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

Cerebral blood flow regulation and cognitive performance in hypertension

We examined the relation between transcranial Doppler (TCD) markers of cerebral blood flow regulation and cognitive performance in hypertension (HT) patients to evaluate the predictive value of these markers for cognitive decline. We assessed dynamic cerebral autoregulation (dCA), vasoreactivity to carbon dioxide, and neurovascular coupling (NVC) in the middle (MCA) and posterior (PCA) cerebral arteries of 52 patients. Neuropsychological evaluation included the Montreal Cognitive Assessment and tests covering attention, executive function, processing speed, and memory. Notably, reduced rate time in the PCA significantly predicted better processing speed (p = 0.003). Furthermore, reduced overshoot systolic cerebral blood velocity in the PCA and reduced phase in the VLF range in the MCA (p = 0.021 and p = 0.017, respectively) significantly predicted better memory. Intriguingly, enhanced dCA in the MCA predicted poorer memory performance, while reduced NVC in the PCA predicted both superior processing speed and memory performance. These findings suggest that HT-induced changes in cerebral hemodynamics impact cognitive performance. Further research should verify these observations and elucidate whether these changes represent adaptive responses or neurovascular inefficiency. TCD markers might provide insights into HT-related cognitive decline.

Orbital and ocular perfusion in thyroid eye disease

Thyroid eye disease (TED) is characterized by enlargement of extraocular muscles, an increase in retrobulbar fat, orbital fibrosis, and fluctuations in plasma thyroid hormone levels in most patients, often associated with raised autoantibody titers. The occurrence of orbital space conflict compromises the orbital perfusion, unchecked progression of which results in irreversible loss of visual acuity and visual fields. The quantitative assessment of orbital perfusion can be done by measurement of blood flow velocities in the superior ophthalmic vein (SOV), ophthalmic artery (OA), central retinal artery (CRA), and posterior ciliary artery by color Doppler imaging. The retinal and choroidal microvasculature is studied by optical coherence tomography and optical coherence tomography angiography. The orbital and ocular perfusion fluctuates during the course of TED. Orbital congestion is reflected by the reduction or reversal of SOV flow and an increase in subfoveal choroidal thickness. The active phase is characterized by high blood flow velocities of the OA and CRA. The onset of dysthyroid optic neuropathy is associated with reduced arterial perfusion and reduction in parafoveal and peripapillary vascular density. Orbital decompression improves the SOV flow and decreases the resistivity index of CRA. Sequential evaluation of orbital hemodynamic changes can thus supplement the clinical scoring systems for monitoring and planning intervention in TED.

Exploration of cerebral hemodynamic pathways through which large artery function affects neurovascular coupling in young women

Background

The interactions between large artery function and neurovascular coupling (NVC) are emerging as important contributors to cognitive health. Women are disproportionally affected by Alzheimer's disease and related dementia later in life. Understanding large artery correlates of NVC in young women may help with preservation of cognitive health with advancing age.

Purpose

To explore the association between large artery function, NVC and cognitive performance in young women.

Methods

Vascular measurements were made in 61 women (21 ± 4 yrs) at rest and during a cognitive challenge (Stroop task). Transcranial Doppler was used to measure left middle cerebral artery (MCA) maximum velocity (Vmax), mean velocity (Vmean), and pulsatility index (PI). NVC was determined as MCA blood velocity reactivity to the Stroop task. Large artery function was determined using carotid-femoral pulse wave velocity (cfPWV) as a proxy measure of aortic stiffness and carotid ultrasound-derived measures of compliance and reactivity (diameter change to the Stroop task). Cognitive function was assessed separately using a computerized neurocognitive battery that included appraisal of response speed, executive function, information processing efficiency, memory, attention/concentration, and impulsivity.

Results

MCA Vmax reactivity was positively associated with executive function (β = 0.26, 95% CI 0.01–0.10); MCA Vmean reactivity was negatively associated with response speed (β = −0.33, 95% CI −0.19 to −0.02) and positively with memory score (β = 0.28, 95% CI 0.01–0.19). MCA PI reactivity was negatively associated with attention performance (β = −0.29, 95% CI −14.9 to −1.0). Path analyses identified significant paths (p < 0.05) between carotid compliance and carotid diameter reactivity to select domains of cognitive function through MCA reactivity.

Conclusions

NVC was associated with cognitive function in young women. Carotid artery function assessed as carotid compliance and carotid reactivity may contribute to optimal NVC in young women through increased blood flow delivery and reduced blood flow pulsatility.

Could salt intake directly affect the cerebral microvasculature in hypertension?

OBJECTIVES

Excess dietary salt and chronic kidney disease (CKD) are acknowledged stroke risk factors. The development of small vessel disease, similarly affecting the cerebral and renal microvasculatures, may be an important mechanistic link underlying this interaction. Therefore, we aimed to evaluate if the dietary salt intake and markers of CKD (estimated glomerular filtration rate, albuminuria) relate to transcranial Doppler (TCD) markers of cerebral small vessel disease (CSVD) in hypertensive patients.

MATERIALS AND METHODS

Fifty-six hypertensive patients (57% with diabetes) underwent TCD monitoring in the middle (MCA) and posterior (PCA) cerebral arteries for evaluating neurovascular coupling (NVC), dynamic cerebral autoregulation (dCA), and vasoreactivity to carbon dioxide (VRCO₂). We investigated the relation between renal parameters and TCD studies using Pearson's correlation coefficient and linear regression analyses.

RESULTS

There were no associations between dCA, VRCO₂, NVC, and renal function tests. However, there was a negative association between the daily salt intake and the natural frequency during visual stimulation (r²=0.101, ß=-0.340, p=0.035), indicative of increased rigidity of the cerebral resistance vessels that react to cognitive activation.

CONCLUSIONS

In this cross-sectional study, we found an association between excess dietary salt consumption and CSVD in hypertensive patients. Future research is needed to evaluate whether the natural frequency could be an early, non-invasive, surrogate marker for microvascular dysfunction in hypertension.

Neurovascular Coupling Is Impaired in Hypertensive and Diabetic Subjects Without Symptomatic Cerebrovascular Disease

The mechanistic link between hypertension, diabetes and cerebral small vessel disease (CSVD) is still poorly understood. We hypothesized that hypertension and diabetes could impair cerebrovascular regulation prior to irreversibly established cerebrovascular disease. In this study, 52 hypertensive patients [54% males; age 64 ± 11 years; 58% with comorbid diabetes mellitus (DM)] without symptomatic cerebrovascular disease underwent transcranial Doppler (TCD) monitoring in the middle (MCA) and posterior (PCA) cerebral arteries, to assess vasoreactivity to carbon dioxide (VRCO₂) and neurovascular coupling (NVC). 1.5T magnetic resonance imaging was also performed and white matter hyperintensity volume was automatically segmented from FLAIR sequences. TCD data from 17 healthy controls were obtained for comparison (47% males; age 60 ± 16 years). Hypertensive patients showed significant impairment of NVC in the PCA, with reduced increment in cerebral blood flow velocity during visual stimulation (22.4 ± 9.2 vs. 31.6 ± 5.7, p < 0.001), as well as disturbed NVC time-varying properties, with slower response (lower rate time: 0.00 ± 0.02 vs. 0.03 ± 6.81, p = 0.001), and reduced system oscillation (reduced natural frequency: 0.18 ± 0.08 vs. 0.22 ± 0.06, p < 0.001), when compared to controls. VRCO₂ remained relatively preserved in MCA and PCA. These results were worse in hypertensive diabetic patients, with lower natural frequency (p = 0.043) than non-diabetic patients. White matter disease burden did not predict worse NVC. These findings suggest that hypertensive diabetic patients may have a precocious impairment of NVC, already occurring without symptomatic CSVD. Future research is warranted to evaluate whether NVC assessment could be useful as an early, non-invasive, surrogate marker for CSVD.

Effects of acute alcohol consumption on neuronal activity and cerebral vasomotor response

Introduction

In the majority of European countries, driving after drinking small-moderate amount of alcohol is legal. Motivated by our previous studies on cerebral hemodynamics, we aimed to study whether a small-moderate blood alcohol content (BAC), at which driving is legal in some countries (0.8 g/L), influences the neuronal activity, neurovascular coupling, and cerebral vasoreactivity.

Methods

Analyses of pattern-reversal visual evoked potential (VEP) and transcranial Doppler (TCD) examinations were performed in thirty young healthy adults before and 30 min after alcohol consumption. Cerebral vasoreactivity was evaluated by breath holding test in both middle cerebral arteries. By using a visual cortex stimulation paradigm, visually evoked flow velocity response during reading was measured in both posterior cerebral arteries (PCA).

Results

The BAC was 0.82 g/L and 0.94 g/L 30 and 60 min after drinking alcohol, respectively. Latency of the VEP P100 wave increased after alcohol consumption. Resting absolute flow velocity values increased, whereas pulsatility indices in the PCA decreased after alcohol ingestion, indicating vasodilation of cerebral microvessels. Breath holding index and the visually evoked maximum relative flow velocity increase in the PCA and steepness of rise of the flow velocity curve were smaller after than before alcohol consumption.

Conclusion

BAC close to a legal value at which driving is allowed in some European countries inhibited the neuronal activity and resulted in dilation of cerebral arterioles. Cerebral vasodilation may explain the decrease of cerebral vasoreactivity and might contribute to the disturbance of visually evoked flow response after alcohol consumption.

Evaluation of Cerebral Microvascular Regulatory Mechanisms with Transcranial Doppler in Fabry Disease

Background: Fabry disease (FD) causes cerebrovascular disease (CVD) even if asymptomatic, and this is why it is important to identify non-invasive methods to monitor the disease. We evaluated the usefulness of the cerebral autoregulation, vasoreactivity, and neurovascular coupling assessed by transcranial Doppler (TCD) in FD. Methods: Ten adult patients with classic phenotype FD, without clinical expression of CVD, and ten healthy controls, were included. We monitored cerebral blood flow velocity with TCD in the middle and posterior cerebral arteries, blood pressure, heart rate, and non-invasive expired carbon dioxide (CO₂). Cerebral autoregulation was calculated from the spontaneous oscillations of blood pressure, cerebral vasoreactivity through CO₂ inhalation and hyperventilation and neurovascular coupling by the flow velocity change to visual stimulation. Results: FD male patients showed blunted vasoreactivity in posterior circulation (0.70 ± 0.36%/mmHg vs. 1.09 ± 0.18%/mmHg CO₂, p = 0.01) and impaired neurovascular coupling (overshoot 15 ± 2.9% vs. 28 ± 6.1%, p < 0.01). Cerebral autoregulation was similar to controls. Conclusion: Male patients with FD classic phenotype and hitherto clinical expression of CVD already show impairment of cerebral vasoreactivity and neurovascular coupling. It supports the notion of an early dysfunction of cerebral microvascular in a presymptomatic stage of CVD in FD and that TCD could be useful in its assessment.

A Prospective Transcranial Doppler Ultrasound-Based Evaluation of the Effects of Repetitive Subconcussive Head Trauma on Neurovascular Coupling Dynamics

OBJECTIVE

To determine the effects of repetitive subconcussive head trauma on neurovascular coupling (NVC) responses.

DESIGN

Prospective cohort study collected between September 2013 and December 2016.

SETTING

University laboratory.

PARTICIPANTS

One hundred seventy-nine elite, junior-level (age, 19.6 ± 1.5 years) contact sport (ice hockey, American football) athletes recruited for preseason testing. Fifty-two nonconcussed athletes returned for postseason testing. Fifteen noncontact sport athletes (age, 20.4 ± 2.2 years) also completed preseason and postseason testing.

EXPOSURE(S)

Subconcussive sport-related head trauma.

MAIN OUTCOME MEASURES

Dynamics of NVC were estimated during cycles of 20 seconds eyes closed and 40 seconds eyes open to a visual stimulus (reading) by measuring cerebral blood flow (CBF) velocity in the posterior (PCA) and middle (MCA) cerebral arteries via transcranial Doppler ultrasound.

RESULTS

Both athlete groups demonstrated no significant differences in PCA or MCA NVC dynamics between preseason and postseason, despite exposure to a median of 353.5 (range, 295.0-587.3) head impacts (>2g) over the course of the season for contact sport athletes.

CONCLUSIONS

Within the context of growing concern over detrimental effects of repetitive subconcussive trauma, the current results encouragingly suggest that the dynamics of NVC responses are not affected by 1 season of participation in junior-level ice hockey or American football. This is an important finding because it indicates an appropriate postseason CBF response to elevated metabolic demand with increases in neural activity.

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.

Primary involvement of neurovascular coupling in cerebral autosomal-dominant arteriopathy with subcortical infarcts and leukoencephalopathy

BACKGROUND

Cerebral autosomal-dominant arteriopathy with subcortical infarcts and leukoencephalopathy (CADASIL) is the most frequent monogenic cause of cerebral ischemia, but reliable biomarkers to monitor the disease are lacking.

AIMS AND OBJECTIVES

To evaluate cerebral autoregulation (CA), vasoreactivity (VR), and neurovascular coupling (NVC) in CADASIL patients through a battery of dynamic transcranial Doppler tests.

METHODS

We screened our database for all pre-dementia CADASIL cases. We monitored cerebral blood flow velocity (CBFV) with transcranial Doppler, blood pressure, and expiratory carbon dioxide (CO₂) non-invasively. CA was assessed by transfer function from the spontaneous oscillations of blood pressure to CBFV, VR with inhalation of CO₂ at 5%, and hyperventilation and NVC by the CBFV response to visual stimulation.

RESULTS

We included 27 CADASIL patients and 20 healthy controls with similar age and sexes. CA and VR were similar between groups. However, NVC was significantly affected in CADASIL patients, with lower magnitudes of CBFV upsurge (overshoot 19 ± 5 vs 26 ± 6%, p = 0.013; gain 12 ± 7 vs 17 ± 5%, p = 0.003) and altered time behavior during visual stimulation (natural frequency 0.18 ± 0.06 vs 0.24 ± 0.06 Hz, p = 0.005; rate time 0.7 ± 1.7 vs 2.7 ± 3.5 s, p = 0.025).

CONCLUSION

Our results express a primary and selective involvement of the neurovascular unit in CADASIL rather than a generalized cerebral vasomotor disturbance. Functional cerebrovascular testing could be useful in patient evaluation and monitoring.

A Comparison of Doppler Flow Parameters in the Ophthalmic Artery and Central Retinal Artery in Patients With Graves' Disease and Toxic Nodular Goiter

Purpose: Despite unquestionable clinical usefulness of Clinical Activity Score, the evaluating system needs frequent supplementation. One of such diagnostic tools is Doppler imaging that is used for the analysis of flow in the retrobulbar vessels. The improvement of the reliability and sensibility of measurements could make Doppler imaging an everyday clinical tool and improve the efficacy of treatment in patients with active thyroid-associated orbitopathy. However, the systemic influence of hyperthyroidism on the orbital vessels can falsify the assessment of local inflammation severity. Methods: To eliminate the influence of systemic hyperthyroidism on orbital vessels, we compared peak systolic velocity (PSV), end-diastolic velocity (EDV), and resistance index (RI) in the central retinal artery (CRA), and ophthalmic artery (OA) in patients with hyperthyroidism in the course of Graves' disease without any detectable orbital changes, (CAS = 0) and toxic nodular goiter. Results: There were no statistically significant differences between the patients with Graves' disease and toxic nodular goiter in terms of the examined parameters in either of the arteries. However, higher PSV and RI and lower EDV in the CRA as well as higher PSV and EDV and unchanged RI in the OA were found in the patients with Graves' diseases and toxic nodular goiter. Conclusion: Hyperthyroidism and hyperthyroidism-induced hyperkinetic flow have a systemic influence on the orbital vessels, irrespective of the cause of hyperthyreosis. Thus, it is necessary to compare the flow parameters in retrobulbar vessels in Graves' patients with the toxic nodular goiter patients to eliminate the systemic influence of hyperthyroidism on the orbital vessels.

Cerebrovascular Hemodynamics in Women

Sex and gender, as biological and social factors, significantly influence health outcomes. Among the biological factors, sex differences in vascular physiology may be one specific mechanism contributing to the observed differences in clinical presentation, response to treatment, and clinical outcomes in several vascular disorders. This review focuses on the cerebrovascular bed and summarizes the existing literature on sex differences in cerebrovascular hemodynamics to highlight the knowledge deficit that exists in this domain. The available evidence is used to generate mechanistically plausible and testable hypotheses to underscore the unmet need in understanding sex-specific mechanisms as targets for more effective therapeutic and preventive strategies.

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.

Age-Related Changes in Ocular Blood Velocity in Suspects with Glaucomatous Optic Disc Appearance. Comparison with Healthy Subjects and Glaucoma Patients

Purpose

To evaluate retrobulbar blood flow characteristics of glaucoma suspects with glaucomatous optic disc appearance (GODA) in comparison to healthy control group (CG) and primary open angle glaucoma patients (POAG) and assess the effect of age.

Methods

145 patients from a single glaucoma clinic were enrolled and classified into two diagnostic groups (GODA and POAG). Third group of subjects consisted of 67 age matched individuals (CG). Retrobulbar blood velocity measurement in central retinal artery was performed using color Doppler imaging (CDI). CDI images were processed in custom software leading a range of parameter estimates from a continuous waveform signal. The effect of age on the estimated parameters was evaluated with the stepwise forward regression and ANCOVA in which age was used as a continuous factor. One-way ANOVA was used to test for the differences in the CDI parameters between the three considered groups. Correlation between restive index (RI) and pulsatility index (PI) was assessed with a bilinear fitting guaranteeing no discontinuities in RI intercept estimate. Fisher test was used to assess the applicability of a bilinear PI/RI relationship, while the statistics of the RI intercept estimate were evaluated using the bootstrap.

Results

ANCOVA showed significant interaction between age and group (p<0.05) for five out of nine considered CDI parameters. The RI intercept for CG and GODA groups was 0.602±0.047, and 0.574±0.044 respectively, while the RI intercept of 0.934±0.066 was found for the POAG.

Conclusions

The observed similarity of CG and GODA group and dissimilarity between GODA and POAG groups in terms of PI/RI relationship is remarkable. Age may play some role in the different mechanisms occurring in blood velocity dynamics in GODA and POAG subjects but it is not a strongly determining factor.

Integrative regulation of human brain blood flow

Herein, we review mechanisms regulating cerebral blood flow (CBF), with specific focus on humans. We revisit important concepts from the older literature and describe the interaction of various mechanisms of cerebrovascular control. We amalgamate this broad scope of information into a brief review, rather than detailing any one mechanism or area of research. The relationship between regulatory mechanisms is emphasized, but the following three broad categories of control are explicated: (1) the effect of blood gases and neuronal metabolism on CBF; (2) buffering of CBF with changes in blood pressure, termed cerebral autoregulation; and (3) the role of the autonomic nervous system in CBF regulation. With respect to these control mechanisms, we provide evidence against several canonized paradigms of CBF control. Specifically, we corroborate the following four key theses: (1) that cerebral autoregulation does not maintain constant perfusion through a mean arterial pressure range of 60-150 mmHg; (2) that there is important stimulatory synergism and regulatory interdependence of arterial blood gases and blood pressure on CBF regulation; (3) that cerebral autoregulation and cerebrovascular sensitivity to changes in arterial blood gases are not modulated solely at the pial arterioles; and (4) that neurogenic control of the cerebral vasculature is an important player in autoregulatory function and, crucially, acts to buffer surges in perfusion pressure. Finally, we summarize the state of our knowledge with respect to these areas, outline important gaps in the literature and suggest avenues for future research.

Effect of non-steroid anti-inflammatory drugs on neurovascular coupling in humans

BACKGROUND/AIMS

Neuronal activation induced cerebral blood flow increase was shown in animal experiments to require the presence of functioning cyclooxygenase. Our aim was to study whether widely used, non-steroid anti-inflammatory drugs (NSAIDs), given orally in usual therapeutic doses, inhibit neurovascular coupling in humans.

METHODS

By using a visual cortex stimulation paradigm, the flow velocity response was measured by transcranial Doppler sonography in both posterior cerebral arteries of fifteen young healthy adults. The investigation was repeated in the same subjects after 2-day administration of 3×25 mg indomethacin (indomethacin phase) and 2×550 mg naproxen (naproxen phase). Visual-evoked-potentials were also recorded during the control phase and after administration of NSAIDs.

RESULTS

Basal flow velocity significantly decreased while the pulsatility index increased after administration of either indomethacin or naproxen (p<0.01). Despite unchanged visual-evoked-potentials, the visually evoked flow velocity increase (26±7% in the control phase) significantly declined after administration of indomethacin (19±5%; p<0.01) or naproxen (20±5%; p<0.02).

CONCLUSION

Oral administration of indomethacin or naproxen in their usual therapeutic doses significantly impaired the resting and the visually evoked blood flow regulations in healthy human subjects. Together with stable evoked potentials, our findings indicate disturbance of neurovascular coupling.

Cerebrovascular function in aging and dementia: a systematic review of transcranial Doppler studies

Background/Aim

The contribution of cerebrovascular dysfunction to the manifestation of dementia and cognitive decline in late life is gaining increased attention. We aimed to systematically review evidence for associations between dementia or aging and cerebrovascular function as measured using transcranial Doppler (TCD) examination.

Methods

A total of 1,172 articles were retrieved from PsychInfo and PubMed searches, and 34 relevant articles were identified using a variety of TCD methods.

Results

The pulsatility index (vessel resistance), spontaneous emboli and cerebrovascular reactivity to hyper-/hypocapnia appeared good discriminators of dementia. Aging was associated with a slowing in blood flow velocity.

Conclusion

TCD ultrasonography is inexpensive, portable and well tolerated by aged and demented subjects. The technique stands to make a valuable contribution to the knowledge regarding the underlying functional biology of age-related cognitive change and dementia.

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.

Hypocapnia induced vasoconstriction significantly inhibits the neurovascular coupling in humans

BACKGROUND/AIMS

Previous studies proved that vasodilation, caused by hypercapnia or acetazolamide, does not inhibit the visually evoked flow velocity changes in the posterior cerebral arteries. Our aim was to determine whether vasoconstriction induced by hypocapnia affects the neurovascular coupling.

METHODS

By using a visual cortex stimulation paradigm, visually evoked flow velocity changes were detected by transcranial Doppler sonography in both posterior cerebral arteries of fourteen young healthy adults. The control measurement was followed by the examination under hyperventilation. Visual-evoked-potentials were also recorded during the control and hyperventilation phases.

RESULTS

The breathing frequency increased from 16 ± 2 to 37 ± 3/min during hyperventilation, resulting in a decrease of the end-tidal CO(2) from 37 ± 3 to 25 ± 3 mm Hg and decrease of resting peak systolic flow velocity from 58 ± 11 to 48 ± 11 cm/s (p<0.01). To allow comparisons between volunteers, relative flow velocity was calculated in relation to baseline. Repeated measures analysis of variance revealed significant difference between the relative flow velocity time courses during hyper- and normoventilation (p<0.001). The maximum changes of visually evoked relative flow velocities were 26 ± 7% and 12 ± 5% during normoventilation and hyperventilation, respectively (p<0.01). Visual-evoked-potentials did not differ in the control and hyperventilation phases.

CONCLUSION

The significantly lower visually evoked flow velocity changes but preserved visual-evoked-potential during hyperventilation indicates that the hypocapnia induced vasoconstriction significantly inhibits the neuronal activity evoked flow response.

Effect of ApoE epsilon4 allele on visual evoked potentials and resultant flow coupling in patients with Alzheimer

The apolipoprotein E epsilon4 (ApoE epsilon4) allele is a strong susceptibility factor for Alzheimer disease, which promotes neurodegeneration and cerebrovascular dysfunction. To address this issue in more detail, we simultaneously obtained visual evoked potentials and resultant hemodynamic responses in newly diagnosed Alzheimer patients without signs of vascular lesions on a cerebral magnetic resonance imaging (MRI) scan. Patients were grouped according to ApoE genotype (n = 19 ApoE epsilon4 carrier and n = 12 noncarrier). ApoE epsilon4 carrier had significantly longer peak latencies and a trend to higher interpeak latencies of late potential components. Potential amplitudes and hemodynamic responses were similar in both groups. At the incidental stage of disease process, it appears that the ApoE epsilon4 allele mainly promotes neuronal dysfunction rather than aggravates neurovascular dysfunction. Studies with larger patient samples are warranted to corroborate the first findings.

Acetazolamide-induced vasodilation does not inhibit the visually evoked flow response

Different methods are used to assess the vasodilator ability of cerebral blood vessels; however, the exact mechanism of cerebral vasodilation, induced by different stimuli, is not entirely known. Our aim was to investigate whether the potent vasodilator agent, acetazolamide (AZ), inhibits the neurovascular coupling, which also requires vasodilation. Therefore, visually evoked flow parameters were examined by transcranial Doppler in ten healthy subjects before and after AZ administration. Pulsatility index and peak systolic flow velocity changes, evoked by visual stimulus, were recorded in the posterior cerebral arteries before and after intravenous administration of 15 mg/kg AZ. Repeated-measures ANOVA did not show significant group main effect between the visually evoked relative flow velocity time courses before and after AZ provocation (P=0.43). Visual stimulation induced significant increase of relative flow velocity and decrease of pulsatility index not only before but also at the maximal effect of AZ. These results suggest that maximal cerebral vasodilation cannot be determined by the clinically accepted dose of AZ (15 mg/kg) and prove that neurovascular coupling remains preserved despite AZ-induced vasodilation. Our observation indicates independent regulation of vasodilation during neurovascular coupling, allowing the adaptation of cerebral blood flow according to neuronal activity even if other processes require significant vasodilation.

The visually-evoked cerebral blood flow response in women with a recent history of preeclampsia and/or eclampsia

Several studies provide evidence for altered cerebral hemodynamics during (pre)eclampsia. Whether (pre)eclampsia has a persistent negative impact on cerebral hemodynamics, possibly contributing to an elevated risk of premature stroke, is unknown. The aims of this study were (i) to refine and apply a control system-based method previously introduced by Rosengarten to quantify the visually-evoked blood flow response of the posterior cerebral artery (PCA); and (ii) to test the hypothesis with this method that cerebral hemodynamics in women with a recent history of (pre)eclampsia is abnormal relative to that in parous controls. Hereto, we recorded cerebral blood flow velocity (CBFV) in the PCA by transcranial Doppler (TCD) sonography during cyclic visual stimulation in 15 former preeclamptics, 13 former eclamptics and 13 controls. The typical CBFV response was fitted with the step response of a second-order-linear model enabling quantification by parameters K (gain), zeta (damping), omega (natural frequency), T(v) (rate time) and T(d) (time delay). The method refinement introduced here consisted of response filtering before quantification and of considering the individual instead of group-averaged response patterns. Application of this refinement reduced the fitting errors (1.4 +/- 1.2 vs. 3.2 +/- 1.8, p < 0.01). Intergroup differences in model parameters were not found. Although statistically not significant, a trend was observed that critical damping (zeta>1) occurred more frequently in the combined group of former patients than in the controls (7 of 28 vs.1 of 13, p = 0.16). Critical damping (zeta>1) reflects an abnormal response, which is either compensated for by a rise in rate time ("intermediate"; zeta>1; T(v) > 20) or remains uncompensated ("sluggish"; zeta>1; T(v) < 20). Critical damping increased significantly (p = 0.039) with (pre-)eclampsia-to-test-interval in the PE+E patients with abnormal responses (zeta>1), suggesting that (pre)eclampsia might induce diminishing cerebral hemodynamic function over time. Based on a system-analytical classification approach, the data of this study provide evidence for individual CBFV responses to be abnormal in former (pre)eclamptics compared with controls. Further study is needed to reveal how the abnormal CBFV response classification reflects cerebrovascular dysfunction.

Neurovascular coupling in Alzheimer patients: effect of acetylcholine-esterase inhibitors

Dualistic effects of acetylcholine-esterase inhibitors on neuronal as well as vasoregulative function have been debated. This study investigated for the first time effects of medication on both components. Visually evoked potentials and resultant hemodynamic responses were assessed in Alzheimer patients (n=31) without vascular lesions in a MRI scan and compared to controls (n=20). After baseline recordings (AD0) tests were repeated under 2x1.5 to 2x3mg (AD1) and 2x4.5 to 2x6mg (AD2) rivastigmine/d. Long-term effects were investigated under 6 months of medication (AD2L). The ADAS, MMSE and DEMTECT were used to assess cognitive function at AD0, AD2 and AD2L. Improvement in vasoregulative function was independent from changes in evoked potentials. Acetylcholine-esterase inhibitors demonstrate substantial vascular effects in humans, which are independent from changes in neuronal function.

Effects of initiation and acute withdrawal of statins on the neurovascular coupling mechanism in healthy, normocholesterolemic humans

BACKGROUND AND PURPOSE

Recent clinical trials imply increased risk of vascular events after statin withdrawal. There is evidence that this observation relates to an impaired nitric oxide system. The present analysis investigates the effect of initiation and withdrawal of statin therapy on resting and functionally activated cerebral hemodynamics in healthy young volunteers.

METHODS

Sixteen healthy students (aged 23.7+/-3.3 years, 10 male) were subjected to a placebo-controlled, double-blind crossover study with a washout phase between blocks of 4 weeks. In the verum group, 20 mg pravastatin was taken for 2 weeks followed by 40 mg for 4 weeks. Withdrawal effects were investigated the day after discontinuation. Total cholesterol levels, blood pressure, resting and evoked hemodynamic responses due to a visual stimulation task in the posterior cerebral artery were obtained at baseline and then weekly and the day after discontinuation.

RESULTS

In the verum group, cholesterol levels significantly decreased after 2 weeks (from 183+/-30 to 150+/-28 mg/dL; P<0.001) and then remained nearly stable (147+/-21 mg/dL after 6 weeks). Blood pressure, resting and evoked hemodynamic responses remained constant throughout the study. The day after statin withdrawal, evoked flow velocity responses were significantly lower (11+/-4% versus 13+/-5% at baseline; P<0.01) indicating inappropriate blood supply of active neurons.

CONCLUSIONS

Reduction in evoked flow velocity responses reflects reduced nitric oxide bioavailability and therefore supports molecular findings of acute statin withdrawal. Questions arise if the present data might give a link to reports of increased vascular events in patients at vascular risk after acute statin withdrawal.

Performance of colour Doppler imaging discriminating normal tension glaucoma from healthy eyes

PURPOSE

Previous studies have shown decreased retrobulbar blood flow in normal tension glaucoma (NTG) compared to healthy controls. This study evaluates the ability of colour Doppler imaging (CDI) to identify patients with NTG.

METHODS

Sixty-two patients with untreated NTG (mean age 57+/-14 years) and 40 age-matched controls (mean age 58+/-9 years) were included in a prospective cross-sectional institutional study. Peak systolic velocity (PSV), end-diastolic velocity (EDV), and resistive indices (RI=(PSV-EDV)/PSV) of the ophthalmic artery (OA), central retinal artery (CRA), and short posterior ciliary arteries (PCAs) were measured by means of CDI. Using receiver operating characteristic (ROC) curves, sensitivity was determined at 90% specificity.

RESULTS

Patients with NTG showed significantly decreased PSV (P<0.0001) and EDV (P<0.0001) of the CRA, significantly decreased EDV of the nasal (P=0.004) and temporal (P=0.002) PCA, and significantly increased RI of the temporal (P=0.003) PCAs compared to healthy controls. Sensitivity values at 90% specificity were calculated: PSV of the CRA, 30.6%; EDV of the CRA, 48.4%; EDV of the nasal PCA, 43.9%; EDV of the temporal PCA, 45.9%; and RI of the temporal PCA, 39.3%.

CONCLUSIONS

The power to identify NTG using CDI reaches 48% sensitivity at 90% specificity. Further longitudinal studies are needed to determine the prognostic value of CDI in glaucoma.

Activation-flow coupling differentiates between vascular and Alzheimer type of dementia

The activation-flow coupling describes a mechanism, which adapts local cerebral blood flow in accordance with the underlying neuronal activity. It was suggested that the mechanism helps in differentiation between Alzheimer and vascular type of dementia. We combined EEG and Doppler techniques and assessed integrity of the activation-flow coupling in the occipital cortex utilizing a visual stimulation task. Alzheimer patients (MMSE: 18+/-8 points, DemTect 5+/-4 points) without signs of vascular lesions on a MRI scan and vascular demented patients (MMSE: 20+/-6 points, DemTect 6+/-3 points; MRI Fazekas score 7+/-3 points) were compared with data from an age-matched control group. Evoked flow velocity responses in the posterior cerebral artery were analysed according to a control system model specifying the parameters gain, attenuation, natural frequency and rate time. Evoked potentials were analysed for the N(75)-P(100) amplitude difference. Vascular demented patients exhibited a significant decreased gain parameter and increased attenuation parameter indicating severe cerebrovascular dysfunction. Also, the potential amplitudes were significantly decreased indicating neuronal damage due to the vascular disease process. Alzheimer patients did not differ in parameters as compared to the control group supporting other reports of intact occipital function at this stage of disease. Simultaneous assessment of electrical as well as vascular integrity might help in differentiating the most frequent forms of dementia.

Sildenafil Improves Dynamic Vascular Function in the Brain: Studies in Patients with Pulmonary Hypertension

BACKGROUND

Prostaglandins and nitric oxide play a pivotal role in the regulation of macro- and microcirculatory blood flow distribution. Interference with both mediator systems have been implicated in cerebrovascular dysfunction. Inhaled iloprost (long-acting prostacyclin analogue) and the phosphodiesterase-5 inhibitor sildenafil have recently shown efficacy in the treatment of chronic pulmonary hypertension. We investigated the impact of these agents on cerebral microcirculatory regulation in patients suffering from this disease.

METHODS

In 11 patients suffering from severe pulmonary hypertension, a functional transcranial Doppler test utilizing a visual stimulation paradigm was undertaken to measure the evoked flow velocity in the posterior cerebral artery. Measurements were performed in parallel to right heart catheterization and pharmacological testing of the pulmonary vasoreactivity. After assessment of baseline measurements, inhaled iloprost and oral sildenafil were given consecutively for testing of cerebral and pulmonary vascular function. The data gained from the Doppler measurements were compared to data from 22 healthy volunteers.

RESULTS

Both substances provoked a significant reduction of pulmonary arterial pressure and vascular resistance, accompanied by minor changes in systemic vascular resistance. In contrast to these superimposable hemodynamic profiles opposite effects were observed regarding cerebral vascular tone: cerebral microvascular reactivity, as assessed by attenuation and time rate parameters, was significantly improved by sildenafil, but slightly worsened by iloprost.

CONCLUSIONS

Sildenafil has beneficial effects on cerebral vascular reactivity indicative of an improvement in neurovascular coupling in patients with pulmonary hypertension. These results warrant further investigations of the influence of sildenafil on dynamic vascular function in the brain independent of the underlying disease.

Acetylcholine esterase inhibitor donepezil improves dynamic cerebrovascular regulation in Alzheimer patients

BACKGROUND

Alzheimer's disease (AD) leads to a degeneration of the nucleus basalis of Meynert and thus to decreased cholinergic tonus in the brain. The transcription of endothelial nitric oxide synthase depends on an adequate cholinergic innervation of microvessels and vasoregulative abnormalities have been reported in AD. We investigated activation-flow coupling to study the role of acetylcholine esterase inhibition (AChEI) on vasoregulative function.

METHODS

A functional transcranial Doppler approach was used to measure the visually evoked flow velocity response in the posterior cerebral artery in AD patients who had no vascular risk factors. The diagnosis of AD was made according to the ICD10/DSMIIIR-criteria. After baseline recording the effect of four weeks 5mg donepezil and then four weeks 10 mg was investigated. Doppler data were evaluated with a control system approach to obtain dynamic properties of vasoregulation and were compared with a healthy control group.

RESULTS

AD patients showed an increased damping (0.64 +/- 0.2; p = 0.007 vs. control) in evoked responses and lower resting flow velocity levels (40 +/- 13 cm/s; p = 0.06 vs. control), which were restored in a dose-dependent manner under AChEI (0.4 +/- 0.2; 44 +/- 11 cm/s).

CONCLUSIONS

AD is associated with a functional vasoregulative deficit possibly due to decreased levels of the endothelial nitric oxide synthase. Augmenting levels with AChEI normalized flow regulation possibly leading to a better blood supply to active neurons.

Comparison of colour Doppler imaging and retinal scanning laser fluorescein angiography in healthy volunteers and normal pressure glaucoma patients

PURPOSE

To correlate retinal circulatory measurements using scanning laser fluorescein angiography and flow velocities of retrobulbar vessels measured by means of colour Doppler imaging.

METHODS

Fifteen patients with normal pressure glaucoma (NPG) and 15 healthy volunteers underwent colour Doppler imaging and fluorescein angiographic studies. Peak systolic velocities (PSVs), end-diastolic velocities (EDVs) and resistive indices (RIs) of the ophthalmic artery (OA) and central retinal artery were obtained. In the fluorescein angiograms arteriovenous passage time (AVP) was quantified by means of digital dye dilution curve analysis.

RESULTS

Arteriovenous passage time was significantly prolonged in NPG patients compared to healthy subjects (p = 0.0026). In the central retinal artery PSV (p = 0.023) and EDV (p < 0.0001) were significantly decreased and RI was increased (p < 0.0001) in patients with NPG. The EDV of the central retinal artery showed a significant correlation with AVP (EDV: r = - 0.53, p = 0.0023). The RI of the central retinal artery correlated significantly to AVP (RI: r = 0.63, p < 0.0001). The AVP did not correlate to EDV or PSV, nor to the RI measured in the ophthalmic artery.

CONCLUSION

Arteriovenous passage time, which represents blood flow in a vascular segment of artery, capillary bed and corresponding vein, was found to be correlated to the EDV and the RI of the central retinal artery. The combination of different techniques allows further interpretation of ocular circulatory responses.

Effect of heart rate on regulative features of the cortical activity-flow coupling

OBJECTIVE

Heart rate plays an important role in compensatory conditions of arterial pressure changes. Very little information, however, exists on its role in the dynamic adjustment of stimulated organ perfusion. We studied the influence of heart rate on the activity-flow coupling mechanism which adapts local cerebral blood flow in accordance with cortical activity. Since it does not affect heart rate or arterial blood pressure by itself, the commonly observed heart rate variability in test conditions was used to compare the flow response between different heart rate groups. For evaluation under stable heart rate conditions we performed a short test paradigm with a transcranial Doppler technique with the necessary high time resolution.

METHODS

168 healthy young volunteers (24 +/- 3 years of age) were grouped according to their heart rate in decade steps from 60 to 100 beats/min (mean: 82 +/- 9 beats/min). The visually evoked flow velocity responses in the posterior cerebral artery were evaluated according to a control system approach. Peak systolic and end diastolic data were evaluated separately.

RESULTS

A correlation analysis between heart rate and baseline flow velocity as well as each of the control system parameters, i.e. gain, attenuation, rate time and natural frequency, revealed no significance. The flow responses did not differ among the heart rate groups as concluded from an ANOVA test.

DISCUSSION

The increase in heart rate and the possible stress factors responsible for this seem to be of no relevance in regulative features of the activity-flow coupling. The almost identical time course of flow velocity responses among the groups showing a heart rate difference of up to 50% indicates an integrative principle in flow regulation supporting simplified concepts of flow adaptation.

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.

Effects of acute hyperhomocysteinemia on the neurovascular coupling mechanism in healthy young adults

BACKGROUND AND PURPOSE

Hyperhomocysteinemia is a vascular risk factor that infers with the nitric oxide signaling pathway of endothelial vasoregulation. Most investigations in young healthy humans on the peripheral vasculature using a standardized methionine challenge demonstrated altered vascular reactivity. In contrast, the cerebral autoregulation mechanism was shown to be unaffected by the same methionine load. To obtain additional insight into the compensatory range of the cerebral vasculature during a methionine challenge, we tested the neurovascular coupling mechanism that adjusts cerebral blood flow in accordance with cortical activity.

METHODS

Fifteen healthy young adults (age, 24.7+/-2.3 years; 7 men) were tested with a functional transcranial Doppler test before and 3, 8, and 24 hours after administration of placebo, 20 mg folic acid, 20 mg folic acid and 0.1 g/kg body weight L-methionine, or L-methionine alone. Evoked blood flow response was evaluated according to a control system approach. Plasma concentrations of homocysteine, resting blood flow velocities, and control system parameters of flow velocity change were compared for each time point using a multiple analysis of variance test.

RESULTS

Homocysteine levels increased significantly compared with baseline (before, 7.6+/-1.9 micromol/L; 3 hours, 22.2+/-6.0 micromol/L [P<0.0001]; 8 hours, 27.9+/-8.6 micromol/L [P<0.0001]; 24 hours, 12.6+/-7.8 micro mol/L [P=NS]). Resting flow velocities and control system parameters remained statistically nonsignificant.

CONCLUSIONS

Compared with the peripheral vasculature, the regulatory mechanisms controlling adequate cerebral blood flow appear to have a wider compensatory range. This is concluded from statistically nonsignificant results comparing the vascular reactivity in young adults undergoing a standardized methionine challenge. Our data confirm indirectly the reports of high concentrations of homocysteine needed to affect the cerebral vasculature in animal experiments.

Splenic artery Doppler in the prediction of the small-for-gestational age infant

OBJECTIVES

We hypothesized that, as with other areas of the peripheral circulation, fetal splenic artery blood flow undergoes changes in small-for-gestational age (SGA) fetuses due to a redistribution of cardiac output, and that the Doppler peak systolic velocity (PSV) reflects such changes and thus may be used to predict fetuses being SGA.

METHODS

Splenic artery Doppler PSV, end-diastolic velocity (EDV), resistance index (RI) and umbilical artery RI were measured prospectively in fetuses at risk for being SGA at birth. Normal reference data were generated from appropriately grown fetuses delivering at > or = 37 weeks without complications, and SGA was defined as birth weight < 10th percentile. The Doppler indices were expressed as multiples of the normal median (MoM) for gestational age. Using receiver operating characteristic curves, optimal Doppler thresholds for the detection of SGA cases were determined and the areas under the curves calculated. The analysis was limited to singleton pregnancies delivered within 2 weeks of the last Doppler examination.

RESULTS

There were 88 study patients of which 60 had SGA babies. The mean gestational age at Doppler examination was 31.4 weeks with a mean interval of 5.6 days from Doppler to delivery. The splenic artery PSV was lower in SGA, compared to normal cases: mean PSV (MoM), 0.93 vs. 1.09, respectively (P = 0.0001). The sensitivity, specificity and area under the curve were 70.0%, 72% and 0.734, respectively (P < 0.003), for the PSV in the prediction of delivery of a SGA fetus. For the splenic artery RI, values were 70%, 46% and 0.539, respectively (not significantly different), and for umbilical artery RI these were 70%, 61% and 0.689, respectively (P < 0.01). Splenic artery EDV was significantly reduced in SGA vs. normally grown fetuses (0.924 MoM vs. 1.145 MoM, P = 0.007).

CONCLUSIONS

Fetal splenic artery PSV decreases in SGA infants, and is a strong predictor of the delivery of a SGA infant. It appears to be superior to the standard Doppler index, the RI, in predicting this outcome.