and CBFv waveform morphology and related metrics. This first in vivo validation of the Openwater optical system highlights its potential as a cerebral hemodynamic monitor, but additional validation is needed in disease states.

Astrocytes modulate cerebral blood flow and neuronal response to cocaine in prefrontal cortex.

Cocaine affects both cerebral blood vessels and neuronal activity in brain. Cocaine can also disrupt astrocytes, which modulate neurovascular coupling-a process that regulates cerebral hemodynamics in response to neuronal activation. However, separating neuronal and astrocytic effects from cocaine's direct vasoactive effects has been challenging, partially due to limitations of neuroimaging techniques able to differentiate vascular from neuronal and glial effects at high temporal and spatial resolutions. Here, we used a newly-developed multi-channel fluorescence and optical coherence Doppler microscope (fl-ODM) that allows for simultaneous measurements of neuronal and astrocytic activities (reflected by the intracellular calcium changes in neurons Ca2+N and astrocytes Ca2+A, respectively) alongside their vascular interactions in vivo to address this challenge. Using green and red genetically-encoded Ca2+ indicators differentially expressed in astrocytes and neurons, fl-ODM enabled concomitant imaging of large-scale astrocytic and neuronal Ca2+ fluorescence and 3D cerebral blood flow velocity (CBFv) in vascular networks in the mouse cortex. We assessed cocaine's effects in the prefrontal cortex (PFC) and found that the CBFv changes triggered by cocaine were temporally correlated with astrocytic Ca2+A activity. Chemogenetic inhibition of astrocytes during the baseline state resulted in blood vessel dilation and CBFv increases but did not affect neuronal activity, suggesting modulation of spontaneous blood vessel's vascular tone by astrocytes. Chemogenetic inhibition of astrocytes during a cocaine challenge prevented its vasoconstricting effects alongside the CBFv decreases, but it also attenuated the neuronal Ca2+N increases triggered by cocaine. These results document a role of astrocytes both in regulating vascular tone and consequently blood flow, at baseline and for modulating the vasoconstricting and neuronal activation responses to cocaine in the PFC. Strategies to inhibit astrocytic activity could offer promise for ameliorating vascular and neuronal toxicity from cocaine misuse.

Cerebral blood flow velocity and oxygenation in neonatal aortic arch repair at two perfusion temperatures.

(i) To monitor cerebral blood flow velocity (CBFv) throughout aortic arch repair surgery and during the recovery period. (ii) To examine the relationship between transcranial doppler ultrasound (TCD) and near-infrared spectroscopy (NIRS) during cardiac surgery. (iii) To examine CBFv in patients cooled to 20°C and 25°C. During aortic arch repair and after surgery, measurements of TCD, NIRS, blood pH, pO2, pCO2, HCO3, lactate, Hb, haematocrit (%) and temperature (core and rectal) were recorded in 24 neonates. General linear mixed models were used to examine differences over time and between two cooling temperatures. Repeated measures correlations were used to determine the relationship between TCD and NIRS. CBFv changed during arch repair (main effect of time: P = 0.001). During cooling, CBFv increased by 10.0 cm/s (5.97, 17.7) compared to normothermia (P = 0.019). Once recovering in paediatric intensive care unit (PICU), CBFv had increased from the preoperative measurement by 6.2 cm/s (0.21, 13.4; P = 0.045). CBFv changes were similar between patients cooled to 20°C and 25°C (main effect of temperature: P = 0.22). Repeated measures correlations (rmcorr) identified a statistically significant but weak positive correlation between CBFv and NIRS (r = 0.25, P≤0.001). Our data suggested that CBFv changed throughout aortic arch repair and was higher during the cooling period. A weak relationship was found between NIRS and TCD. Overall, these findings could provide clinicians with information on how to optimise long-term cerebrovascular health.

Continuous fetal cerebral blood flow monitoring during labor: A feasibility study

BackgroundCurrent methods for fetal surveillance during labor have significant limitations. Since continuous fetal cerebral blood flow velocity (CBFV) monitoring during labor may add valuable information about fetal well-being, we developed a new ultrasound system called VisiBeam. VisiBeam consists of a flat probe (diameter 11 mm) with a cylindric plane wave beam, a vacuum attachment (diameter 40 mm), a scanner, and a display. AimsTo assess the feasibility of VisiBeam for continuous fetal CBFV monitoring during labor, and to study changes in CBFV during uterine contractions. Study designDescriptive observational study. SubjectsTwenty-five healthy women in labor with a singleton fetus in cephalic presentation at term. A transducer was placed over a fontanelle and attached to the fetal head with vacuum suction. Outcome measuresAchievement of continuous good quality fetal CBFV measures, such as peak systolic velocity, time averaged maximum velocity and end diastolic velocity. Trend plots of velocity measures display changes in CBFV between and during uterine contractions. ResultsGood quality recordings during and between contractions were achieved in 16/25 fetuses. In twelve fetuses, CBFV measures were stable during uterine contractions. Four fetuses showed patterns of reduced CBFV velocity measures during contractions. ConclusionsContinuous fetal CBFV monitoring by VisiBeam was feasible in 64 % of the subjects during labor. The system displayed variations of fetal CBFV not available by today's monitoring techniques and motivates for further studies. However, improvement of the probe attachment is required to ensure good quality signal in a higher proportion of fetuses during labor.

Cerebral blood flow velocity progressively decreases with increasing levels of verticalization in healthy adults. A cross-sectional study with an observational design.

Autoregulation of the cerebral vasculature keeps brain perfusion stable over a range of systemic mean arterial pressures to ensure brain functioning, e.g., in different body positions. Verticalization, i.e., transfer from lying (0°) to upright (70°), which causes systemic blood pressure drop, would otherwise dramatically lower cerebral perfusion pressure inducing fainting. Understanding cerebral autoregulation is therefore a prerequisite to safe mobilization of patients in therapy. We measured the impact of verticalization on cerebral blood flow velocity (CBFV) and systemic blood pressure (BP), heart rate (HR) and oxygen saturation in healthy individuals. We measured CBFV in the middle cerebral artery (MCA) of the dominant hemisphere in 20 subjects using continuous transcranial doppler ultrasound (TCD). Subjects were verticalized at 0°, -5°, 15°, 30°, 45° and 70° for 3-5 min each, using a standardized Sara Combilizer chair. In addition, blood pressure, heart rate and oxygen saturation were continuously monitored. We show that CBFV progressively decreases in the MCA with increasing degrees of verticalization. Systolic and diastolic BP, as well as HR, show a compensatory increase during verticalization. In healthy adults CBFV changes rapidly with changing levels of verticalization. The changes in the circulatory parameters are similar to results regarding classic orthostasis. ClinicalTrials.gov, identifier: NCT04573114.

Aerobic exercise training reduces cerebrovascular impedance in older adults: a 1-year randomized controlled trial.

Older adults have higher cerebrovascular impedance than young individuals which may contribute to chronic brain hypoperfusion. Besides, middle-aged athletes exhibit lower cerebrovascular impedance than their sedentary peers. We examined whether aerobic exercise training (AET) reduces cerebrovascular impedance in sedentary older adults. We conducted a proof-of-concept trial that randomized 73 older adults to 1 yr of AET (n = 36) or stretching and toning (SAT, n = 37) interventions. Cerebrovascular impedance was estimated from simultaneous recordings of carotid artery pressure (CAP) via applanation tonometry and cerebral blood flow velocity (CBFV) in the middle cerebral artery via transcranial Doppler using transfer function analysis. Fifty-six participants completed 1-yr interventions, and 41 of those completed cerebrovascular impedance measurements. AET group showed a significant increase in V̇o2peak after the intervention [estimated marginal mean (95% confidence interval); from 22.8 (21.6 to 24.1) to 24.9 (23.6 to 26.2) mL·kg-1·cm-1, P < 0.001], but not SAT [from 21.7 (20.5 to 22.9) to 22.3 (21.1 to 23.7) mL·kg-1·cm-1, P = 0.114]. Coherence between changes in CBFV and CAP was >0.90 in the frequency range of 0.78-3.12 Hz. The averaged cerebrovascular impedance modulus (Z) in this frequency range decreased after 1-yr AET [from 1.05 (0.96 to 1.14) to 0.95 (0.92 to 1.06) mmHg·s·cm-1, P = 0.023], but not SAT [from 0.96 (0.87 to 1.04) to 1.01 (0.92 to 1.10) mmHg·s·cm-1, P = 0.138]. Reductions in Z were correlated positively with reductions in carotid pulse pressure (r = 0.628, P = 0.004) and inversely with mean CBFV (r = -0.563, P = 0.012) in the AET group. One-year AET reduces cerebrovascular impedance in older adults, which may benefit brain perfusion.NEW & NOTEWORTHY Estimation of cerebrovascular impedance is essential for understanding dynamic cerebral blood flow regulation. This randomized controlled trial demonstrated that aerobic exercise training reduced cerebrovascular impedance in older adults, which may benefit brain perfusion.

Orthostatic systemic and cerebral hemodynamics in newly injured patients with spinal cord injury.

Low blood pressure (BP) may lead to reduced cerebral blood flow velocity (CBFv) during an orthostatic challenge in newly injured patients with traumatic spinal cord injury (SCI), which, may relate to the neurological level of injury (NLI) as documented on the International Standards for the Neurologic Classification of SCI (ISNCSCI), or to evidence of cardiovascular autonomic impairment as determined by the International Standards to document remaining Autonomic Function after SCI (ISAFSCI). To examine the influence of patient demographics, ISNCSCI and ISAFSCI scores on hemodynamic responses to a bedside sit-up test in newly injured patients with SCI. Cross-sectional, prospective analysis. A modified sit-up test was conducted at the bedside with the hips at an angle between 45° and 90° and the legs horizontal, level with the hips. Heart rate, BP, and CBFv were recorded for 10min in the supine and seated positions. Fifty-three newly injured patients (median 39.5days post-injury) with traumatic SCI were enrolled. Overall, 28 (53%) patients met ISAFSCI criteria, and the number of criteria met (total score) was significantly related to orthostatic changes in CBFv. Change in SBP and change in CBFv were not significantly related, but NLI and total sensory score from the ISNCSCI were significant predictors of the change in CBFv. Total ISAFSCI score, NLI and sensory scores were positively associated with orthostatic changes in CBFv. Long term follow-up should be considered to determine the consequences of diminished CBFv on cognitive function and quality of life following SCI.

Dynamics of the cerebral autoregulatory response to paced hyperventilation assessed using subcomponent and time-varying analyses.

Cerebral blood flow (CBF) can be altered by a change in partial pressure of arterial CO2 (Pco2), being reduced during hyperventilation (HPV). Critical closing pressure (CrCP) and resistance area product (RAP) are parameters that can be studied to understand this change, but their dynamic response has not been investigated during paced HPV (PHPV). Seventy-five participants had recordings at rest and during PHPV. Blood pressure (BP) (Finometer), bilateral CBF velocity (CBFV) (transcranial Doppler), end-tidal CO2 (capnography), and heart rate (HR) were recorded continuously. Subcomponent analysis (SCA) and time-varying CrCP, RAP, and dynamic cerebral autoregulation (autoregulation index, ARI) were estimated by comparing PHPV with poikilocapnia. PHPV caused a change in CBFV (P < 0.01), EtCO2, (P < 0.01), HR (P < 0.001), and RAP (P < 0.01). SCA demonstrated RAP was the main parameter explaining the changes in CBFV due to PHPV. The time-varying step responses for CBFV and RAP during PHPV demonstrated considerable nonstationarity compared with poikilocapnia (P < 0.00001). Although time-varying ARI was temporarily depressed, after 60 s of PHPV it was significantly higher (6.81 ± 1.88) (P < 0.0001) than in poikilocapnia (5.08 ± 1.86). The mean plateau of the RAP step response was -98.3 ± 58.8% 60 s after the onset of PHPV but -71.7 ± 45.0% for poikilocapnia (P = 0.0026), with no corresponding changes in CrCP (P = 0.6). Further work is needed to assess the role of sex and aging in our findings, and the potential for using RAP and CrCP to improve the sensitivity and specificity of CO2 reactivity studies in cerebrovascular conditions.NEW & NOTEWORTHY The dynamic response of critical closing pressure (CrCP) and resistance-area product (RAP) of the cerebral circulation to a step change in mean arterial pressure can shed light on the nonstationary changes induced by paced hyperventilation and the effects of hypocapnia on the autoregulation of cerebral blood flow. Contrary to hypercapnia, where the response is dominated by CrCP, hypocapnia shows an initial depression of cerebral autoregulation, followed by improvements controlled by changes in RAP.

Cerebrovascular responses to somatomotor stimulation in Parkinson’s disease: A multivariate analysis

Parkinson’s disease (PD) is a common neurodegenerative disorder, yet little is known about cerebral haemodynamics in this patient population. Previous studies assessing dynamic cerebral autoregulation (dCA), neurovascular coupling (NVC) and vasomotor reactivity (VMR) have yielded conflicting findings. By using multi-variate modelling, we aimed to determine whether cerebral blood flow (CBF) regulation is impaired in PD patients. 55 healthy controls (HC) and 49 PD patients were recruited. PD subjects underwent a second recording following a period of abstinence from their anti-Parkinsonian medication. Continuous bilateral transcranial Doppler in the middle cerebral arteries, beat-to-beat mean arterial blood pressure (MAP; Finapres), heart rate (HR; electrocardiogram), and end-tidal CO2 (EtCO2; capnography) were measured. After a 5-min baseline period, a passive motor paradigm comprising 60 s of elbow flexion was performed. Multi-variate modelling quantified the contributions of MAP, ETCO2 and neural stimulation to changes in CBF velocity (CBFV). dCA, VMR and NVC were quantified to assess the integrity of CBF regulation. Neural stimulation was the dominant input. dCA, NVC and VMR were all found to be impaired in the PD population relative to HC (p < 0.01, p = 0.04, p < 0.01, respectively). Our data suggest PD may be associated with depressed CBF regulation. This warrants further assessment using different neural stimuli.

Principal component analysis to identify the major contributors to task-activated neurovascular responses.

Consensus on the optimal metrics for neurovascular coupling (NVC) is lacking. The aim of this study was to use principal component analysis (PCA) to determine the most significant contributors to NVC responses in healthy adults (HC), Alzheimer's disease (AD), and mild cognitive impairment (MCI). PCA was applied to three datasets: 1) 69 HC, 2) 30 older HC, 34 AD, and 22 MCI, 3) 1&2 combined. Data were extracted on peak percentage change in cerebral blood flow velocity (CBFv), variance ratio (VR), cross-correlation function peak (CCF), and blood pressure, for five cognitive tasks. An equamax rotation was applied and factors were significant where the eignevalue was ≥1. Rotated factor loadings ≥0.4 determined significant NVC variables. PCA identified 12 significant factors accounting for 78% of variance (all datasets). Contributing variables loaded differently on the factors across the datasets. In datasets 1&2, peak percentage change in CBFv contributed to factors explaining the most variance (45-58%), whereas cognitive test scores, fluency and memory domains contributed the least (15-37%). In the combined dataset, CBFv, CCF and fluency domain contributed the majority (33-43%), whereas VR and attention the least (6-24%). Peak percentage change in CBFv and the visuospatial task consistently accounted for a large proportion of the variance, suggesting these are robust NVC markers for future studies.

Leptomeningeal collateral flow in patients with middle cerebral artery occlusion assessed by transcranial Doppler.

To explore the application value and clinical significance of transcranial Doppler(TCD)in assessing leptomeningeal collateral flow (LMF) status in patients with unilateral middle cerebral artery (MCA) occlusion. Medical records of patients with unilateral MCA occlusion confirmed by digital subtraction angiography (DSA) were analyzed retrospectively. The patients were divided into three groups according to LMF status, and the laboratory and imaging results were collected. Cerebral blood flow velocity (CBFV) of MCA, anterior cerebral artery (ACA), and posterior cerebral artery (PCA) on the affected side (ipsilateral, i) and the healthy side (contralateral, c) were measured and recorded by TCD. The results of CBFV changes detected by TCD were compared with those of DSA, and the correlation between CBFV changes and LMF status was analyzed. Eighty-four patients with unilateral MCA occlusion were included. CBFViACA and CBFViPCA were significantly faster than CBFVcACA and CBFVcPCA in patients with good LMF status (p<.05). There was a significant positive correlation between CBFViACA and LMF status (r = 0.697, p<.001). There was statistical significance in receiver operating characteristic curve analysis of CBFViACA and CBFViPCA (p<.05). The area under the curve of CBFViACA and CBFViPCA, respectively, was 0.879 and 0.678, and the best cutoff value was 82 and 60.5 cm/s. TCD can assess LMF status by detecting the changes of flow velocity of intracranial vessels. CBFV of ACA and PCA in patients with MCA occlusion is significantly correlated with LMF status by DSA. Assessing LMF status, CBFViACA, CBFViACA/CBFVcACA, and CBFViACA/CBFViMCA has the great diagnostic value, which is of great significance in guiding MCA occlusion patients to choose individualized treatment.

Efficacy of Cranial Doppler ultrasound in Diagnosis of Early-onset neonatal sepsis

Background: Neonatal sepsis is considered a major neonatal issue and associated with a high rate of mortality in newborns. Because there is a lack of studies concerning early hemodynamic changes of cerebral circulation in neonatal sepsis, so our research aims to assess the cerebral blood flow (CBF) changes in newborns with early-onset neonatal sepsis (EONS) and to clarify the role of cranial Doppler Ultrasound in EONS diagnosis. Objective: To evaluate cerebral hemodynamic changes using transcranial Doppler in newborns with EONS. Methods: This cross-sectional study with 50 neonates involved in the study. The neonates were classified into 2 groups: Case Group (25 neonates) with a diagnosis of early-onset neonatal sepsis and control group (25 neonates) without clinical manifestations of sepsis. Examination of all neonates using Transcranial Doppler (TCD) was performed within seventy-two hours after birth. Measurement of cerebral blood flow velocity (CBFV) in two major cerebral vessels: The anterior cerebral artery (ACA) and middle cerebral artery (MCA) with documentation of doppler data: The peak systolic velocity (PSV), pulsatility index (PI), and resistive index (RI). Data were statistically analyzed followed by calculation of specificity, sensitivity, positive and negative predictive value at selected cut-off values of CBFV parameters. Results: Sepsis group showed statistically significant changes in doppler indices with increased PSV and deceased PI and RI in both arteries. Conclusion: the present study demonstrates changes in CBFV measured by transcranial Doppler in newborns with EONS which can be used as noninvasive bedside investigation.

Functional Transcranial Doppler Ultrasound for Monitoring Cerebral Blood Flow.

Functional transcranial Doppler ultrasound (fTCD) is the use of transcranial Doppler ultrasound (TCD) to study neural activation occurring during stimuli such as physical movement, activation of tactile sensors in the skin, and viewing images. Neural activation is inferred from an increase in the cerebral blood flow velocity (CBFV) supplying the region of the brain involved in processing sensory input. For example, viewing bright light causes increased neural activity in the occipital lobe of the cerebral cortex, leading to increased blood flow in the posterior cerebral artery, which supplies the occipital lobe. In fTCD, changes in CBFV are used to estimate changes in cerebral blood flow (CBF). With its high temporal resolution measurement of blood flow velocities in the major cerebral arteries, fTCD complements other established functional imaging techniques. The goal of this Methods paper is to give step-by-step instructions for using fTCD to perform a functional imaging experiment. First, the basic steps for identifying the middle cerebral artery (MCA) and optimizing the signal will be described. Next, placement of a fixation device for holding the TCD probe in place during the experiment will be described. Finally, the breath-holding experiment, which is a specific example of a functional imaging experiment using fTCD, will be demonstrated.

Determinants of cerebral blood flow velocity change during squat-stand maneuvers.

The large changes in mean arterial blood pressure (MABP) and cerebral blood flow velocity (CBFV) induced by squat-stand maneuvers (SSM) make this approach particularly suited for studying dynamic cerebral autoregulation (CA). However, the role of other systemic determinants of CBFV has not been described and could provide alternative physiological interpretations of SSM results. In 32 healthy subjects (16 female), continuous recordings of MABP (Finometer), bilateral CBFV (transcranial Doppler, MCA), end-tidal CO2 (EtCO2; capnography), and heart rate (HR; electrocardiogram) were performed for 5 min standing at rest, and during 15 SSM at the frequency of 0.05 Hz. A time-domain, multivariate dynamic model estimated the CBFV variance explained by different inputs, corresponding to significant contributions from MABP (P < 0.00001), EtCO2 (P < 0.0001), and HR (P = 0.041). The autoregulation index (ARI; range 0-9) was estimated from the CBFV response to a step change in MABP. At rest, ARI values (typically 5.7) were independent of the number of model inputs, but during SSM, ARI was reduced compared with baseline (P < 0.0001), and the three input model yielded lower values for the right and left MCA (3.4 ± 1.2, 3.1 ± 1.3) when compared with the single-input MABP-CBFV model (4.1 ± 1.1, 3.9 ± 1.0; P < 0.0001). The high coherence of the MABP-CBFV transfer function at 0.05 Hz (typically 0.98) was considerably reduced (around 0.71-0.73; P < 0.0001) when the contribution of CBFV covariates was taken into account. Not taking into consideration other determinants of CBFV, in addition to MABP, could be misleading and introduce biases in physiological and clinical studies.

Effects of tilt on cerebral hemodynamics measured by NeoDoppler in healthy neonates

BackgroundToday, there are conflicting descriptions of how neonates respond to tilt. Examining physiologic responses of cerebral blood flow velocities (BFVs) in challenging situations like a tilt requires equipment that can cope with positional changes. We aimed to characterize how healthy term neonates respond to mild cerebral hemodynamic stress induced by a 90° tilt test using the recently developed NeoDoppler ultrasound system.MethodsA small ultrasound probe was fixated to the neonatal fontanel by a cap, and measured cerebral BFV in healthy neonates during and after a 90° head-up tilt test, five min in total, at their first and second day of life. Unsupervised k-means cluster analysis was used to characterize common responses.ResultsFifty-six ultrasound recordings from 36 healthy term neonates were analyzed. We identified five distinct, immediate responses that were related to specific outcomes in BFV, heart rate, and pulsatility index the next two min. Among 20 neonates with two recordings, 13 presented with different responses in the two tests.ConclusionsInstant changes in cerebral BFV were detected during the head-up tilt tests, and the cluster analysis identified five different hemodynamic responses. Continuous recordings revealed that the differences between groups persisted two min after tilt.ImpactNeoDoppler is a pulsed-wave Doppler ultrasound system with a probe fixated to the neonatal fontanel by a cap that can measure continuous cerebral blood flow velocity.Healthy neonates present with a range of normal immediate cerebral hemodynamic responses to a 90° head-up tilt, categorized in five groups by cluster analysis.This paper adds new knowledge about connection between immediate responses and prolonged responses to tilt.We demonstrate that the NeoDoppler ultrasound system can detect minute changes in cerebral blood flow velocity during a 90° head-up tilt.

Older age and male sex are associated with higher cerebrovascular impedance.

Cerebral blood flow (CBF) becomes pulsatile in response to the pulsatile change in perfusion pressure that is regulated by cerebrovascular impedance. In this study, we aimed to characterize age-related differences in cerebrovascular impedance across the adult lifespan. Carotid artery pressure [(CAP), via applanation tonometry] and CBF velocity (CBFV) in the middle cerebral artery (via transcranial Doppler) were measured in 148 healthy adults (21-79 yr, 62% women). Cerebrovascular impedance was quantified using transfer function analysis. Coherence between changes in CBFV and CAP was >0.90 in the frequency range of 0.78-2.73 Hz, suggesting a linear dynamic relationship between these two variables. Impedance modulus at the first harmonics (0.78-1.56 Hz) of CBFV and CAP oscillations (Z1), reflecting mainly heart rate frequency, was 20% higher in the old (>64 yr, P = 0.002) and 13% higher in the middle-aged (45-64 yr, P = 0.08) than in young individuals (<45 yr). In addition, Z1 was 24% higher in men than in women (P < 0.001). Multiple linear regression analysis revealed that Z1 is negatively associated with systolic (β = -0.470), diastolic (β = -0.418), pulsatile (β = -0.374), and mean CBFV (β = -0.473; P < 0.001 for all) after adjustment for age, sex, and body mass index (BMI). These results suggest that older age and male sex are associated with higher cerebrovascular impedance than young individuals, which may contribute to brain hypoperfusion.NEW & NOTEWORTHY Impedance modulus at the first harmonics of cerebral blood flow velocity (CBFV) and carotid artery pressure oscillations (Z1) was higher in the old (>64 yr) than in the young individuals (<45 yr), and it was higher in men than in women. Z1 is negatively associated with CBFV after adjustment for age, sex, and body mass index. Increases in cerebrovascular impedance with age may buffer systemic arterial pressure fluctuations at the cost of increased brain hypoperfusion risk.

Cerebral blood flow alterations associated with high volume caudal block in infants

BackgroundHigh-volume (1.5 ml kg−1) caudal block in infants results in major reductions of cerebral blood flow velocity (CBFV) and cerebral oxygenation, caused by rostral CSF movement which increases intracranial pressure. The primary aim of this study was to determine the relationship between injected volume and CBFV changes. We hypothesised that this volume–blood flow relationship would have a similar albeit inverted shape to the well-known intracranial pressure volume–pressure curve. MethodsFifteen subjects, age 0–6 months, mean (range) weight 4.9 (2.1–6.4) kg, were studied. A 1.5 ml kg−1 caudal injection of 0.2% ropivacaine was administered in three phases separated by two pauses. Subjects were randomised into five groups, in whom the pauses were implemented at different pre-set proportions of the total injected volume. Middle cerebral artery Doppler ultrasonography was used for CBFV measurements (Vmax, peak CBF velocity; Vmin, lowest CBF velocity; velocity time index). Mean flow velocity, pulsatility index, and resistivity index were calculated, and haemodynamic parameters were recorded. ResultsCBFV parameters decreased in all patients. The most affected parameter, Vmin, was reduced by ∼50% (range 15–68%) compared with baseline. There was a nonlinear relationship between the volume of the first phase injection and the CBFV measurement during the first pause. Across all time points, there was a linear relationship between volume administered and CBFV. Systemic haemodynamic parameters remained stable throughout the study. ConclusionsInjection pauses appear to attenuate adverse CBFV increases during administration of a high-volume caudal block.

Cerebral blood flow changes during palpation of external airway structures in healthy volunteers.

Previous studies demonstrate increased intracranial pressure (ICP) during direct laryngoscopy in patients with traumatic brain injury (TBI). Worse outcomes in TBI have been associated with increased ICP. It remains unclear if the same effect occurs during cricothyrotomy. We evaluated changes in cerebral blood flow and hemodynamic changes that occurred during preparation for cricothyrotomy in healthy volunteers. An emergency medicine trainee performed routine anatomical procedural palpation with simultaneous transcranial doppler (TCD) measurements of cerebral blood flow velocities (CBFV) from bilateral middle cerebral arteries (MCAs). Mean arterial pressure (MAP) and heart rate (HR) were recorded throughout event. Our primary outcome was changes in pulsatility index (PI) and CBFV by TCD during palpation. TCD measurements were used as a surrogate for ICP. We enrolled 20 healthy volunteers for this study. No significant differences were found in pulsatility index [Right MCA -0.02 (95% confidence interval, -0.09 to 0.06), left MCA -0.02 (95% confidence interval, -0.011 to 0.07)] or mean CBFV [right MCA -0.70 mm/s (95% confidence interval, -10.15 to 8.75) left MCA -1.20 mm/s (95% confidence interval, -10.68 to 8.28)] during palpation. No significant change in HR was found [-1.1 bpm ((95% confidence interval, -2.4 to 0.1)]. A change in MAP was observed [1.3 mmHg (95% confidence interval, -0.1 to 2.4)]. In healthy individuals, no clinically significant change in cerebral blood flow velocities, ICP, or change heart rate was observed during palpation for cricothyrotomy.

Acute Adolescent Concussion: Cerebrovascular Reactivity, Symptom Burden, And Exercise Response

Cerebral vasoreactivity (CVR) disruptions have been observed following concussion, which may exacerbate concussion-related symptoms. These CVR disruptions may also influence the cerebral blood flow (CBF) responses during aerobic exercise and thus limit exercise capacity post-concussion. PURPOSE: To examine the relationships between concussion-related symptoms, cerebral vasculature’s ability to respond to changes in CO2 (vasoreactivity), and CBF responses during sub-maximal aerobic exercise in adolescents post-concussion. METHODS: : Adolescents less than two weeks post-concussion completed the Post-Concussive Symptom Checklist (PCSC), cerebral vascular assessments, and a modified YMCA exercise protocol. CVR at rest was estimated from the slope (cm/s/mmHg) of the relationship between the increases in breath-by-breath end-tidal CO2 and responses of CBF velocity during an air rebreathing task (i.e., increasing end-tidal CO2). CBF velocity was measured via transcranial Doppler ultrasonography at the M1 segment of the middle cerebral artery unilaterally during the air rebreathing task and during the aerobic exercise test. Only physical and vestibular-related symptoms were included in analysis: headache, nausea, balance problems, dizziness, fatigue, and % activity level. Two stepwise linear regressions were conducted to test (1) if concussion symptoms are related to CVR and (2) if CVR predicted relative change in CBF velocity during the aerobic exercise test. RESULTS: The majority of the variance in CVR was explained by symptoms of nausea, dizziness, as well as current physical activity levels. Age, sex, resting heart rate, balance, and fatigue also contributed to a lesser degree. Exercise caused a significant increase in MCA blood flow (p<0.001). This exercise induced increase in CBF was largely explained by CVR and estimated VO2peak, with resting heart rate, prior concussion(s), and time since injury also contributing. CONCLUSIONS: Our findings suggests that symptom burden is related to disruptions in CVR in adolescents in the early weeks after a concussion. In turn, the cerebral blood flow response to exercise was related to both CVR and aerobic capacity, suggesting that disruptions in CVR may impact exercise tolerance post-concussion.

Estimation of Cerebral Vasomotor Reactivity with Near Infrared Spectroscopy in Young Adults

Cerebral vasomotor reactivity (CVMR) reflects the ability of cerebral blood vessels to regulate cerebral blood flow (CBF) in response to changes in carbon dioxide (CO2). Transcranial Doppler (TCD) measurement of CBF velocity (CBFV) in the large cerebral arteries has been widely used to evaluate CVMR. However, assessment of CVMR in the cerebral microcirculation using near infrared spectroscopy (NIRS) has not been fully explored. The aims of this study are to characterize CVMR during hypo‐ and hypercapnia using NIRS, and to investigate the relationship between CVMRs assessed by NIRS and TCD. Thirty‐six healthy adults aged 21 to 45 (women: n=23) underwent CBFV and brain tissue oxygenation measurements using both NIRS and TCD during hyperventilation (hypocapnia) and modified CO2 rebreathing (hypercapnia). NIRS was placed on the forehead to measure brain tissue oxygenation index (TOI), TCD was used to measure CBFV in the middle cerebral artery, and capnography was used to measure end‐tidal CO2 (EtCO2). The percentage changes in TOI and CBFV relative to the baseline level in response to changes in EtCO2 were calculated to assess CVMR. Both TOI (−6±2 %) and CBFV (−41±10 %) were reduced during hypocapnia, and elevated during hypercapnia (TOI: 6±2 %, CBFV: 62±20 %) (Fig. 1). CVMR assessed by NIRS was significantly lower than that measured with TCD in both hypocapnia (0.33±0.16 vs. 2.21±0.43 %/mmHg, p&lt; 0.001) and hypercapnia (0.52±0.19 vs. 3.88±1.22 %/mmHg, p&lt; 0.001, Fig. 2). CVMRs assessed by NIRS and TCD were correlated in hypocapnia (r=0.40, p=0.02), but not in hypercapnia (r=0.10, p=0.55). These findings suggest that NIRS measurement of TOI in responses to changes in CO2 likely reflects CVMR of the cerebral microcirculation, which is in difference from that measured in the large cerebral arteries using TCDSupport or Funding InformationThe study was supported by National Institutes of Health (R01AG033106 and R01HL102457). JingShang was supported by the China Scholarship Council.The typical recordings of hyperventilation (hypocapnia) and the modified CO2 rebreathing (hypercapnia).Figure 1Average values of the relative change in CBFV and TOI in 4‐mmHg bins of ΔEtCO2 during hypercapnia induced by CO2 rebreathing. Error bars indicate standard deviation. ΔEtCO2 refers to change in EtCO2 from baseline.Figure 2