Dynamic cerebral autoregulation in acute lacunar and middle cerebral artery territory ischemic stroke

Background and Purpose - We addressed whether dynamic cerebral autoregulation (dCA) is affected in middle cerebral artery (MCA) territory (MCAS) and lacunar ischemic stroke (LS). Methods - Blood pressure (MAP) and MCA velocity (V) were measured in 10 patients with large MCAS (National Institutes of Health Stroke score, 17 +/- 2; mean +/- SEM), in 10 with LS (score, 9 +/- 1), and in 10 reference subjects. dCA was evaluated in time (delay of the MCA V-mean counter-regulation during changes in MAP) and frequency domains (cross-spectral MCA V-mean-to-MAP phase lead). Results - In reference subjects, latencies for MAP increments (5.3 +/- 0.5 seconds) and decrements (5.6 +/- 0.5 seconds) were comparable, and low frequency MCA V-mean-to-MAP phase lead was 56 +/- 5 and 59 +/- 5 degrees (left and right hemisphere). In MCAS, these latencies were 4.6 +/- 0.7 and 5.6 +/- 0.5 seconds in the nonischemic hemisphere and not detectable in the ischemic hemisphere. In the unaffected hemisphere, phase lead was 61 +/- 6 degrees versus 26 +/- 6 degrees on the ischemic side (P <0.05). In LS, no latency and smaller phase lead bilaterally (32 +/- 6 and 33 +/- 5 degrees) conformed to globally impaired dCA. Conclusions - In large MCAS infarcts, dynamic cerebral autoregulation was impaired in the affected hemisphere. In LS, dynamic cerebral autoregulation was impaired bilaterally, a finding consistent with the hypothesis of bilateral small vessel disease in patients with lacunar infarct

Phase Transformation for the Large-Scale Synthesis and Assembly via Welding of Metal Silicide Nanowires for Thermoelectric Applications

Solid-state thermoelectrics that convert thermal energy into electricity have the potential to increase the efficiencies of existing process and systems (e.g., automobiles). The large-scale deployment of thermoelectrics for terrestrial use requires the following: a) enhancing their efficiencies beyond that are currently possible, and b) their fabrication from non-toxic, inexpensive, earth-abundant elements. Recent studies have determined that nanostructuring of earth-abundant materials, such as magnesium silicide (Mg2Si), is a possible pathway for accomplishing this task. Contextually, the overall aim of this dissertation is to engineer highly efficient Mg2Si nanowire-based thermoelectrics. This was achieved through the design of strategies for (1) the large-scale synthesis of a form of nanostructured Mg2Si, nanowires of Mg2Si, and (2) the interface-engineered assembly of the synthesized nanowires into welded nanowire networks that do not have any insulating MgO at the nanowire interfaces. Together, these strategies are intended to offer the ability to control thermal and electrical transport through Mg2Si. For the large-scale synthesis of Mg2Si nanowires, a phase transformation strategy that converts pre-synthesized silicon nanowires into Mg2Si nanowires was engineered. Experimentation performed indicated that 20 to 300 nm-thick, 5 to 20 µm-long silicon nanowires obtained by electroless etching can be phase transformed into polycrystalline Mg2Si nanowires by reacting them with magnesium supplied via the vapor phase. To prevent the formation of multiple nuclei within each silicon nanowire during the phase transformation process and the formation of polycrystalline Mg2Si nanowires, a solid-state phase transformation process was engineered. Here, the solid-state reaction of sharp-tipped silicon nanowires with magnesium foils was employed for obtaining single-crystalline Mg2Si nanowires. To assemble the nanowires, the solid-state phase transformation strategy was extended and the phase transformation of silica nanoparticle coated silicon nanowires was employed. This procedure led to the formation of welded Mg2Si nanowire networks, where both the nanowires and the bridges connecting the nanowires were composed of Mg2Si. Thermoelectric performance evaluation of these networks and microcrystalline Mg2Si devices proved our hypothesis and indicated a 2-fold increase in the power factors. The high power factor of 0.972 x 10^-3 Wm^-1K^-2 achieved at 875 K is twice that reported in the literature for undoped Mg2Si

Reproducibility of twenty-four-hour finger arterial blood pressure, variability and systemic hemodynamics

At present, non-invasive continuous monitoring of finger arterial blood pressure by the volume-clamp technique is considered the best approach to obtain reliable assessments of beat-to-beat blood pressure. However, data on the reproducibility (accuracy and precision) of prolonged recordings and of the hemodynamics derived from wave-form analysis are not available. Ten patients with untreated essential hypertension and eight normotensive subjects were monitored by Portapres over 24 h in the hospital on two occasions with 1-4 weeks in-between. Physical and mental activities were standardized as far as possible to minimize intra- and intersubject biological variability. Stroke volume was obtained by the Modelflow method. Differences between the two recordings were computed separately for the day (0700 to 2300 h) and the night (2300 to 0700 h) and for all hours. Differences in stroke volume were calculated as percentage change from the first recording. Accuracy was good in both groups and bias was close to zero. Precision was also remarkable in the daytime, and at least as good as values reported in studies that used the standard intra-arterial recording. The SD of the differences in systolic and diastolic pressure in the hypertensives in the daytime were 6.6 and 4.7 mmHg, respectively. At night, precision was less good, possibly because of the 30 min finger-cuff switching: 12.5 and 6.5 mmHg for systolic and diastolic pressure, respectively. The average stroke volume did not change more than 8% at most between the first and the second recordings. These results indicate that the Finapres and Portapres devices are a reliable substitute for intra-arterial recording, and are most useful instruments for the study of blood pressure regulatio

Effects of acute and chronic angiotensin converting enzyme inhibition by spirapril on cardiovascular regulation in essential hypertensive patients : assessment by spectral analysis and haemodynamic measurements

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