Recordings Of Cerebral Blood Flow Velocity Research Articles

The upper frequency limit of dynamic cerebral autoregulation.

Dynamic cerebral autoregulation (CA) is expressed by the temporal pattern of cerebral blood flow (CBF) recovery following a sudden change in arterial blood pressure (BP). Transfer function analysis of BP as input and CBF velocity as output can express dynamic CA through its amplitude (or gain) and phase frequency responses. The upper frequency limit (FupLim ) at which dynamic CA can operate is of considerable physiological interest and can also provide additional information about worsening CA due to disease processes. In healthy subjects FupLim was strongly dependent on arterial changes induced by four different breathing manoeuvres. The considerable intersubject variability in FupLim suggests that fixed frequency bands should not be adopted for averaging values of gain and phase in studies of dynamic CA. Dynamic cerebral autoregulation (CA) can be expressed in the frequency domain by the amplitude and phase frequency responses calculated by transfer function analysis of arterial blood pressure (BP) and cerebral blood flow velocity (CBFV). We studied the effects of arterial ( ) on the upper frequency limit (FupLim ) of these responses and its intersubject variability. Twenty-four healthy subjects (11 female, age 36.0±13.4years) were recruited. Recordings of CBFV (transcranial Doppler ultrasound), BP (Finometer) and end-tidal CO2 ( , capnography) were performed during 5min at rest (normocapnia) and during four breathing manoeuvres: 5% and 8% CO2 in air and hyperventilation targeting reductions of 5 and 10mmHg compared to normocapnia. FupLim was determined by the break point of the autoregulation index (ARI) curve as a function of frequency when the phase response was gradually set to zero. The five breathing conditions led to highly significant differences in (p<0.0001), CBFV (P<0.0001), ARI (p<0.0001) and FupLim (p<0.0001). FupLim ranged from 0.167±0.036Hz at the lowest values of hypocapnia (28.1±1.9mmHg) to 0.094±0.040Hz at the highest level of hypercapnia (41.7±5.4mmHg), showing a correlation of r=-0.53 (p<0.001) with . These findings reinforce the key role of in CBF regulation. The considerable intersubject variability of FupLim suggests that fixed frequency bands should not be adopted for averaging values of gain and phase in dynamic CA studies, and that the higher frequency band (0.20-0.40Hz), in particular, does not contain relevant information about dynamic CA. Further investigations are needed to assess the information value of FupLim as a marker of dynamic CA efficiency in physiological and clinical studies.

PaCO2 measurement in cerebral haemodynamics: face mask or nasal cannulae?

Objective. PaCO2 affects cerebral blood flow (CBF) and its regulatory mechanisms, but the effects of CO2 measurement technique on cerebrovascular parameters are unknown. In order to determine if the two most commonly used approaches, face mask (FM) or nasal cannulae (NC), are interchangeable or not, we tested the hypothesis that the use of FM versus NC does not lead to significant differences in CO2-related systemic and cerebrovascular parameters. Approach. Recordings of CBF velocity (CBFV), blood pressure (BP), heart rate, and end-tidal CO2 (EtCO2) were performed in 42 subjects during normocapnia (FM or NC) and 5% CO2 inhalation (FM) or hyperventilation (NC). Dynamic cerebral autoregulation was assessed with the autoregulation index (ARI), derived by transfer function analysis from the CBFV response to a hypothetical step change in BP. Main Results. Significant differences in physiological parameters were seen between FM and NC: EtCO2 (37.40 versus 35.26 mmHg, p = 0.001) and heart rate (69.6 versus 66.7 bpm, p = 0.001) respectively. No differences were observed for mean BP, CBFV or the ARI index. Significance. Use of FM or NC for measurement of EtCO2 leads to physiological changes and differences in parameter values that need to be taken into consideration when interpreting and/or comparing results in studies of cerebral haemodynamics.

The Longitudinal Evolution of Cerebral Blood Flow Regulation after Acute Ischaemic Stroke

Background: Acute stroke is known to impair cerebral blood flow (CBF) regulation, but the longitudinal changes of these effects have been poorly reported. The main CBF regulatory mechanisms [cerebral autoregulation (CA) and neurovascular coupling (NVC)] were assessed over 3 months after acute ischaemic stroke. Methods: Recordings of CBF velocity (CBFv), blood pressure (BP), and end-tidal CO₂ were performed during 5 min baseline and 1 min passive movement of the elbow. Stroke patients were assessed <72 h of stroke onset, and at 2 weeks, 1 and 3 months after stroke. Results: Fifteen acute stroke subjects underwent all 4 sessions and were compared to 22 control subjects. Baseline recordings revealed a significantly lower CBFv in the affected hemisphere within 72 h after stroke compared to controls (p = 0.02) and a reduction in CA index most marked at 2 weeks (p = 0.009). CBFv rise in response to passive arm movement was decreased bilaterally after stroke, particularly in the affected hemisphere (p < 0.01). Both alterations in CA and NVC returned to control levels during recovery. Conclusion: The major novel finding of this study was that both CA and NVC regulatory mechanisms deteriorated initially following stroke onset, but returned to control levels during the recovery period. These findings are relevant to guide the timing of interventions to manipulate BP and potentially for the impact of intensive rehabilitation strategies that may precipitate acute physiological perturbations but require further exploration in a larger population that better reflects the heterogeneity of stroke. Further, they will also enable the potential influence of stroke subtype to be investigated.

Improving the sensitivity of neurovascular coupling assessment using motor‐cognitive paradigms post‐stroke (1068.4)

Neurovascular coupling (NVC) has been used as an index of synaptic activity in health and disease. Cerebral blood flow velocity (CBFv) responses to neural stimulation can be misleading due to influences from peripheral co‐variates, leading to inaccurate NVC assessment. A dynamic multivariate autoregressive‐moving average (ARMA) model was used to calculate the contributions of blood pressure (BP), PaCO2 and the stimulus itself to changes in CBFv during active (AP), passive (PP) and motor imagery (MIP) paradigms of elbow flexion/ extension. Continuous recordings of CBFv, BP, heart rate and end‐tidal CO2 were performed in 20 stroke patients and 17 control subjects during the three paradigms. Though the paradigms led to a significant bilateral CBFv increase in both groups, differences between stroke and controls were found only during the PP (p=0.02). ARMA analysis revealed a marked BP contribution to the initial peaked CBFv response in both groups during all paradigms, as well as a decrease in the amplitude of the stimulation component after stroke (p&lt;0.001). PaCO2 did not interfere with the CBFv responses during either the MIP in controls or any of the paradigms in the stroke group. However, the responses to AP and MIP in controls were influenced by a significant reduction in PaCO2 (AP reduction of 2%; MIP reduction of 2.5%). By removing the influences of BP and PaCO2, it was possible to detect differences in the effects of stimulation between controls and strokes. We concluded that, independent of the paradigm adopted for NVC assessment, factoring out the influences of BP and PaCO2 can lead to improvements in sensitivity.Grant Funding Source: CAPES ‐ Ministry of Education of Brazil

An analysis of cerebral blood flow from middle cerebral arteries during cognitive tasks via functional transcranial Doppler recordings

Functional transcranial Doppler (fTCD) is a useful medical imaging technique to monitor cerebral blood flow velocity (CBFV) in major cerebral arteries. In this paper, CBFV changes in the right and left middle cerebral arteries (MCA) caused by cognitive tasks, such as word generation tasks and mental rotation tasks, were examined using fTCD. CBFV recordings were collected from 20 healthy subjects (10 females, 10 males). We obtained both the raw CBFV signal and the envelope CBFV signal, which is the maximal velocity to gain more information about the changes and hemisphere lateralization in cognitive tasks compared to the resting state. Time, frequency, time–frequency, and information-theoretic features were calculated and compared. Sex effects were also taken into consideration. The results of our analysis demonstrated that the raw CBFV signal contained more descriptive information than the envelope signals. Furthermore, both types of cognitive tasks produced higher values in most signal features. Geometric tasks were more distinguished from the rest-state than verbal tasks and the lateralization was exhibited in right MCA during geometric tasks. Our results show that the raw CBFV signals provided valuable information when studying the effects of cognitive tasks and lateralization in the MCA.

Cerebral Hemodynamics During Spinal Cord Stimulation

An increase of cerebral blood flow (CBF) during spinal cord stimulation (SCS) has been shown in man using the 133Xenon inhalation technique. We report the effects of SCS on cerebral hemodynamics studied with transcranial Doppler sonography (TCD). Twenty-six patients with epidural electrodes implanted at the cervical level (11 patients) and at the thoracic level (15 patients) were investigated with recordings of CBF velocity in the middle cerebral arteries before and during SCS. Sixty-three and two-thirds percent of patients with cervical stimulation and 29.4% of patients with thoracic stimulation showed a decrease of cerebral vascular resistance simultaneously with an increase of the velocity of CBF.

Cerebral blood flow velocity recordings and the prediction of intracranial haemorrhage and ischaemia

Doppler ultrasound was used to study prospectively cerebral and cardiovascular hemodynamics in a cohort of 120 preterm infants to see whether it was possible to predict infants at increased risk of developing cerebral pathology. The infants were divided into four outcome groups: Group I (n = 65, median gestation = 30 weeks) did not develop periventricular haemorrhage (PVH) nor periventricular leukomalacia (PVL) Group II (n = 43, median gestation = 28 weeks) developed PVH as the first or only cerebral lesion Group III (n = 7, median gestation = 29 weeks) developed PVL as the first or only cerebral lesion Group IV (n = 5, median gestation = 28 weeks) developed PVH and PVL simultaneously. Cerebral blood flow velocity (CBFV) and aorta blood flow velocity (ABFV) recordings made before the onset of PVH or PVL were compared between the four groups on each postnatal day but it was not possible to demonstrate a statistically significant difference between these variables in the four outcome groups. We conclude, therefore, that it is not possible to identify the infants who will go on to develop haemorrhage or ischaemic lesions on the basis of Doppler cerebral haemodynamic studies.

Severe birth asphyxia and abnormal cerebral blood-flow velocity.

Thirty-four fullterm infants with postasphyxial encephalopathy (PAE) were studied with ultrasound to assess cerebral blood-flow velocity (CBFV). A control group of 126 healthy infants also had CBFV recordings during the first week of life. Measurements for the majority of the asphyxiated group fell into two abnormal patterns in relation to the control group: four had low CBFV (less than 2 SD below the mean) and 17 had high CBFV (greater than or equal to 2 SD above the mean); all had severe PAE. Median age at the first high measurement was 26 hours. There were no statistically significant differences between infants with normal and abnormal CBFV in measurements of PaCO2 or mean arterial blood-pressure. A Pourcelot Resistance Index (PRI) less than 0.55 was common in the presence of high CBFV, but never seen with low measurements. 21 of the 34 infants died of their asphyxial insult and four of the survivors have severe cerebral palsy at a median age of two years. Three of the four with low CBFV died, and no infant with CBFV greater than 3 SD survived without severe impairment. The positive predictive value of CBFV measurements less than 2 SD or greater than 3 SD for death or severe impairment is 94 per cent, compared with 83 per cent for low PRI alone. The authors believe that high CBFV is the result of vasoparalysis of cerebral arterioles and that it represents a form of irreversible cerebral injury.

Quantification of Cardiovascular Instability in Premature Infants Using Spectral Analysis of Waveforms

Spectral analysis was applied to blood pressure and cerebral blood flow velocity recordings in premature infants with respiratory distress in order to quantify respiration-induced cardiovascular variability. Aortic blood pressure was transduced via an umbilical arterial catheter and cerebral blood flow velocity measured in the anterior cerebral artery using a 10 MHz continuous wave Doppler velocimeter in 16 infants less than or equal to 32 wk gestational age. Spectral analysis of the resulting waveforms revealed heart rate and respiratory rate components whose relative amplitudes (heart rate/respiratory rate amplitude ratio) represent an index of that component of variability induced by respiratory events. The mean (heart rate/respiratory rate amplitude) ratio was 47.2 in spontaneously breathing infants and rose to 165.9 in infants who were ventilated during muscle paralysis (p = 0.0003). Cerebral blood flow velocity recordings showed R components in only 22 of 38 simultaneous recordings. This method can be used to quantify respiration-induced cardiovascular variability and its response to therapy, and may provide a means of identifying infants at risk from brain injury due to an inability to regulate cerebral blood flow.