Why do nanowires grow with their c-axis vertically-aligned in the absence of epitaxy?

Images of uniform and upright nanowires are fascinating, but often, they are quite puzzling, when the substrate is clearly not an epitaxial template. Here, we reveal the physics underlying one such hidden growth guidance mechanism through a specific example - the case of ZnO nanowires grown on silicon oxide. We show how electric fields exerted by the insulating substrate may be manipulated through the surface charge to define the orientation and polarity of the nanowires. Surface charge is ubiquitous on the surfaces of semiconductors and insulators, and as a result, substrate electric fields need always be considered. Our results suggest a new concept, according to which the growth of wurtzite semiconductors may often be described as a process of electric-charge-induced self-assembly, wherein the internal built-in field in the polar material tends to align in parallel to an external field exerted by the substrate to minimize the interfacial energy of the system.

Probing unfolded acoustic phonons with X rays.

Ultrafast laser excitation of an InGaAs/InAlAs superlattice (SL) creates coherent folded acoustic phonons that subsequently leak into the bulk (InP) substrate. Upon transmission, the phonons become "unfolded" into bulk modes and acquire a wave vector much larger than that of the light. We show that time-resolved x-ray diffraction is sensitive to this large-wave vector excitation in the substrate. Comparison with dynamical diffraction simulations of propagating strain supports our interpretation.

Observation of surface-avoiding waves: a new class of extended states in periodic media.

Coherent time-domain optical experiments on GaAs-AlAs superlattices reveal the existence of an unusually long-lived acoustic mode at approximately 0.6 THz which couples weakly to the environment by evading the sample boundaries. Classical as well as quantum states that steer clear of surfaces are generally shown to occur in the spectrum of periodic structures, for most boundary conditions. These surface-avoiding waves are associated with frequencies outside forbidden gaps and wave vectors in the vicinity of the center and edge of the Brillouin zone. Possible consequences for surface science and resonant-cavity applications are discussed.

Generation and propagation of a picosecond acoustic pulse at a buried interface: time-resolved x-ray diffraction measurements.

We report on the propagation of coherent acoustic wave packets in (001) surface oriented Al0.3Ga0.7As/GaAs heterostructure, generated through localized femtosecond photoexcitation of the GaAs. Transient structural changes in both the substrate and film are measured with picosecond time-resolved x-ray diffraction. The data indicate an elastic response consisting of unipolar compression pulses of a few hundred picosecond duration traveling along [001] and [001] directions that are produced by predominately impulsive stress. The transmission and reflection of the strain pulses are in agreement with an acoustic mismatch model of the heterostructure and free-space interfaces.

Assessment: transcranial Doppler ultrasonography: report of the Therapeutics and Technology Assessment Subcommittee of the American Academy of Neurology.

OBJECTIVE: To review the use of transcranial Doppler ultrasonography (TCD) and transcranial color-coded sonography (TCCS) for diagnosis. METHODS: The authors searched the literature for evidence of 1) if TCD provides useful information in specific clinical settings; 2) if using this information improves clinical decision making, as reflected by improved patient outcomes; and 3) if TCD is preferable to other diagnostic tests in these clinical situations. RESULTS: TCD is of established value in the screening of children aged 2 to 16 years with sickle cell disease for stroke risk (Type A, Class I) and the detection and monitoring of angiographic vasospasm after spontaneous subarachnoid hemorrhage (Type A, Class I to II). TCD and TCCS provide important information and may have value for detection of intracranial steno-occlusive disease (Type B, Class II to III), vasomotor reactivity testing (Type B, Class II to III), detection of cerebral circulatory arrest/brain death (Type A, Class II), monitoring carotid endarterectomy (Type B, Class II to III), monitoring cerebral thrombolysis (Type B, Class II to III), and monitoring coronary artery bypass graft operations (Type B to C, Class II to III). Contrast-enhanced TCD/TCCS can also provide useful information in right-to-left cardiac/extracardiac shunts (Type A, Class II), intracranial occlusive disease (Type B, Class II to IV), and hemorrhagic cerebrovascular disease (Type B, Class II to IV), although other techniques may be preferable in these settings.

Continuous quality improvement in dialysis units: basic tools.

Physicians and allied health professionals are expected to understand and participate in the assessment and improvement of the quality of care delivered to patients in end-stage renal disease (ESRD) treatment centers. Participating in the quality improvement process will bring clinicians into contact with special knowledge and skills drawn from the areas of statistical process control and industrial engineering. Some of the more frequently encountered of these concepts and tools are described.

Improving serum albumin levels in hemodialysis patients by a continuous quality improvement project.

Protein nutrition and serum albumins predict present and future mortality and morbidity. The North East Albuquerque Dialysis Center undertook a quality improvement project to improve serum albumin levels in hemodialysis patients. The dialysis facility's quality assurance team evolved into a continuous quality improvement team just before this project. The timeline and steps in this evolution are presented. The continuous quality improvement project process analysis, interventions, and outcome rates are also described. The project has resulted in a rate of less than or equal to 10% of hemodialysis patients with serum albumins less than 3.4 for the past 3 years.

Reduced anterior cingulate gyrus volume correlates with executive dysfunction in men with first-episode schizophrenia.

Although frontal lobe structural and functional abnormalities have been identified in schizophrenia, their relationship remains elusive. Because the frontal lobes are both structurally and functionally heterogeneous, it is possible that some measures of frontal lobe structure may not have accurately identified relevant frontal lobe subregions. The authors hypothesized that the volumes of two dorsal, 'archicortical' subregions (i.e. superior frontal gyrus and anterior cingulate gyrus), but not a ventral, 'paleocortical' subregion (i.e. orbital frontal region) would be significantly and selectively correlated with executive and motor dysfunction in patients with schizophrenia as previously reported for the anterior hippocampal region. Volumes of these frontal lobe subregions were measured from magnetic resonance images based on sulcal anatomy in 20 men and 15 women with first-episode schizophrenia. All patients completed a comprehensive neuropsychological test battery while clinically stabilized that encompassed six domains of functioning: attention, executive, motor, visuospatial, memory and language. Findings indicated that reduced anterior cingulate gyrus volume was significantly correlated with worse executive functioning in men; among women, there were no significant correlations. Among men, anterior cingulate gyrus volume was significantly more strongly correlated with executive functioning than with attention, visuospatial, memory, language and general intellectual functioning. Neither executive nor motor functioning was significantly more strongly correlated with the dorsal 'archicortical' volumes than with orbital frontal volume. These findings suggest a link between executive deficits and dysfunction of the dorsal 'archicortical' system and implicate sex differences in their relationship in first-episode schizophrenia.

Neuropsychology of first-episode schizophrenia: initial characterization and clinical correlates.

OBJECTIVE: Neuropsychological impairments are well documented in schizophrenia and are important targets of treatment. Information about the severity and pattern of deficits after treatment for the first psychotic episode and about relationships between these deficits and syndromal characteristics remains limited. METHOD: Comprehensive neuropsychological assessments including 41 individual tests were given to 94 patients with first-episode schizophrenia after initial stabilization of psychosis and to a comparison group of 36 healthy volunteers. Profiles of neuropsychological deficits and the relationship of deficits to sex and handedness were examined. Correlations of neuropsychological deficit with a broad range of historical and clinical characteristics, including outcome, were explored. RESULTS: Patients had a large generalized neuropsychological deficit (1.5 standard deviations compared to healthy volunteers). Patients also had, superimposed on the generalized deficit, subtle relative deficits (less than 0.5 standard deviation compared to their own average profile) in memory and executive functions. Learning/memory dysfunction best distinguished patients from healthy individuals; after accounting for this difference, only motor deficits further distinguished the groups. Patients with higher neuropsychological ability had only memory deficits, and patients with lower ability had both memory and executive deficits. No sex differences were observed beyond the normal advantage for men in motor speed. Dextral patients had less severe generalized deficit. Severity of residual symptoms was associated with greater generalized deficit. Executive and attentional deficits were most linked to global functional impairment and poor outcome. CONCLUSIONS: The results document a large generalized deficit, and more subtle differential deficits, in clinically stabilized first-episode patients. Learning/memory deficits were observed even in patients with less severe generalized deficit, but the pattern was unlike the amnestic syndrome and probably reflects different mechanisms. Executive and attentional deficits marked the more severely disabled patients, and may portend relatively poor outcome. Failure to develop typical patterns of cerebral dominance may increase the risk for greater generalized deficit.

Proline in vasoactive peptides: consequences for peptide hydrolysis in the lung.

To examine the hypothesis that trans isomers of bradykinin and [Gly6]bradykinin are preferentially hydrolyzed by lung peptidases, we studied the fractional inactivation of these peptides in the perfused rat lung using a bioassay after a single-pass bolus injection and high-performance liquid chromatography after lung recirculation. In the bioassay studies, when the peptides passed through the lung, 25.6-fold more bradykinin or 7-fold more [Gly6]bradykinin was required to elicit a contraction equivalent to that produced when the peptides did not pass through the lung. In the recirculation studies, hydrolysis progress curves with rapid and slow phases were observed, with a higher fraction of bradykinin than [Gly6]bradykinin hydrolyzed in the rapid phase. Cyclophilin increased the hydrolysis rate during the slow phase for both peptides. Kinetic analysis indicated that the slowly hydrolyzed peptide fraction, presumably the cis fraction, was 0.13 for bradykinin and 0.43 for [Gly6]bradykinin with cis-trans isomerization rate constants of 0.074 and 0.049 s-1, respectively, consistent with published nuclear magnetic resonance studies.

Meta-analysis of symptom factors in schizophrenia.

Factor and correlational analyses have been used to characterize symptom dimensions in schizophrenia, though they have yielded divergent models. This study used meta-analysis of published work to determine the number and composition of symptom dimensions. Principal components analysis of data from 10 empirical studies (pooled n = 896) yielded three factors, 'positive', 'negative' and 'conceptual disorganization'. The findings suggest that a three-factor solution is a relatively stable outcome of studies assessing these symptoms in chronic patients, and that some symptoms (alogia, attentional impairment) are less likely to load uniquely on a single factor.

Continuous quality improvement in ESRD: the role of networks, the United States Renal Data System, and facility-specific reports.

End-stage renal disease (ESRD) outcome improvement involves many different private and governmental entities. Networks have fulfilled a pivotal role in ESRD quality improvement by facilitating the change from quality assurance (QA) to continuous quality improvement (CQI) methodology, providing the collection and dissemination of outcome measures to facilities and developing quality improvement projects (QIPs) that interface directly with facilities. Improving outcomes in hemodialysis is generally limited to adequacy of hemodialysis and anemia management. Opportunities in peritoneal dialysis, nutrition, vascular access, and quality-of-life outcomes persist. Interaction between facilities and Medical Review Boards (MRBs) using workshops, site visits, and facility report cards can provide continuing ESRD outcome improvement. Every facility has unique people, procedures, and equipment producing their processes of care. Therefore, a certain amount of autonomy is required to encompass individual variation. Quality improvement methodology, although less rigorous than traditional outcome research, provides efficient and effective intervention when a rapid response is required to improve clinical outcomes. The two methods are not mutually exclusive but require distinct methodology to accomplish.

An analysis of the determinants of urinary urea and creatinine clearance in patients on continuous peritoneal dialysis.

The relative contribution of urinary volume (UV) and urine-to-plasma concentration ratios for urea (U/PUr) and creatinine (U/PCr) to urinary Kt/V urea (Kt/VU) and urinary uncorrected creatinine clearance (CCrU), respectively, was studied by simple and multiple linear regression analysis in 236 urea kinetic studies and 233 creatinine kinetic studies performed in 135 patients on continuous peritoneal dialysis (CPD). The following simple regressions were obtained: Kt/VU = 0.09 + 0.72 (UV), r = 0.75; Kt/VU = -0.01 + 0.11 (U/PUr), r = 0.55; CCrU = 12.06 + 56.46 + 46.46 (UV), r = 0.62; CCrU = 3.51 + 3.40 (U/PCr), r = 0.58. All r values were significant (p < 0.001). According to these regressions, a loss of 0.2 L/24 hours in UV leads to a loss of 0.15 weekly in Kt/VU and 11.3 L/1.73 m2 weekly in corrected CCrU (approximately 8 L/1.73 m2 weekly in corrected CCrU). By multiple linear regression, (1) Kt/VU = -0.38 + 0.70 (UV) + 0.10 (U/PUr). Standardized coefficients were 0.72 for UV and 0.51 for U/PUr (2) CCrU = -33.36 + 59.83 (UV) + 3.63 (U/PCr). Standardized coefficients were 0.65 for UV and 0.61 for U/PCr. UV is the most important determinant of both urea and creatinine urinary clearances in CPD patients. The contribution of the U/P ratios to the urinary clearances is important, but less than that of UV. The primary dependence of urinary clearances on UV allows the use of UV, which can be easily monitored by patients, as a first approximation index of changing residual renal function in CPD.

Serum albumin in continuous peritoneal dialysis: its predictors and relationship to urea clearance.

We investigated the predictors of serum albumin and the relationship between serum albumin and urea kinetic indices in continuous peritoneal dialysis (CPD). In a training set (TS) of 143 urea kinetic studies performed in 92 CPD patients, stepwise logistic regression identified high/high-average peritoneal solute transport, diabetes, advanced age and high daily drain volume normalized by body water as predictors of low serum albumin (< 35 g/liter). This analysis was then substantiated in a validation set (VS) of 187 kinetic studies performed in another 102 CPD patients. The calculated area under the receiver operating characteristic (ROC) curve by this logistic regression model was 0.782 (95% CI, 0.745 to 0.819). Logistic regression was repeated in the TS using only the first kinetic study from each patient, and it identified high/high-average peritoneal solute transport, diabetes, and advanced age as predictors of low albumin. Using only the first kinetic study from each patient in the VS, the second logistic regression model calculated an area under the ROC curve equal to 0.850 (95% CI, 0.810 to 0.890). The relative risk (RR) of serum albumin < 35 g/liter was computed for all kinetic studies after combining the TS and the VS and using non-diabetic CPD subjects aged < or = 61 years with low/low average peritoneal solute transport as the reference group. The RR with only one risk factor present ranged from 1.076 (age > 61 years) to 6.792 (high/high-average transport). The RR with two risk factors present ranged from 5.200 to 9.729. The RR with all three risk factors present was 9.100 (95% CI, range 3.923 to 21.111). A subset of 37 CPD patients had a second urea kinetic study 8 +/- 5 months after an increase in the amount of dialysis due to low urea clearance and/or uremic symptoms. The weekly KT/V urea increased from 1.40 +/- 0.24 to 2.10 +/- 0.31 after the increase in the CPD dose. With the increase in dialysis, the protein catabolic rate increased substantially; however, the mean serum albumin remained stable (from 33.9 +/- 4.6 to 33.3 +/- 6.2 g/liter; decrease 18; increase 15; same 4). In comparison to the subjects who had a decrease in serum albumin after the increase in KT/V, those with the increase in serum albumin were younger (44.2 +/- 11.2 vs. 54.3 +/- 16.2 years, P = 0.044) and had a higher serum urea after the increase in the dose of CPD (22.4 +/- 7.8 vs. 17.0 +/- 6.0 mmol/liter, P = 0.037). We conclude that the major predictors of low serum albumin in CPD are advanced age, diabetes, and high/high-average peritoneal solute transport, but not urea kinetic studies. An increase in the dose of dialysis does not cause a consistent rise in serum albumin in underdialyzed CPD subjects. However, a subset of younger patients may be able to increase their serum albumin in response to the increase in KT/V.

Discriminability of the Wisconsin Card Sorting Test using the standardization sample.

The four neurological patient groups and the normals from the Wisconsin Card Sorting Test (WCST) standardization sample were used to examine the discriminability of the WCST's indices. Results reveal consistent differentiation of normals from the patient groups on all WCST variables, with classification rates averaging 71% accuracy. However, patient groups with frontal, diffuse, and nonfrontal lesions were not consistently discriminable from each other. The results suggest that the WCST is most usefully conceptualized as a measure of executive abilities that involves the frontal lobes, but should not be considered solely as a marker of isolated frontal lobe pathology.

Results of a computerized screening of stroke patients for unjustified hospital stay.

BACKGROUND AND PURPOSE: Effective methods to monitor length of stay can help reduce unnecessary hospital stay without adversely affecting the quality of care. In this study a clinical algorithm for assessing unjustified hospital stay in stroke patients was computerized and tested. METHODS: An algorithm was developed by the authors to estimate the number of medically justified and unjustified hospital days for patients admitted with a primary diagnosis of ischemic stroke. Data for the algorithm were obtained from 177 stroke patients from an acute-care teaching hospital. The performance of the algorithm was evaluated on a subset of 46 patients by comparing the number of medically unjustified hospital days determined by the algorithm with the consensus determination of two neurologists. RESULTS: The algorithm classified 68% of the 177 patients as having some unjustified hospital days and 41% of all hospital days as unjustified. With the neurologists as the gold standard, the sensitivity of the algorithm was .89 and the specificity was .91. The correlation between the number of unjustified days determined by the algorithm and the neurologists was .76. CONCLUSIONS: There is considerable unjustified length of stay for stroke patients. Physicians can develop simple clinical algorithms for detecting unjustified hospital stay in stroke patients that provide a reasonable approximation of complex clinical judgment.

Protein catabolic rate in patients on continuous peritoneal dialysis. A multivariate predictive model.

Protein catabolic rate (PCR) and PCR normalized to standard weight (PCRN) are important indices of nutrition in patients on continuous peritoneal dialysis. The purpose of this study was to test whether urea clearance is among the predictors of PCR and PCRN in a multivariate analysis. Stepwise logistic regression was used to develop separate models for low PCR and low PCRN on a set of 143 urea kinetic studies in 92 patients on continuous peritoneal dialysis. The regression models were tested on an independent sample of 189 urea kinetic studies in 102 patients on continuous peritoneal dialysis by deriving the area under a receiver operating characteristic curve. In the derivation set, low serum urea, high serum creatinine, low urine and dialysate drain volumes, and low body surface area were identified as predictors of PCR < or = 50 g daily. The area under the receiver operating characteristic curve in the validation set was 0.930 (95% confidence interval: 0.915-0.945). Low serum urea, male gender, high body mass index and low urea fractional clearance (KT/V) were predictors of PCRN < or = 0.80 g/kg daily. The receiver operating characteristic area for this model was 0.948 (95% confidence interval: 0.926-0.970). Logistic regression analysis was repeated twice after adding urea nitrogen excretion normalized to standard weight (UNEN) as a candidate variable. This process identified low UNEN, male gender, and obesity as the predictors of low PCRN, and low UNEN, male gender, low urine volume, low drain volume normalized by body water, and high serum albumin as predictors of low KT/V urea. The authors conclude that PCR and PCRN can be predicted by models that incorporate serum azotemic indices, body size and composition, and direct or indirect measurements of urea clearance. Small body size and lean body composition predict low PCR but high PCRN values. Both PCRN and KT/V urea are predicted by UNEN. Multivariate analysis cannot, therefore, rule out the hypothesis that PCRN and KT/V are linked mathematically.