Inspiratory resistance, cerebral blood flow velocity, and symptoms of acute hypotension

INTRODUCTION: Symptoms of orthostatic intolerance, e.g., following prolonged bed rest and microgravity exposure, are associated with reductions in cerebral blood flow. We tested the hypothesis that spontaneously breathing through an impedance threshold device (ITD) would attenuate the fall in cerebral blood flow velocity (CBFV) during a hypotensive orthostatic challenge and reduce the severity of reported symptoms. METHODS: While breathing through either an active ITD (-7 cm H2O inspiratory impedance) or a sham ITD (no impedance), 19 subjects performed a squat stand test (SST). Symptoms upon stand were recorded on a 5-point scale (1 = normal; 5 = faint) of subject-perceived rating (SPR). To address our hypothesis, only data from symptomatic subjects (SPR > 1 during the sham trial) were analyzed (N = 9). Mean arterial blood pressure (MAP) and mean CBFV were measured continuously throughout the SST and analyzed in time and frequency domains. RESULTS: Breathing with the active ITD during the SST reduced the severity of orthostatic symptoms in eight of the nine symptomatic subjects (sham ITD SPR, 1.9 +/- 0.1; active ITD SPR, 1.1 +/- 0.1), but there was no statistically distinguishable difference in the reduction of mean CBFV between the two trials (sham ITD, -39 +/- 3% vs. active ITD, -44 +/- 3%). High frequency oscillations in mean CBFV, however, were greater during the active ITD trial (7.8 +/- 2.6 cm x s(-2)) compared with the sham ITD trial (2.5 +/- 0.9 cm x s(-2)). CONCLUSIONS: Higher oscillations in CBFV while breathing with the active ITD may account for the reduction in symptom severity during orthostatic hypotension despite the same fall in absolute CBFV.     

Voluntary respiratory control and cerebral blood flow velocity upon ice-water immersion

INTRODUCTION: In non-habituated subjects, cold-shock response to cold-water immersion causes rapid reduction in cerebral blood flow velocity (approximately 50%) due to hyperventilation, increasing risk of syncope, aspiration, and drowning. Adaptation to the response is possible, but requires several cold immersions. This study examines whether thorough instruction enables non-habituated persons to attenuate the ventilatory component of cold-shock response. METHODS: There were nine volunteers (four women) who were lowered into a 0 degrees C immersion tank for 60 s. Middle cerebral artery mean velocity (CBFV) was measured together with ventilatory parameters and heart rate before, during, and after immersion. RESULTS: Within seconds after immersion in ice-water, heart rate increased significantly from 95 +/- 8 to 126 +/- 7 bpm (mean +/- SEM). Immersion was associated with an elevation in respiratory rate (from 12 +/- 3 to 21 +/- 5 breaths, min(-1)) and tidal volume (1022 +/- 142 to 1992 +/- 253 ml). Though end-tidal carbon dioxide tension decreased from 4.9 +/- 0.13 to 3.9 +/- 0.21 kPa, CBFV was insignificantly reduced by 7 +/- 4% during immersion with a brief nadir of 21 +/- 4%. DISCUSSION: Even without prior cold-water experience, subjects were able to suppress reflex hyperventilation following ice-water immersion, maintaining the cerebral blood flow velocity at a level not associated with impaired consciousness. This study implies that those susceptible to accidental cold-water immersion could benefit from education in cold-shock response and the possibility of reducing the ventilatory response voluntarily.     

Effects of inhaled stable xenon on cerebral blood flow velocity

The effects of inhaled stable xenon gas on cerebral blood flow were studied with 23 transcranial Doppler examinations performed in 13 normal volunteers while breathing, 25, 30, or 35% xenon for 5 min. Doppler velocities from the middle cerebral artery rose significantly during inhalation in 85% of subjects and 78% of studies and decreased significantly in 15% of subjects and 17% of studies. These different velocity responses may represent different responses of pial vasculature to xenon. The mean velocity rise among those studies showing a significant increase was 38 +/- 3.6% (SEM). The velocity rise began 2 min after the start of xenon inhalation and increased rapidly, so that the velocities measured at the four times at which scans were obtained in our xenon CT protocol (0, 1.5, 3, and 5 min after the start of xenon inhalation) were significantly different. A consistent fall in the pulsatility of the Doppler waveform as the velocity increased provided evidence for xenon-induced vasodilation of the small-resistance vessels as the cause of the increase in flow velocity. Most subjects became mildly hyperventilated, so that the observed changes could not be attributed to hypercapnia. Inhalation of 25, 30, or 35% xenon for 5 min induces a delayed but significant rise in cerebral blood velocity. This suggests that cerebral blood flow itself may be rapidly changing during the process of xenon CT scanning. These changes may compromise the ability of the xenon CT technique to provide reliable quantitative measurements of cerebral blood flow.     

Cerebral blood flow,blood volume,and vascular permeability of cerebral glioma assessed with dynamic CT perfusion imaging

We report dynamic CT perfusion imaging assessment of hemodynamics in a patient with a high-grade cerebral glioma and compare our results to those of previously published studies.     

Relationship between aerobic endurance training and dynamic cerebral blood flow regulation in humans

The incidence of orthostatic intolerance is elevated in endurance‐trained individuals. We sought to test the hypothesis that aerobic endurance training is associated with an attenuated control of the cerebral vasculature. Endurance trained (ET, n = 13) and age‐matched untrained (UT, n = 11) individuals (peak O₂ consumption, mean ± SEM; 63 ± 1 vs 42 ± 1 mL/min/kg, P < 0.05) were examined while supine and seated upright. Dynamic cerebral autoregulation (CA) was assessed by calculation of the rate of regulation (RoR) from the arterial blood pressure (ABP) and middle cerebral artery (MCA) mean blood velocity (V_mean) responses to a bilateral thigh cuff release, which evoked a transient hypotension. Cerebral oxygenation (oxyhemoglobin; HbO₂) was determined with near‐infrared spectroscopy. When seated upright, cuff release evoked a greater decrease in ABP (P < 0.001), MCA V_mean (P = 0.096) and HbO₂ (P < 0.001) in ET compared with UT. However, RoR was similar in ET and UT individuals while seated upright (to 0.193 ± 0.039 vs 0.129 ± 0.029/s, P > 0.05), and there was no significant difference in the relative change in RoR from the supine to upright positions (ΔRoR: ?65 ± 7 and ?69 ± 7%, for ET and UT, respectively). These findings suggest that aerobic endurance training is not associated with an attenuation in dynamic CA.     

Combined arterial spin label and dynamic susceptibility contrast measurement of cerebral blood flow

Dynamic susceptibility contrast (DSC) and arterial spin labeling (ASL) are both used to measure cerebral blood flow (CBF), but neither technique is ideal. Absolute DSC‐CBF quantitation is challenging due to many uncertainties, including partial‐ volume errors and nonlinear contrast relaxivity. ASL can measure quantitative CBF in regions with rapidly arriving flow, but CBF is underestimated in regions with delayed arrival. To address both problems, we have derived a patient‐specific correction factor, the ratio of ASL‐ and DSC‐CBF, calculated only in short‐arrival‐time regions (as determined by the DSC‐based normalized bolus arrival time [Tmax]). We have compared the combined CBF method to gold‐standard xenon CT in 20 patients with cerebrovascular disease, using a range of Tmax threshold levels. Combined ASL and DSC CBF demonstrated quantitative accuracy as good as the ASL technique but with improved correlation in voxels with long Tmax. The ratio of MRI‐based CBF to xenon CT CBF (coefficient of variation) was 90 ± 30% (33%) for combined ASL and DSC CBF, 43 ± 21% (47%) for DSC, and 91 ± 31% (34%) for ASL (Tmax threshold 3 sec). These findings suggest that combining ASL and DSC perfusion measurements improves quantitative CBF measurements in patients with cerebrovascular disease. Magn Reson Med 63:1548–1556, 2010. © 2010 Wiley‐Liss, Inc.     

Quantitative cerebral blood flow in dynamic susceptibility contrast MRI using total cerebral flow from phase contrast magnetic resonance angiography

Dynamic susceptibility contrast magnetic resonance imaging during bolus injection of gadolinium contrast agent is commonly used to investigate cerebral hemodynamics. The large majority of clinical applications of dynamic susceptibility contrast magnetic resonance imaging to date have reported relative cerebral blood flow values because of dependence of the result on the accuracy of determining the arterial input function, the robustness of the singular value decomposition algorithm, and others. We propose a calibration approach that directly measures the total (i.e., whole brain) cerebral blood flow in individual subjects using phase contrast magnetic resonance angiography. The method was applied to data from 11 patients with intracranial pathology. The sum of squares variance about the mean (uncorrected: white matter = 105.6, gray matter = 472.2; corrected: white matter = 34.1, gray matter = 99.8) after correction was significantly lower for white matter (P = 0.045) and for gray matter (P = 0.011). However, the mean gray and white matter cerebral blood flow in the contralateral hemisphere were not significantly altered by the correction. The proposed phase contrast magnetic resonance angiography calibration technique appears to be one of the most direct correction schemes available for dynamic susceptibility contrast magnetic resonance imaging cerebral blood flow values and can be performed rapidly, requiring only a few minutes of additional scan time. Magn Reson Med, 2011. © 2011 Wiley‐Liss, Inc.     

Optimizing dynamic T2* MR imaging for measurement of cerebral blood flow using infusions for cerebral blood volume

We describe an approach to measuring cerebral blood flow (CBF) based on independent measurements of cerebral blood volume (CBV) and mean transit time (MTT) with calculation of CBF by using the central volume theorem: CBF = CBV / MTT. This permits optimization of the individual acquisitions and analyses. In particular, measurement of CBV during contrast infusion, rather than simultaneously with MTT from a single bolus, yields values more consistent with those of other methods.     

Asymmetry in cerebral blood flow velocity with processing of facial images during head-down rest

INTRODUCTION: Ability to interpret facial expression is crucial for non-verbal communication among humans, and could be affected by changes in cerebral circulation during exposure to microgravity or its simulation. METHODS: There were 16 subjects (8 men and 8 women) who were exposed to 24 h of -6 degrees head-down rest (HDR). Transcranial Doppler ultrasonography was used to monitor mean blood flow velocity (MBFV) in the middle cerebral arteries bilaterally during processing of facial images before, at 6 and 24 h of HDR, and after HDR (Pre-, 6H-, 24H-, and Post-HDR, respectively). The laterality index was assessed as side-to-side differences in MBFV relative to Pre-HDR for each condition. RESULTS: For Pre-HDR, both objects and faces were right lateralized in men (p < 0.001) and showed a left lateralization tendency in women (p > 0.05). At 6H-HDR, both object and faces were left lateralized in men (p < 0.05), but right lateralized in women (p < 0.001). At 24H-HDR, both men and women were left lateralized (p < 0.05). For Post-HDR, both remained left lateralized for all tasks (p < 0.05). DISCUSSION: HDR alters cerebral lateralization for object and facial stimuli, with opposing tendencies in men and women. The gender differences may reflect peculiarities in processing strategy for object and faces between men and women. Men use a right hemisphere processing strategy for faces and women a left hemisphere strategy. The superiority of processing of faces by women compared with men has been attributed to left hemisphere based strategy. HDR alters lateralization patterns and may thus alter processing strategies for faces.     

Maximum likelihood estimation of cerebral blood flow in dynamic susceptibility contrast MRI.

For quantification of cerebral blood flow (CBF) using dynamic susceptibility contrast magnetic resonance imaging (DSC-MRI), knowledge of the tissue response function is necessary. To obtain this, the tissue contrast passage measurement must be corrected for the arterial input. This study proposes an iterative maximum likelihood expectation maximization (ML-EM) algorithm for this correction, which takes into account the noise in T2- or T2*-weighted image sequences. The ML-EM algorithm does not assume a priori knowledge of the shape of the response function; it automatically corrects for arrival time offsets and inherently yields positive response values. The results on synthetic image sequences are presented, for which the recovered flow values and the response functions are in good agreement with their expectation values. The method is illustrated by calculating the gray and white matter flow in a clinical example.     

Middle cerebral artery blood velocity during running

Running induces characteristic fluctuations in blood pressure (BP) of unknown consequence for organ blood flow. We hypothesized that running‐induced BP oscillations are transferred to the cerebral vasculature. In 15 healthy volunteers, transcranial Doppler‐determined middle cerebral artery (MCA) blood flow velocity, photoplethysmographic finger BP, and step frequency were measured continuously during three consecutive 5‐min intervals of treadmill running at increasing running intensities. Data were analysed in the time and frequency domains. BP data for seven subjects and MCA velocity data for eight subjects, respectively, were excluded from analysis because of insufficient signal quality. Running increased mean arterial pressure and mean MCA velocity and induced rhythmic oscillations in BP and in MCA velocity corresponding to the difference between step rate and heart rate (HR) frequencies. During running, rhythmic oscillations in arterial BP induced by interference between HR and step frequency impact on cerebral blood velocity. For the exercise as a whole, average MCA velocity becomes elevated. These results suggest that running not only induces an increase in regional cerebral blood flow but also challenges cerebral autoregulation.     

Theoretical basis of hemodynamic MR imaging techniques to measure cerebral blood volume, cerebral blood flow, and permeability

Cerebrovascular hemodynamic assessment adds new information to standard anatomic MR imaging and improves patient care. This article reviews the theoretic underpinnings of several potentially quantitative MR imaging-based methods that shed light on the hemodynamic status of the brain, including cerebral blood flow (CBF), cerebral blood volume (CBV), and contrast agent permeability. Techniques addressed include dynamic susceptibility contrast (which most simply and accurately estimates CBV), arterial spin labeling (a powerful method to measure CBF), and contrast-enhanced methods to derive permeability parameters such as the transport constant Ktrans.     

与二巯基丁二酸（DMSA）核素显像和最后诊断对比评价综合肾脏超声检查在儿童急性尿道感染评估肾脏损害中的价值

本研究的目的是通过与①^99TC^m，DMSA核素显像及②最后诊断进行对比，评价综合肾脏超声即结合灰阶超声和幅度编码彩色多普勒超声成像（aCDS）在评估婴幼儿尿道感染（UTI）中的价值。对287例UTI患儿进行了综合肾脏超声检查和DMSA核素显像，检查结果以①DMSA核素显像和②最后诊断为金标准，比较其诊断肾脏损害的可靠性。     

Dose reduction in dynamic perfusion CT of the brain: effects of the scan frequency on measurements of cerebral blood flow, cerebral blood volume, and mean transit time

The influence of the frequency of computed tomography (CT) image acquistion on the diagnostic quality of dynamic perfusion CT (PCT) studies of the brain was investigated. Eight patients with clinically suspected acute ischemia of one hemisphere underwent PCT, performed on average 3.4 h after the onset of symptoms. Sixty consecutive images per slice were obtained with individual CT images obtained at a temporal resolution of two images per second. Eight additional data sets were reconstructed with temporal resolutions ranging from one image per second to one image per 5 s. Cerebral blood flow (CBF), cerebral blood volume (CBV) and mean transit time (MTT) measurements were performed in identical regions of interest. Two neuroradiologists evaluated the PCT images visually to identify areas of abnormal perfusion. Perfusion images created up to a temporal resolution of one image per 3 s were rated to be diagnostically equal to the original data. Even at one image per 4 s, all areas of infarction were identified. Quantitative differences of CBF, CBV and MTT measurements were ≤10% up to one image per 3 s. For PCT of the brain, temporal resolution can be reduced to one image per 3 s without significant compromise in image quality. This significantly reduces the radiation dose of the patient.     

Microdroplet tracking using biplane digital subtraction angiography for cerebral arteriovenous malformation blood flow path and velocity determinations.

High-speed biplane angiography is used to determine the path and velocity of microdroplets of contrast material in three dimensions. By allowing more accurate determination of detailed blood flow in feeding vessels and draining veins of cerebral arteriovenous malformations than available with standard angiography, the new method offers the potential for more accurate treatment and further study of neurovascular/cerebrovascular hemodynamics. The first study of the method is presented.     

Hyperacute stroke: simultaneous measurement of relative cerebral blood volume, relative cerebral blood flow, and mean tissue transit time

PURPOSE: To investigate additional information provided by maps of relative cerebral blood flow in functional magnetic resonance (MR) imaging of human hyperacute cerebral ischemic stroke. MATERIALS AND METHODS: Diffusion-weighted and hemodynamic MR imaging were performed in 23 patients less than 12 hours after the onset of symptoms. Maps of relative cerebral blood flow and tracer mean tissue transit time were computed, as were maps of apparent diffusion and relative cerebral blood volume. Acute lesion volumes on the maps were compared with follow-up imaging findings. RESULTS: In 15 of 23 subjects (65%), blood flow maps revealed hemodynamic abnormalities not visible on blood volume maps. A mismatch between initial blood flow and diffusion findings predicted growth of infarct more often (12 of 15 subjects with infarcts that grew) than did a mismatch between initial blood volume and diffusion findings (eight of 15). However, lesion volumes on blood volume and diffusion maps correlated better with eventual infarct volumes (r > 0.90) than did those on blood flow and tracer mean transit time maps (r approximately 0.6), likely as a result of threshold effects. In eight patients, blood volume was elevated around the diffusion abnormality, suggesting a compensatory hemodynamic response. CONCLUSION: MR imaging can delineate areas of altered blood flow, blood volume, and water mobility in hyperacute human stroke. Predictive models of tissue outcome may benefit by including computation of both relative cerebral blood flow and blood volume.