Visually evoked blood flow response assessed by simultaneous two-channel transcranial Doppler using flow velocity averaging

Cerebrovascular Reactivity and Neurovascular Coupling in Multiple Sclerosis-A Systematic Review

The inflammatory processes observed in the central nervous system in multiple sclerosis (MS) could damage the endothelium of the cerebral vessels and lead to a dysfunctional regulation of vessel tonus and recruitment, potentially impairing cerebrovascular reactivity (CVR) and neurovascular coupling (NVC). Impaired CVR or NVC correlates with declining brain health and potentially plays a causal role in the development of neurodegenerative disease. Therefore, we examined studies on CVR or NVC in MS patients to evaluate the evidence for impaired cerebrovascular function as a contributing disease mechanism in MS. Twenty-three studies were included (12 examined CVR and 11 examined NVC). Six studies found no difference in CVR response between MS patients and healthy controls. Five studies observed reduced CVR in patients. This discrepancy can be because CVR is mainly affected after a long disease duration and therefore is not observed in all patients. All studies used CO₂ as a vasodilating stimulus. The studies on NVC demonstrated diverse results; hence a conclusion that describes all the published observations is difficult to find. Future studies using quantitative techniques and larger study samples are needed to elucidate the discrepancies in the reported results.

The scalability of common paradigms for assessment of cognitive function: A functional transcranial Doppler study

Cognitive paradigms induce changes in cerebral blood flow (CBF) associated with increased metabolic demand, namely neurovascular coupling (NVC). We tested the hypothesis that the effect of complexity and duration of cognitive paradigms will either enhance or inhibit the NVC response. Bilateral CBF velocity (CBFV) in the middle cerebral arteries (MCAs) via transcranial Doppler ultrasound (TCD), blood pressure (BP), electrocardiogram (ECG) and end-tidal CO₂ (EtCO₂) of 16 healthy participants (aged 21–71 years) were simultaneously recorded at rest and during randomized paradigms of different complexities (naming words beginning with P-,R-,V- words and serial subtractions of 100–2,100–7,1000–17), and durations (5s, 30s and 60s). CBFV responses were population mean normalized from a 30-s baseline period prior to task initiation. A significant increase in bilateral CBFV response was observed at the start of all paradigms and provided a similar pattern in most responses, irrespective of complexity or duration. Although significant inter-hemispherical differences were found during performance of R-word and all serial subtraction paradigms, no lateralisation was observed in more complex naming word tasks. Also, the effect of duration was manifested at late stages of 100–7, but not for other paradigms. CBFV responses could not distinguish different levels of complexity or duration with a single presentation of the cognitive paradigm. Further studies of the ordinal scalability of the NVC response are needed with more advanced modelling techniques, or different types of neural stimulation.

The effects of acute incremental hypocapnia on the magnitude of neurovascular coupling in healthy participants

The high metabolic demand of cerebral tissue requires that local perfusion is tightly coupled with local metabolic rate (neurovascular coupling; NVC). During chronic altitude exposure, where individuals are exposed to the antagonistic cerebrovascular effects of hypoxia and hypocapnia, pH is maintained through renal compensation and NVC remains stable. However, the potential independent effect of acute hypocapnia and respiratory alkalosis on NVC remains to be determined. We hypothesized that acute steady-state hypocapnia via voluntary hyperventilation would attenuate the magnitude of NVC. We recruited 17 healthy participants and insonated the posterior cerebral artery (PCA) with transcranial Doppler ultrasound. NVC was elicited using a standardized strobe light stimulus (6 Hz; 5 × 30 s on/off) where absolute delta responses from baseline (BL) in peak, mean, and total area under the curve (tAUC) were quantified. From a BL end-tidal (PET )CO2  level of 36.7 ± 3.2 Torr, participants were coached to hyperventilate to reach steady-state hypocapnic steps of Δ-5 Torr (31.6 ± 3.9) and Δ-10 Torr (26.0 ± 4.0; p < 0.001), which were maintained during the presentation of the visual stimuli. We observed a small but significant reduction in NVC peak (ΔPCAv) from BL during controlled hypocapnia at both Δ-5 (-1.58 cm/s) and Δ-10 (-1.37 cm/s), but no significant decrease in mean or tAUC NVC response was observed. These data demonstrate that acute respiratory alkalosis attenuates peak NVC magnitude at Δ-5 and Δ-10 Torr PET CO2 , equally. Although peak NVC magnitude was mildly attenuated, our data illustrate that mean and tAUC NVC are remarkably stable during acute respiratory alkalosis, suggesting multiple mechanisms underlying NVC.

Cerebral blood flow response rate to task-activation using a novel method can discriminate cognitive impairment from healthy aging

Objective.A new method to classify individuals as 'responders' to task-activated cerebral blood flow velocity (CBFv) has recently been developed. This study investigated whether CBFv response rate to task-activation is affected by Alzheimer's disease (AD) and mild cognitive impairment (MCI).Approach.The 95th thresholds for cross correlation function peak and variance ratio were derived from 270 unstimulated, healthy hemispheres, and were used to classify the presence of a response to task-activation. Thresholds were applied to five cognitive tasks (attention, verbal fluency, language, visuospatial, memory) in CBFv data from 30 healthy older adults (HC), 35 AD and 22 MCI participants. Cumulative response rate (CRR) was calculated from the sum of responses across five tasks, for both hemispheres. Area under the curve (AUC) was derived from receiver operating characteristic (ROC) curve analysis.Main results. The number of responders differed significantly between tasks (p < 0.005) and diagnostic groups (p = 0.011). On post hoc tests there were more responders in the visuospatial (79%-90%) compared to fluency (45%-80%), language (50%-77%), and memory (44%-70%) tasks bilaterally, and responders were greater in the HC (70%) compared to AD (41%) and MCI (23%) groups to at least eight out of ten tasks. At an optimal threshold of 7.5 out of 10 responses, the AUC-ROC distinguished HC from AD and MCI with a; sensitivity 66% and specificity 70% (AUC = 0.72).Significance. Using a novel method to classify responders to cognitive task-activation, HC demonstrated a higher CRR than those with MCI or AD, and a threshold of <8 responses distinguished healthy ageing from dementia.

Effect of reading with direct or indirect light on the visually evoked flow response in the posterior cerebral artery

PURPOSE

Reading with direct light from computer monitors or tablets may cause visual fatigue and hamper reading comprehension. Our aim was to compare the blood flow response in the supplying artery of the visual cortex when reading from tablet screen or from paper. The neurovascular coupling was tested also after 15-minute reading from either monitor or paper.

METHODS

Flow velocity responses evoked by reading from paper and from monitor were measured by transcranial Doppler sonography in a random sequence in both posterior cerebral arteries (PCAs) of 20 young healthy adults. Afterward, PCA flow response evoked by reading from paper was also investigated after 15 minutes reading on the same tablet or paper, in a random order.

RESULTS

Reading from monitor with its own source of light and reading from paper with indirect light caused very similar PCA flow response. Moreover, the flow velocity increase, evoked by reading form paper did not differ after 15-minute reading from monitor or from paper.

CONCLUSIONS

Reading with direct or indirect light produces similar flow response in the occipital cortex.

Cerebral blood flow modulations during antisaccade preparation in chronic hypotension

In addition to symptoms including fatigue, dizziness, reduced drive, or mood disturbance, individuals with chronic low blood pressure (hypotension) frequently report cognitive complaints. While attentional deficits have been empirically confirmed, it is still unknown whether the impairments also encompass executive functions. This study investigated cerebral blood flow modulations in hypotension during a precued antisaccade/prosaccade task requiring the executive function of proactive inhibition in addition to preparatory attention. Using functional transcranial Doppler sonography, bilateral blood flow velocities in the middle cerebral arteries (MCA) were recorded in 39 hypotensive and 40 normotensive participants. In the task, a stimulus appeared left or right of a fixation point 5 s after a cuing stimulus; subjects had to move their gaze to the mirror image position of the stimulus (antisaccade) or toward it (prosaccade control condition). Video-based eye tracking was used for ocular recording. A right dominant MCA blood flow increase arose during task preparation, which was smaller in hypotensive than normotensive participants. In addition, hypotensive participants exhibited lower peak velocity of the saccadic response. The extent of the reductions in blood flow and task performance in hypotension did not differ between antisaccade and prosaccade conditions. The smaller MCA flow increase may reflect reduced activity in the dorsolateral prefrontal and inferior parietal cortices during proactive inhibition and preparatory attention in hypotension. Given that group differences in blood flow and performance arose independent of task complexity and executive function load, hypotension may be characterized by basic attentional impairments rather than particular executive function deficits.

Stimulus-evoked changes in cerebral vessel diameter: A study in healthy humans

The high metabolic demand of neuronal tissue, coupled with its relatively low energy storage capacity, requires that increases in neuronal activation are quickly matched with increased blood flow to ensure efficient supply of oxygen and nutrients to the tissue. For this to occur, dilation of nearby arterioles must be coordinated with the dilation of larger upstream feeding arteries. As it stands, the exact spatial extent of such dilation in humans is unknown. Using non-invasive time-of-flight magnetic resonance angiography in healthy participants, we developed an automatic methodology for reconstructing cerebral arterial vessels and quantifying their diameter on a voxel-by-voxel basis. Specifically, we isolated the posterior cerebral artery (PCA) supplying each occipital lobe and quantified its vasodilation induced by visual stimulation. Stimulus-induced changes were strongest (∼30%) near primary visual cortex and progressively decreased in a non-linear fashion as a function of distance. Surprisingly, weak - albeit significant - changes (∼2%) were observed ∼70 mm from the visual cortex. This demonstrates that visual stimulation modulates vascular tone along the bulk of the PCA segment, and thus may have important implications for our understanding of functional hyperemia in healthy and diseased states.

Functional Transcranial Doppler Ultrasound for Measurement of Hemispheric Lateralization During Visual Memory and Visual Search Cognitive Tasks

Functional transcranial Doppler ultrasound (fTCD) is a noninvasive sensing modality that measures cerebral blood flow velocity (CBFV) with high temporal resolution. CBFV change is correlated to changes in cerebral oxygen uptake, enabling fTCD to measure brain activity and lateralization with high accuracy. However, few studies have examined the relationship of CBFV change during visual search and visual memory tasks. Here a protocol to compare lateralization between these two similar cognitive tasks using fTCD is demonstrated. Ten healthy volunteers (age 21±2 years) were shown visual scenes on a computer and performed visual search and visual memory tasks while CBFV in the bilateral middle cerebral arteries was monitored with fTCD. Each subject completed 40 trials, consisting of baseline (25 s), calibration (variable), instruction (2.5 s), and task (20 s) epochs. Lateralization was computed for each task by calculating the bilateral CBFV envelope percent change from baseline and subtracting the right side from the left side. The results showed significant lateralization ( ) of the visual memory and visual search tasks, with memory reaching lateralization of 1.6% and search reaching lateralization of 0.5%, suggesting that search is more right lateralized (and therefore may be related to "holistic" or global perception) and memory is more left lateralized (and therefore may be related to local perception). This method could be used to compare cerebral activity for any related cognitive tasks as long as the same stimulus is used in all tasks. The protocol is straightforward and the equipment is inexpensive, introducing a low-cost high temporal resolution technique to further study lateralization of the brain.

Cerebrovascular Response to Repetitive Visual Stimulation in Interictal Migraine with Aura

Cortical hypersensitivity and absent habituation to different stimuli have been observed in migraine patients. These features might also be transmitted to the cerebral vasoreactivity, but results are conflicting so far. Transcranial Doppler ultrasound (TCD) was used to assess cerebral blood flow velocity (CBFV) changes in the middle (MCA) and posterior cerebral arteries (PCA) in relation to repetitive checkerboard visual stimulation. Stimulation consisted of 10 consecutive cycles, each comprising 10 s stimulation and 10 s rest. TCD recordings were analysed using stimulus-related averaging algorithm. Data of 19 interictal migraineurs with aura were compared to those of 19 headache-free healthy volunteers. The CBFV increase in PCA and in MCA during visual stimulation was significantly larger and steeper in migraineurs than in controls ( P = 0.017 and P = 0.005). The response in PCA remained stable over the 10 stimulation cycles, both in migraineurs and in controls. The response in MCA was stable only in migraineurs. In controls it decreased over the last 5 stimulation cycles compared with the first 5 cycles ( P = 0.04). Migraineurs with aura exhibit a larger cerebrovascular response to repetitive visual stimulation compared to headache-free subjects. A reduced adaptation to environmental stimuli in migraine is suggested, since there was no habituation in migraineurs in contrast to healthy controls.

Neurovascular coupling in patients with relapsing-remitting multiple sclerosis

OBJECTIVES

Also to the inflammatory demyelinating lesions and degenerative process, altered cerebrovascular reactivity or neurovascular coupling (NVC) might be considered as playing another role in the pathogenesis of multiple sclerosis. The objective of this study is to assess the NVC of patients with relapsing-remitting multiple sclerosis (RRMS) during the acute exacerbation period.

PATIENTS AND METHODS

Four hundred fifty-eight patients with RRMS and 160 healthy subjects were screened for this study during the last 14 years. We performed transtemporal transcranial Doppler recordings from the P2-segments of both posterior cerebral arteries simultaneously during simple or complex visual stimulation. The NVC was defined as a relative increase of the blood flow velocities as a percentage change of the baseline values during visual stimulation.

RESULTS

The NVC to simple visual stimulation was significantly higher in the patients on both sides (37.2±13.5% and 36.0±14.8%; right and left side, respectively) from those of the controls (30.9±9.9% and 30.0±8.8%; right and left side, respectively) (p<0.01). Similarly, the NVC to complex visual stimulation was significantly higher in the patients (43.3±14.1% and 41.7±13.5%; right and left side, respectively) from those of the controls (38.6±14.2% and 37.6±14.1%; right and left side, respectively) (p<0.05).

CONCLUSION

Our results suggest that patients with RRMS during exacerbation period have more reactive neurovascular units in the occipital cortex.

Effect of reading on blood flow changes in the posterior cerebral artery in early blind and sighted people--A transcranial Doppler study

BACKGROUND

Neuroimaging studies proved that Braille reading resulted in visual cortex activation in blind people, however, very few data are available about the measure of flow increase in these subjects. Therefore, we investigated the flow response in the posterior cerebral artery (PCA) of eleven early blind and ten sighted subjects induced by reading Braille and print, respectively.

METHODS

Two experimental protocols were used in both groups: PCA flow velocity during reading was compared to the resting phase and "NLC" phase (volunteers "read" non-lexical characters; e.g. .,-.:,-.:...,). The use of these experimental protocols allowed to investigate separately the effect of "light stimulus+print reading" versus "print reading alone" in sighted, and "hand/finger movement+Braille reading" versus "Braille reading alone" in blind subjects.

RESULTS

The flow response in the PCA evoked by "Braille reading alone" in blind (10.5±4.5%) and "print reading alone" in sighted subjects (8.1±3.5%) was similar. The flow increase induced by "hand/finger movement+Braille reading" and by "Braille reading alone" did not differ in blind people, however, "light stimulus+print reading" in sighted subjects caused higher PCA flow increase (25.9±6.9%) than "print reading alone" (8.1±3.5%).

CONCLUSION

The similar PCA flow response induced by Braille and print reading alone suggested a similar degree of occipital cortex activation in blind and sighted subjects. In sighted people, the 3-times higher flow velocity increase induced by "light stimulus+print reading" compared with "print reading alone" indicated that 2/3 of PCA flow increase during reading was due to the light stimulus and only 1/3 of flow response was caused by reading alone.

Cerebrovascular reactivity and neurovascular coupling in patients with obstructive sleep apnea

AIM

Obstructive sleep apnea syndrome (OSAS) has been implicated as an independent risk factor for stroke. There are data suggesting the presence of lower cerebrovascular reactivity (CVR) as determined by transcranial Doppler (TCD) in patients with OSAS. We concurrently investigated neurovascular coupling (NVC) with visual stimulation, and CVR using breath-holding (BH) test on TCD in patients with OSAS.

MATERIALS AND METHODS

Data were collected in 49 patients with moderate to severe OSAS, and compared to 15 healthy subjects matched for age and risk factors. The CVR to hypercapnia was measured by BH test, and the NVC was performed with visual stimulation.

RESULTS

There were no significant differences in baseline characteristics of patients and controls, except for BMI, which was significantly higher in patients with OSAS (p = 0.036). OSAS patients showed significantly lower reactivity during BH in comparison to controls (36.9% ± 14.0% vs. 46.6% ± 20.1%; p = 0.019). The reactivity time was also significantly shorter in the OSAS group (8.0 ± 4.2 s) when compared to controls (10.1 ± 4.3 s; p = 0.015). The visual stimulation produced similar reactivity in patients (27.7% ± 9.4%) and controls (29.1 ± 13.9; p > 0.05).

CONCLUSIONS

Our data demonstrate a diminished vasodilator response capacity only to a strong stimulator such as hypercapnia in OSAS patients. However, the NVC, as shown by the TCD, is quite normal, suggesting that a weak or mild stimulation produces a proper reactivity among OSAS patients.

Influence of Hypoxia on Cerebral Blood Flow Regulation in Humans

The brain is a vital organ that relies on a constant and adequate supply of blood to match oxygen and glucose delivery with the local metabolic demands of active neurones. It is well established that cerebral blood flow is altered in response to both neural activity and humoral stimuli. Thus, augmented neural activation (e.g. visual stimulation) leads to locally increased cerebral blood flow via functional hyperaemia, whereas humoral stimuli (i.e. alterations in arterial PO2 and PCO2) produce global increases in cerebral blood flow. Perhaps not surprisingly, cerebrovascular responses to neural activity and humoral stimuli may not be highly correlated because they reflect different physiological mechanisms for vasodilation. Exquisite regulation of cerebral blood flow is particularly important under hypoxic conditions when cerebral PO2 can be reduced substantially. Indeed, cerebrovascular reactivity to hypoxia determines the capacity of cerebral vessels to respond and compensate for a reduced oxygen supply. This reactivity is dynamic, changing with prolonged exposure to hypoxic environments, and in patients and healthy individuals exposed to chronic intermittent periods of hypoxia. More recently, a number of animal studies have provided evidence that glial cells (i.e. astrocytes) play an important role in regulating cerebral blood flow under normoxic and hypoxic conditions. This review aims to summarize our current understanding of cerebral blood flow control during hypoxia in humans and put into context the underlying neurovascular mechanisms that may contribute to this regulation.

The Posterior Cerebral Artery and its Main Cortical Branches Identified with Noninvasive Transcranial Color-Coded Duplex Sonography

PURPOSE

To differentiate PCA segments and cortical branches by means of transcranial color-coded duplex sonography (TCCD) and to measure flow parameters at rest and during visual stimulation.

MATERIALS AND METHODS

60 healthy subjects with a good acoustic temporal bone window were examined. The main stem of the PCA (P1, P2 and P3) and 4 main cortical branches - the anterior temporal artery (ATA), the occipital temporal artery (OTA), the parietooccipital artery (POA) and the calcarine artery (CA) - were assessed using an axial transtemporal approach. Systolic and diastolic blood flow velocities (BFVs) were recorded at rest and during visual stimulation.

RESULTS

Identification of the P1 segment of the PCA was successful in 97.5% (117/120) of cases. The P2 and P3 segments were visualized in all cases. The 4 main cortical branches could be identified to varying degrees: ATA in 88%, OTA in 96%, POA in 69% and CA in 62%. There was an evoked flow response in the P2 main stem and in all cortical branches. The most pronounced increase in diastolic/systolic BFV after visual stimulation test was seen in the CA (42%/35%), followed by P2 (30%/24%), the POA (27%/27%), the OTA (16%/13%) and the ATA (9%/8%).

CONCLUSION

Insonation through the temporal bone window with TCCD confidently allows the assessment of the P1 to P3 segments of the PCA as well as the 2 proximal branches, the ATA and the OTA. An ultrasound-based classification of PCA anatomy and its cortical branches may be used as a noninvasive method for the evaluation of posterior circulation pathology.

Effects of aging on the association between cerebrovascular responses to visual stimulation, hypercapnia and arterial stiffness

Aging is associated with decreased vascular compliance and diminished neurovascular- and hypercapnia-evoked cerebral blood flow (CBF) responses. However, the interplay between arterial stiffness and reduced CBF responses is poorly understood. It was hypothesized that increased cerebral arterial stiffness is associated with reduced evoked responses to both, a flashing checkerboard visual stimulation (i.e., neurovascular coupling), and hypercapnia. To test this hypothesis, 20 older (64 ± 8 year; mean ± SD) and 10 young (30 ± 5 year) subjects underwent a visual stimulation (VS) and a hypercapnic test. Blood velocity through the posterior (PCA) and middle cerebral (MCA) arteries was measured concurrently using transcranial Doppler ultrasound (TCD). Cerebral and systemic vascular stiffness were calculated from the cerebral blood velocity and systemic blood pressure waveforms, respectively. Cerebrovascular (MCA: young = 76 ± 15%, older = 98 ± 19%, p = 0.004; PCA: young = 80 ± 16%, older = 106 ± 17%, p < 0.001) and systemic (young = 59 ± 9% and older = 80 ± 9%, p < 0.001) augmentation indices (AI) were higher in the older group. CBF responses to VS (PCA: p < 0.026) and hypercapnia (PCA: p = 0.018; MCA: p = 0.042) were lower in the older group. A curvilinear model fitted to cerebral AI and age showed AI increases until ~60 years of age, after which the increase levels off (PCA: R (2) = 0.45, p < 0.001; MCA: R (2) = 0.31, p < 0.001). Finally, MCA, but not PCA, hypercapnic reactivity was inversely related to cerebral AI (MCA: R (2) = 0.28, p = 0.002; PCA: R (2) = 0.10, p = 0.104). A similar inverse relationship was not observed with the PCA blood flow response to VS (R (2) = 0.06, p = 0.174). In conclusion, older subjects had reduced neurovascular- and hypercapnia-mediated CBF responses. Furthermore, lower hypercapnia-mediated blood flow responses through the MCA were associated with increased vascular stiffness. These findings suggest the reduced hypercapnia-evoked CBF responses through the MCA, in older individuals may be secondary to vascular stiffening.

Methodological aspects of functional transcranial Doppler sonography and recommendations for simultaneous EEG recording

The neurovascular coupling describes a vasoregulative principle of the brain that adapts local cerebral blood flow in accordance with the underlying neuronal activity. It is the basis of modern indirect brain imaging techniques. Because of its wide availability and high tolerability the functional transcranial Doppler has been often used to assess brain function in clinical conditions. In the present paper we will give an overview of the current understanding of the coupling, explain basic principles of the Doppler technique and summarize relevant findings of functional Doppler tests in the different vascular territories of the brain. Finally, the concept of a combined functional electroencephalogram and transcranial Doppler technique will be outlined, which allows simultaneous investigation of the neuronal and vascular responses of neurovascular coupling.

The effect of the cold pressor test on a visually evoked cerebral blood flow velocity response

We investigated the hypothesis that during tonic pain stimulus, neurovascular coupling (NVC) decreases, measuring visually evoked cerebral blood flow velocity response (VEFR) during cold pressor test (CPT) in healthy human subjects as a test. VEFR was calculated as a relative increase in blood flow velocity in the posterior cerebral artery from average values during the last 5 s of the stimulus-OFF period to average values during the last 10 s of the stimulus-ON period. Three consecutive experimental phases were compared: basal, CPT and recovery. During CPT, end-diastolic and mean VEFR increased from 20.2 to 23.6% (p < 0.05) and from 17.5 to 20.0% (p < 0.05), respectively. In recovery phase, end-diastolic and mean VEFR decreased to 17.7% and 15.5%, respectively. Both values were statistically significantly different from CPT phase (p < 0.05). Compared with the basal phase, only end-diastolic VEFR was statistically significantly different in the recovery phase (p < 0.05). Our results are consistent with the assumption that there is a change in the activity of NVC during CPT because of the modulatory influence of subcortical structures activated during tonic pain. Contrary to our expectations, the combined effect of such influences increases rather than decreases NVC.

Hemispheric asymmetry of visual cortical response by means of functional transcranial Doppler

We assessed the visual evoked response and investigated side-to-side differences in mean blood flow velocities (MBFVs) by means of functional transcranial Doppler (fTCD) in 49 right-handed patients with severe internal carotid artery (ICA) stenosis and 30 healthy volunteers, simultaneously in both posterior cerebral arteries (PCAs) using 2 MHz probes, successively in the dark and during the white light stimulation. Statistically significant correlation (P = 0.001) was shown in healthy and in patients (P < 0.05) between MBFV in right PCA in physiological conditions and MBFV in right PCA during the white light stimulation and in the dark. The correlation between MBVF in right PCA and contralateral left PCA was not statistically significant (P > 0.05). The correlation between ipsilateral left PCA was significantly higher than the one with contralateral right PCA (P < 0.05). There is a clear trend towards the lateralisation of the visual evoked response in the right PCA.

Prolonged mean reaction time in posterior cerebral artery during visual stimulation in patients with severe carotid stenosis

While the mean increase in flow velocities in posterior cerebral artery (PCA) as a response to visual stimuli is well documented, the data on the reaction time as a measurement of the vasomotor response of the posterior part of the circle of Willis are still sparse. The aim was to assess the visual evoked response in PCA during white light stimulation by means of functional transcranial doppler in patients with severe internal carotid artery (ICA) stenosis, to introduce a real-time haemodynamic changes as a measurement of the effect of severe carotid disease on the posterior circulation. The measurements were taken in 49 right-handed patients with severe ICA stenosis or occlusion and 30 healthy volunteers, simultaneously in left and right PCA using 2-MHz probes, successively in the dark and during the white light stimulation, during three consecutive repetitive periods of 1 min each. Mean values of mean blood flow velocities (MBFV) and mean reaction time (MRT) with and without visual stimuli were analysed. Linear regression analysis showed no statistically significant correlation between the age, MBFV and a degree of left and right carotid stenosis, and MRT in left and right PCA either in the group of healthy subjects or in the group of patients with severe carotid stenosis, in both test conditions. MRT could be an indicator of compromised cerebral circulation in the presence of haemodynamic significant carotid stenosis as well as an additional and independent haemodynamic parameter of the cerebral visual evoked response.

Basal cerebral blood volume during the poststimulation undershoot in BOLD MRI of the human brain

One of the characteristics of the blood oxygenation level-dependent (BOLD) magnetic resonance imaging (MRI) response to functional challenges of the brain is the poststimulation undershoot, which has been suggested to originate from a delayed recovery of either cerebral blood volume (CBV) or cerebral metabolic rate of oxygen to baseline. Using bolus-tracking MRI in humans, we recently showed that relative CBV rapidly normalizes after the end of stimulation. As this observation contradicts at least part of the blood-pool contrast agent studies performed in animals, we reinvestigated the CBV contribution by dynamic T1-weighted three-dimensional MRI (8 seconds temporal resolution) and Vasovist at 3 T (12 subjects). Initially, we determined the time constants of individual BOLD responses. After injection of Vasovist, CBV-related T1-weighted signal changes revealed a signal increase during visual stimulation (1.7% ± 0.4%), but no change relative to baseline in the poststimulation phase (0.2% ± 0.3%). This finding renders the specific nature of the contrast agent unlikely to be responsible for the discrepancy between human and animal studies. With the assumption of normalized cerebral blood flow after stimulus cessation, a normalized CBV lends support to the idea that the BOLD MRI undershoot reflects a prolonged elevation of oxidative metabolism.

A new visually evoked cerebral blood flow response analysis using a low-frequency estimation

Transcranial Doppler (TCD) has been widely used to monitor cerebral blood flow velocity (BFV) during the performance of cognitive tasks compared with repose periods. Although one of its main advantages is its high temporal resolution, only some of the previous functional TCD studies have focused on the analysis of the temporal evolution of the BFV signal and none of them has performed a spectral analysis of the signal. In this study, maximum BFV data in both posterior cerebral arteries was monitored during a visual perception task (10 cycles of alternating darkness and illumination) for 23 subjects. A peak was located in the low-frequency band of the spectrum of the maximum BFV of each subject both during visual stimulation and repose periods. The frequency of this peak was in the range between 0.037 and 0.098Hz, depending on the subject, the vessel and the experimental condition. The component of the signal at this frequency, which is associated with the slow variations caused by the visual stimuli, was estimated. That way, the variations in BFV caused by the experimental stimuli were isolated from the variations caused by other factors. This low-frequency estimation signal was used to obtain parameters about the temporal evolution and the magnitude variations of the BFV in a reliable way, thus, characterizing the neurovascular coupling of the participants.

Impaired visual evoked flow velocity response in cerebral amyloid angiopathy

OBJECTIVE

Animal models of cerebral amyloid angiopathy (CAA) exhibit abnormal vascular reactivity. We determined whether vascular reactivity, measured by transcranial Doppler ultrasound (TCD), is reduced in humans with CAA.

METHODS

Cases were recruited from an established prospective study of CAA. Healthy controls were recruited from a study of normal aging. Evoked mean flow velocity increase in the posterior cerebral artery (PCA) was measured while subjects viewed a flashing alternating checkerboard stimulus. In a separate but partially overlapping cohort we measured the mean flow velocity increase in the middle cerebral artery (MCA) while subjects inhaled carbon dioxide.

RESULTS

The visual evoked mean flow velocity increase was 8.0 +/- 6.1% in CAA (n = 11) compared to 17.4 +/- 5.7% in controls (n = 9, p = 0.002). The PCA pulsatility index, a marker of distal vascular resistance, was higher in CAA (CAA 1.35 +/- 0.35, control 1.04 +/- 0.14, p = 0.03). Among CAA subjects, lower visual evoked mean flow velocity increase was associated with a higher number of hemorrhages seen on MRI (r = -0.87, p = 0.0005) and higher MRI white matter hyperintensity volume (r = -0.67, p = 0.02). The MCA response to carbon dioxide did not differ between CAA and control in 20 subjects (9 CAA, 11 control, p = 0.54).

CONCLUSIONS

Cerebral amyloid angiopathy (CAA) was associated with decreased vascular reactivity in response to visual stimulation, possibly reflecting the occipital predilection of the disease. The association of posterior cerebral artery (PCA) evoked flow velocity response with elevated PCA pulsatility index and MRI markers of small vessel disease suggests that abnormal PCA evoked flow velocity in CAA is caused by pathology of the distal resistance vessels.

Hemispheric asymmetries in blood flow during color stimulation

Visual stimulation influences mean blood flow velocities (MBFV) in posterior cerebral arteries (PCA). In 51 healthy, right-handed volunteers MBFV were measured in PCA with opened and closed eyes and while watching colored light (red, yellow, green, blue) for 1 minute. Volunteers had eyes closed 2 minutes between different colors. MBFV in left PCA was 41.2 +/- 8.6 cm/s (mean +/- 2SD) and 27.8 +/- 8.5 cm/s with eyes opened and closed, respectively. For red light MBFV in left PCA was 31.4 +/- 7.1 cm/s, for yellow 31.4 +/- 7.2 cm/s, for green 32.0 +/- 8.3 cm/s, and for blue 33.0 +/- 7.6 cm/s. MBFV in right PCA 41.7 +/- 8.9 cm/s and 28.2 +/- 9.1 cm/s with eyes opened and closed, respectively. For red light MBFV in right PCA was 39.4 +/- 8.4 cm/s, for yellow 38.9 +/- 9.2 cm/s, for green 37.8 +/- 9.4 cm/s and for blue 38.0 +/- 8.8 cm/s. There was no significant difference in MBFV between left and right PCA with eyes opened and closed, but MBFV were significantly higher in right PCA for each color than corresponding MBFV in left PCA. These findings could indicate the greater metabolism of visual cortex in right occipital lobe while watching colors. Visual cortex of right occipital lobe could have greater importance in color perception than visual cortex of left occipital lobe.

The effect of methylprednisolone treatment on cerebral reactivity in patients with multiple sclerosis

We assessed the effect of intravenous high-dose methylprednisolone (IVMP) on cerebral reactivity in multiple sclerosis (MS) patients during exacerbations by means of functional transcranial Doppler imaging. Forty-eight clinically defined MS patients were evaluated with visual evoked potentials (VEP) and functional transcranial Doppler sonography (TCD) of both posterior cerebral arteries before and after 5 days of 1000 mg IVMP. After treatment, mean Expanded Disability Status Scale score, mean blood flow velocities and mean blood flow velocities at rest and at stimulation, significantly decreased (p < 0.0001, for each). The change in cerebral blood flow velocity ratio (CBFv) with visual stimulation after treatment increased slightly (p = 0.20). All TCD parameters were not significantly correlated with VEP changes. In conclusion, we observed significant changes in CBFv with a non-significant increase in vascular reactivity after treatment with IVMP in exacerbations of MS. Case-control studies are necessary to draw conclusions regarding the beneficial effects of IVMP treatment.

Coupling between visual evoked cerebral blood flow velocity responses and visual evoked potentials in migraneurs

Neurovascular coupling may be altered in migraneurs. Therefore, visual evoked potentials (VEP) and visually evoked cerebral blood flow velocity responses (VEFR) were simultaneously recorded in 30 healthy controls and 30 migraneurs interictally using a checkerboard stimulus with visual contrasts of 1%, 10% and 100%. The VEFR were measured in the posterior cerebral artery using transcranial Doppler and VEP were recorded from occipital leads. We found an increase in VEFR and VEP in both the healthy and migraneur groups (P < 0.01). VEFR were significantly higher in migraneurs (P < 0.01), while VEP did not significantly differ between the groups (P > 0.05). Regression showed a significant association between VEP and VEFR in both healthy controls (r = 0.66, P < 0.01) and migraneurs (r = 0.63, P < 0.01). The regression coefficient of migraneurs (b = 0.88, SE = 0.08) was significantly higher than that of healthy controls (b = 0.55, SE = 0.07) (P = 0.04). We conclude that neurovascular coupling is increased in migraneurs interictally.

Correlation between blood flow velocity in the middle cerebral artery and EEG during cognitive effort

Cognitive effort modifies blood flow velocity (BFV) in the middle cerebral artery (MCA) which can be recorded by transcranial Doppler sonography (TCD). EEG parameters can be used as indicators of cortical activation. To find temporal and spatial relation between circulatory and bioelectric phenomena, we used combined EEG and TCD measurements during cognitive experiments. Bilateral BFV in the MCAs and 16-channel scalp EEG were recorded during mental arithmetic (MA) and verbal fluency (VF) tests in 12 healthy volunteers. Temporal profile of BFV, heart rate (HR), EEG central frequency (CF), relative alpha power (ralphap), and laterality index (Li) for BFV and CF were statistically analysed. During mental effort, BFV changes showed a reproducible pattern, which was different in MA and VF tests. The Li(BFV) correlated with handedness in 9/12 subjects (75%) in the VF, and in 6/12 subjects (50%) in the MA test. Significant correlation was found between Li(BFV) and Li(CF) during VF (r(2) = 0.69). Li was more indicative for the hemispheric dominance in the VF than in the MA test. During VF test, correlation between HR and BFV was significant in 7/12 subjects. CF and ralphap provide real time assessment of the functional state of the brain tissue during cognition. The correlation between CF and BFV during mental activity suggests a short latency neurogenic and a long latency, supposedly chemical regulation of regional blood flow. Parallel analysis of EEG and flow parameters increases the confidence of determining hemispheric dominance and provides an alternative to study physiological consequences of cognitive processes.

Cognitive performance and cerebral blood flow in essential hypotension

In the present study cerebral blood flow was assessed in 40 subjects with chronically low blood pressure and 40 normotensive controls at resting conditions and during the execution of a cued reaction time task. Blood flow velocities were recorded by means of transcranial Doppler sonography in both middle cerebral arteries. In hypotensives flow velocity at rest was reduced bilaterally. During the anticipation of the stimuli, which the subjects had to respond to, a predominantly right hemispheric increase of flow velocity was observed in both groups. This increase was significantly less pronounced in the hypotensive group. Hypotensives showed longer reaction times, and there was a negative correlation between the extent of the flow velocity increase and the reaction times. This study is the first to demonstrate a reduced cerebral perfusion and maladaption of blood flow to cognitive demands due to essential hypotension.

Functional Imaging with Intraoperative Ultrasound: Detection of Somatosensory Cortex in Dogs with Color-duplex Sonography

OBJECTIVE

To evaluate the capability of intraoperative color-duplex sonography to detect eloquent flow-activated areas and their anatomic relationship in dogs.

METHODS

After craniotomy, the sensory cortex of eight dogs was identified by recording the highest amplitude detected with a grid electrode evoked with somatosensory evoked potential stimulation of the nervus ischiadicus. A 7.5-MHz linear array transducer was placed on the dura, and eight images were taken in color-coded capture mode during baseline and somatosensory evoked potential stimulation of the ipsilateral (nonevoked) and contralateral (evoked) sensory cortex. The differences in flow velocity intensities were statistically compared (Wilcoxon test) in three arbitrary velocity ranges and across all colored pixels in a region of interest between baseline and stimulation in both hemispheres.

RESULTS

Comparing both hemispheres during stimulation, the evoked sensory cortex demonstrated an increase of 10% in the number of counted colored pixels during stimulation, whereas the number of counted colored pixels in the ipsilateral sensory cortex decreased by 2% (P < 0.05), indicating an overall increase in measured flow during stimulation. Comparing differences during nonstimulation and stimulation in single hemispheres, the lowest of the three velocity ranges (approximately 10-20 mm/s) demonstrated a statistically significant (P = 0.01) increase during stimulation, whereas no change was observed during stimulation in the ipsilateral hemisphere. This increase has been confirmed by regional cerebral blood flow measurement with colored microspheres.

CONCLUSION

This study indicates, for the first time, the capability of intraoperative ultrasound to detect functionally important areas during evoked stimulation.

Cerebral and systemic hemodynamic changes during cognitive and motor activation paradigms

Cognitive and/or sensorimotor stimulations of the brain induce increases in cerebral blood flow that are usually associated with increased metabolic demand. We tested the hypothesis that changes in arterial blood pressure (ABP) and arterial Pco(2) also take place during brain activation protocols designed to induce hemispheric lateralization, leading to a pressure-autoregulatory response in addition to the metabolic-driven changes usually assumed by brain stimulation paradigms. Continuous recordings of cerebral blood flow velocity [CBFV; bilateral, middle cerebral artery (MCA)], ABP, ECG, and end-tidal Pco(2) (Pet(CO(2))) were performed in 15 right-handed healthy subjects (aged 21-43 yr), in the seated position, at rest and during 10 repeated presentations of a word generation and a constructional puzzle paradigm that are known to induce differential cortical activation. Derived variables included heart rate, cerebrovascular resistance, critical closing pressure, resistance area product, and the difference between the right and left MCA recordings (CBFV(R-L)). No adaptation of the CBFV(R-L) difference was detected for the repeated presentation of 10 activation tasks, for either paradigm. During activation with the word generation tasks, CBFV changed by (mean +/- SD) 9.0 +/- 3.7% (right MCA, P = 0.0007) and by 12.3 +/- 7.6% (left MCA, P = 0.0007), ABP by 7.7 +/- 6.0 mmHg (P = 0.0007), heart rate by 7.1 +/- 5.3 beats/min (P = 0.0008), and Pet(CO(2)) by -2.32 +/- 2.23 Torr (P = 0.002). For the puzzle paradigm, CBFV changed by 13.9 +/- 6.6% (right MCA, P = 0.0007) and by 11.5 +/- 6.2% (left MCA, P = 0.0007), ABP by 7.1 +/- 8.4 mmHg (P = 0.0054), heart rate by 7.9 +/- 4.6 beats/min (P = 0.0008), and Pet(CO(2)) by -2.42 +/- 2.59 Torr (P = 0.001). The word paradigm led to greater left hemispheric dominance than the right hemispheric dominance observed with the puzzle paradigm (P = 0.004). We concluded that significant changes in ABP and Pet(CO(2)) levels occur during brain activation protocols, and these contribute to the evoked change in CBFV. A pressure-autoregulatory response can be observed in addition to the hemodynamic changes induced by increases in metabolic demand. Simultaneous changes in Pco(2) and heart rate add to the complexity of the response, indicating the need for more detailed modeling and better understanding of brain activation paradigms.

Visually evoked blood flow responses and vasoneuronal coupling in partial epilepsy

OBJECTIVES

Increased metabolic demand is coupled with increased regional blood flow. The decreased vasoreactivity in epileptic patients however, prompts an impact of epileptic dysfunction on vasoneuronal coupling.

MATERIAL AND METHODS

Blood flow velocities during visual stimulation were monitored by TCD in both posterior cerebral arteries in 20 epileptic patients and 20 control persons, response-amplitudes (RA) and pulsatility indices (PI) were analyzed.

RESULTS

The RAs were significantly smaller in patients than in controls (28.4 +/- 5.7% vs 38.4 +/- 10.2%; P < 0.001). RAs were larger in the right side and these right-sided responses were significantly smaller in patients with right-sided vs left-sided epileptic foci (27.9 +/- 5.5% vs 36.1 +/- 4.5%; P < 0.005). The PI during stimulation was significantly larger in patients than in controls (0.92 +/- 0.11 vs 0.74 +/- 0.15; P < 0.001).

CONCLUSION

Our data suggest an impaired vasoneuronal coupling in focal epilepsy, and support the view that the right hemisphere might be more important for color processing.

The relationship between visually evoked cerebral blood flow velocity responses and visual-evoked potentials

A noninvasive assessment of neurovascular coupling would be of great importance. For this reason, we simultaneously studied graded responses of visually evoked cerebral blood flow (CBF) velocity responses (VEFR) and visual-evoked potentials (VEP) to visual contrasts. The records were made from 30 healthy volunteers aged 38.0 +/- 9.6 years. The stimulus was a black-and-white checkerboard with visual contrasts (VC) of 1%, 10%, and 100%. The VEFR were measured in the posterior cerebral artery using transcranial Doppler, and the VEP were recoded from the scalp from occipital leads. To test the relationship between the VEFR and the VEP, a linear regression analysis was performed. We found that the VEFR at 100% VC were 36% higher than those at 10% VC (P < 0.01). The VEFR at 10% VC were 81% higher than those at 1% VC (P < 0.01). The VEP at 100% VC were 76% higher than those at 10% VC (P < 0.01). The VEP at 10% VC were 184% higher than those at 1% VC (P < 0.01). The linear regression showed a significant, moderate association between the VEP and the VEFR (r = 0.66, P < 0.01). The analysis of the regression slopes (b = 0.48 in older subjects vs. b = 0.58 in younger subjects) between two different age subgroups (P < 0.01) did not show any significant difference (P = 0.035). We concluded that a simultaneous recording of VEFR and VEP to graded visual contrasts could allow an assessment of neurovascular coupling.

The influence of visual contrast on visually evoked cerebral blood flow responses

Transcranial Doppler (TCD) could be used to evaluate the visually evoked cerebral blood flow responses (VEFRs) during graded visual cortex activity. The purpose of the study was to evaluate the effects of visual contrasts on VEFR. The records were made from 30 healthy volunteers aged 38.0 +/- 9.6 years. The stimulus was a black-and-white checkerboard with visual contrasts of 1%, 10% and 100%. The VEFRs were measured in the posterior cerebral artery using TCD. We found that the VEFRs at 100% visual contrast were 36% higher than those at 10% visual contrast (p < 0.01). The VEFRs at 10% visual contrast were 81% higher than those at 1% visual contrast (p < 0.01). The linear regression showed significant relationships between the visual contrast and the VEFR (r = 0.61, p < 0.01). We have concluded that TCD monitoring of VEFR detects the changes of the blood flow in the visual cortex and that TCD could allow an assessment of neurovascular coupling.

Acupuncture using laser needles modulates brain function: first evidence from functional transcranial Doppler sonography and functional magnetic resonance imaging

Acupuncture using laser needles is a new totally painless stimulation method which has been described for the first time. This paper presents an experimental double-blind study in acupuncture research in healthy volunteers using a new optical stimulation method. We investigated 18 healthy volunteers (mean age +/- SD: 25.4 +/- 4.3 years; range: 21-30 years; 11 female, 7 male) in a randomized controlled cross-over trial using functional multidirectional transcranial ultrasound Doppler sonography (fTCD; n = 17) and performed functional magnetic resonance imaging (fMRI) in one volunteer. Stimulation of vision-related acupoints resulted in an increase of mean blood flow velocity in the posterior cerebral artery measured by fTCD [before stimulation (mean +/- SE): 42.2 +/- 2.5; during stimulation: 44.2 +/- 2.6; after stimulation: 42.3 +/- 2.4 cm/s, n.s.]. Mean blood flow velocity in the middle cerebral artery decreased insignificantly. Significant changes (p < 0.05) of brain activity were demonstrated in the occipital and frontal gyrus by fMRI. Optical stimulation using properly adjusted laser needles has the advantage that the stimulation cannot be felt by the patient (painless and no tactile stimulation) and the operator may also be unaware of whether the stimulation system is active. Therefore true double-blind studies in acupuncture research can be performed.

Functional transcranial Doppler sonography as a tool in psychophysiological research

Functional transcranial Doppler sonography (fTCD) allows the noninvasive and uncomplicated registration of intracranial blood flow parameters under defined conditions of stimulation. Although local distribution patterns of regional blood perfusion can be measured with high spatial resolution through neuroimaging methods (e.g., PET or SPECT), these methods are limited by their low temporal resolution. The high temporal resolution provided by fTCD, however, allows the recording of the dynamic component of cerebral blood perfusion by continuously measuring the cerebral blood flow velocity in the basal cerebral arteries. Hence, this method is especially appropriate for the investigation of fast neuronal activation processes, which are generally accompanied by changes in local blood perfusion. In this review, we present methodical issues regarding fTCD, as well its application in the field of psychology, especially psychophysiology. The relevant studies available to date investigate processes of attention and perception, higher cognitive functions, and emotional and psychomotor processes. Considering the current state of methodology and research, fTCD can be seen to be an important complement to the other psychophysiological methods for studying brain function.

Cerebrovascular reactivity in adolescents with migraine and tension-type headache during headache-free interval and attack

BACKGROUND

Migraine is a common cause of headache in adolescents. Assuming that the cerebral vasculature is involved in the pathophysiology of migraine, we compared cerebral vasoreactivity in adolescents both during a migraine attack and a headache-free interval.

METHODS

A functional transcranial Doppler test utilizing a visual stimulation paradigm was undertaken to measure the evoked flow velocity in the posterior cerebral artery of adolescents suffering from a migraine without aura or a tension-type headache. To serve as a control, data previously obtained from age-matched adolescents with no primary headache disorder were used. The flow curves were evaluated by determining the maximal flow velocity increase and by modeling their time course according to a control system analysis. In that analysis, the main dynamic features of the flow response were described mathematically in terms of a control system model of low order. The parameters were time delay, gain, attenuation, rate time, and natural frequency.

RESULTS

The attenuation parameter (P<.005), indicative of an increased tone of the vessel, and the resting absolute flow velocity (P<.01) both showed a significant increase during an attack in the adolescents with migraine; the gain parameter showed a trend towards similar increase (P =.07). The maximal flow velocity did not increase significantly during an attack.

CONCLUSIONS

The control system approach utilized here appears to be more sensitive for detecting migraine-associated changes in cerebral vasoreactivity than examination of the maximal flow velocities alone.

TCD study of hemodynamic changes in PCA response to photic stimulation

OBJECTIVES

During visual stimulation, the elevated metabolism rate will couple with increase of blood flow velocity(BFV) in posterior cerebral artery(PCA). This study with TCD was aimed to investigate whether the coupling might change according to the different vasoneuronal conditions.

METHODS

Ninety-nine volunteers including 24 hypertension(HT) patients and 2 patients suffering from both HT and diabetes mellitus (DM) were enrolled in this trial. BFV and pulse indexes(PI) in P2 segments of PCA on both sides were monitored during visual stimulation.

RESULTS

In all subjects, Mean BFV increased and PI went down in response to visual stimulation. The percentages of changes (deltaV and deltaP) of both mean BFV and PI were larger in young group( < 55 years old) than in old one(> or = 55 years old). There was significant positive correlation between deltaV and deltaP. Multivariated regression analysis did not show HT and DM, but age related to deltaV(deltaP). We did not find significant difference of deltaV(deltaP) between left and right sides.

CONCLUSIONS

Blood flow velocity in PCA P2 segment increased due to decreased cerebrovascular resistance during visual stimulation and the response weakened with aging of the patient.

Control system analysis of visually evoked blood flow regulation in humans under normocapnia and hypercapnia

OBJECTIVE

Among other factors, the cerebral blood flow (CBF) is regulated in accordance to the arterial CO(2) tension and the cortical activity. The CO(2) test is commonly used to measure the vascular reserve capacity. Most functional imaging studies rely on the activity-flow coupling (AFC). We aimed to combine both challenges in order to increase the insight into mechanisms of CBF regulation.

METHODS

Fifteen healthy students underwent a functional transcranial Doppler test using a visual stimulation paradigm: firstly under normocapnia and secondly under conditions of hypercapnia. Hypercapnia was induced by breathing a carbogene gas mixture of 5% CO(2) and 95% O(2). The entire time course of flow velocity adaptation in the posterior cerebral artery (PCA) was analyzed mathematically using a control system approach.

RESULTS

Resting CBF velocities increased by nearly 26% under conditions of hypercapnia, whereas the slight increase in arterial blood pressure (ABP) and the decrease in the Pourcelot-Pulsatility index (PI) were statistically not significant. From the control system parameters which were time delay, rate time, gain, attenuation and natural frequency, only the parameter rate time, indicative for the initial steepness of flow velocity increase, showed a statistically significant decrease, consistently for the peak systolic and enddiastolic flow velocity data. As concluded from the unchanged gain parameter the absolute amount of blood flow evoked by the same visual stimulus increased also by 26%.

CONCLUSION

Evaluated by Doppler measurements hypercapnia seems to influence the AFC in two ways: It decreases the steepness of the initial increase in blood flow velocity and enhances the absolute amount of blood flow evoked by the same stimulus.

Cerebral hemodynamic patterns with technetium Tc 99m exametazime single photon emission computed tomography and transcranial Doppler sonography: a validation study using visual stimulation

OBJECTIVE

To assess whether a correlation between perfusion changes to visual stimulus on the bilateral occipital areas and blood flow velocity changes to visual stimulus in both posterior cerebral arteries is present.

METHODS

Nine right-handed healthy subjects (4 women and 5 men; mean +/- SD age, 58.0 +/- 5.6 years) were included in the study. Visual stimulation was performed in room light with the subject's eyes open and looking around versus eyes closed as the stimulus-off condition. The blood flow velocities were recorded using transcranial Doppler sonography, and the regional cerebral blood flow measurements were recorded with the use of technetium Tc 99m exametazime and a single photon emission computed tomographic gamma camera system. Individual reactivity was defined as a relative increase of blood flow velocity and perfusion, which were calculated as percentage changes of baseline values.

RESULTS

Visual stimuli produced a marked increase of blood flow velocity in both posterior cerebral arteries (35.2 +/- 2.3 cm/s; P < .001) without a significant side-to-side difference in all subjects as well as a marked increase of perfusion on both occipital areas (24.9 +/- 4.8 cm/s; P < .01). Moreover, there was a positive correlation between blood flow velocity changes and perfusion changes on both sides (r = 0.833; P < .01).

CONCLUSIONS

The use of bilateral simultaneous Doppler recordings by means of a flow velocity averaging algorithm to a specific stimulus allows quantitative assessment of blood flow responses, and simple visual stimuli can be applied for different disorders to assess the vasomotor regulation that may result in measurable abnormal cerebral flow regulation even when clinically stabilized.

The influence of brightness, colour and complexity on visual evoked doppler flow responses

The purpose of this study was to evaluate specific influence of colour, brightness and complexity on visual evoked flow responses (VEFRs). A total of 31 healthy subjects aged 35.1 +/- 7.7 years participated in the study. Mean arterial velocity was measured in the right posterior cerebral artery (v(pca)) and in the left middle cerebral artery (v(mca)) by Multi-DopX4 (DWL). Simple-white (SW), red (R) and complex-checkerboard (C) stimuli were used. VEFRs were determined by the difference of the v(pca):v(mca) ratio before and after stimulation. The VEFRs of SW with brightness of 21.4 cd/m(2), 10.5 cd/m(2) and 2 cd/m(2) were 8.7 +/- 3.4%, 9.1 +/- 3.0% and 8.0 +/- 3.7%, respectively (p < 0.001). The VEFRs of R and C stimuli were 10.4 +/- 6.5% and 12.4 +/- 6.1%, respectively (p < 0.001). ANOVA for repeated measurements did not show significant variances (p = 0.295) between VEFRs of SW of different brightness, but variances between VEFRs of SW, R and C stimuli were significant (p < 0.001). We found significant differences between VEFRs of SW and of C stimuli (3.8 +/- 1.9%, p < 0.001), VEFRs of SW and of R stimuli (1.8 +/- 2.4%, p = 0.008) as well as between VEFRs of C and of R stimuli (2.0 +/- 2.5%, p = 0.010). We have concluded that SW, R and C stimuli have a specific influence on VEFRs. Brightness does not appear to affect VEFRs.

Vinpocetine increases cerebral blood flow and oxygenation in stroke patients: a near infrared spectroscopy and transcranial Doppler study

OBJECTIVE

To investigate the effect of vinpocetine on cerebral blood flow (CBF) in the compromised circulation of a stroke affected hemisphere using transcranial Doppler (TCD) and near infrared spectroscopy (NIRS) methods.

METHODS

43 patients with ischemic stroke were randomized into vinpocetine (VP) and placebo group in a double blind, placebo-controlled study of the effect of a single-dose i.v. infusion of vinpocetine on cerebral blood perfusion and oxygenation. In the VP group 20 mg VP in 500 ml saline, in the placebo group 500 ml saline alone were administered. The concentrations of oxy-, reduced- and total hemoglobin were measured by NIRS frontolaterally on the side of lesion while the mean cerebral blood flow velocity (CBFV), the pulsatility index (PI) and Doppler spectral intensity (DSI) were monitored by TCD in the middle cerebral artery on the same side. Values were averaged for the first 5 min prior to the infusion and for the last 5 min of infusion and they were compared between groups.

RESULTS

The concentration of all three chromophores increased during infusion in the VP group (mean dHbT = 1.03, CI(95) = 0.84, P = 0.058; mean dHbO = 0.92, CI(95) = 0.91, P = 0.071; mean dHb = 0.10, CI(95) = 0.21, P = 0.297). The HbT and HbO showed a substantially smaller increase in the placebo group (mean dHbT = 0.31, CI(95) = 0.74, P = 0.22; mean dHbO = 0.57, CI(95) = 0.80, P = 0.094) while the Hb decreased (mean dHb = -0.26, CI(95) = 0.29, P = 0.05). Comparing to the placebo group Hb increased significantly in the VP group (P = 0.027) while the increase of HbO and HbT did not reach the level of significance (P = 0.29 and 0.11). DSI showed a significantly larger increase in the VP than in placebo group (dDSI=25.8 CI(95)=8.8 [VP]; dDSI =3.3, CI(95) = 3.7 [Placebo], P < 0.005). The CBFV and PI did not differ significantly between groups. (dVm = 5.0+/-2.98 cm/s [VP], dVm = 4.1+/-2.57 cm/s [Placebo], P = 0.28; dPI = 0.08 [VP], dPI = 0.09 [Placebo]; P = 0.47).

CONCLUSION

VP increases cerebral perfusion and parenchymal oxygen extraction as well. The increased perfusion was indicated by NIRS and by TCD measurement of DSI while conventional velocity and pulsatility measurements failed to detect theses effects. NIRS is a sensitive, feasible method of measuring changes in regional blood flow and tissue oxygenation in the superficial cortex.

Cerebral blood flow velocity changes to visual stimuli in patients with multiple sclerosis

We assessed the blood flow velocity (BFv) changes to visual stimuli using transcranial Doppler (TCD) in patients with multiple sclerosis (MS) during an exacerbation period by means of vasoneuronal coupling. Eighty-four patients (19 men, 75 women) and 45 healthy subjects (14 men, 31 women) were studied. Both posterior cerebral arteries (PCAs) were simultaneously monitored by TCD sonography during 10 cycles of 20 s eyes open observing complex moving visual images, and 20 s eyes closed at the end of every cycle. TCD sonography was performed at least at the first 2 days of exacerbation. Mean cerebral BFv throughout the procedure (p=0.003, p=0.001; right and left sides, respectively), velocity at rest (p=0.001, p<0.001), and velocity at stimulation (p=0.021, p=0.01) on both PCAs were significantly lower in patients than controls. However, BFv changes to visual stimulation on both sides were significantly higher in patients (p=0.01, p=0.031) compared to controls. There were negative correlations between P100 latencies and relative blood flow changes on both sides, but it was not significant on the left side. These results may suggest that patients with MS during exacerbation have more reactive vessels in the posterior circulation and! or more reactive neurons in the occipital cortex.

Changes in blood flow velocity in the middle and anterior cerebral arteries evoked by walking

PURPOSE

Transcranial Doppler sonography (TCD) is an established method for assessing changes in blood flow velocity (BFV) coupled to brain activity. Our objective was to investigate whether walking induces measurable changes in BFV in healthy subjects.

METHODS

Changes in BFV in both middle cerebral arteries (MCAs) of 40 healthy adult subjects during walking on a treadmill were measured using bilateral TCD. In 8 of the 40 subjects, 1 anterior cerebral artery (ACA) was monitored simultaneously with the contralateral MCA. The percentage increase in BFV (BFVI%) compared with the baseline velocity (V(0)), the percentage decrease in BFV (BFVD%) compared with the V(0), and the normalized ACA-MCA ratio were analyzed.

RESULTS

The overall mean (+/- standard deviation [SD]) V(0) was 59.9 +/- 11.6 cm/second in the left MCA and 60.1 +/- 12.9 cm/second in the right MCA. Women had higher V(0) values than men had. Walking evoked an initial mean overall BFVI% in both left (8.4 +/- 5.1%) and right MCAs (9.1 +/- 5.1%), followed by a decrease to below baseline values in 38 of 40 subjects. A statistically significant increase of the normalized ACA-MCA ratio was measured, indicating that changes in BFV in the ACA territory were coupled to brain activation during walking.

CONCLUSIONS

The use of functional TCD showed different changes in BFV in the ACAs and MCAs during walking. This method may be an interesting tool for monitoring progress in patients with motor deficits of the legs, such as paresis.

Comparison of visually evoked peak systolic and end diastolic blood flow velocity using a control system approach

Transcranial Doppler sonography measures blood flow velocity in basal cerebral vessels with high accuracy. For quantification, time averaged mean blood flow velocities are used most because the peak systolic and end diastolic blood flow velocities mark the velocity extremes of one heart cycle. It is known, from hemodynamic measurements of the neurovascular coupling mechanism, that the end diastolic velocity is more sensitive for change in hemodynamics than the peak systolic velocity. Thus, we used a recently introduced control system approach to compare both indices for their use in functional transcranial Doppler tests focusing on hemodynamics of blood flow velocity change. We enrolled 65 healthy young volunteers without a medical history of cardiovascular risk factors, and performed a visual stimulation test. Peak systolic and end diastolic maximal blood flow velocities were used after transformation to relative data for control-system analysis. Due to Doppler artefacts, 95% of peak systolic and 86% of end diastolic data sets were analyzed. Results showed statistically significant differences for resting blood flow velocity and the control system parameter gain, attenuation and rate time, whereas the parameters' natural frequency and time delay were equal. Increase in relative blood flow velocity in the posterior cerebral artery due to visual-cortical stimulation was higher in end diastolic values than peak systolic data. Using a complex visual stimulation paradigm, the higher sensitivity of the end diastolic index is of no practical use. Being less influenced by Doppler artefacts, the peak systolic velocity index is more feasible for control-system analysis of dynamic blood flow regulation.