A class of exact Navier-Stokes solutions for homogeneous flat-plate boundary layers and their linear stability

We introduce a new boundary layer formalism on the basis of which a class of exact solutions to the Navier–Stokes equations is derived. These solutions describe laminar boundary layer flows past a flat plate under the assumption of one homogeneous direction, such as the classical swept Hiemenz boundary layer (SHBL), the asymptotic suction boundary layer (ASBL) and the oblique impingement boundary layer. The linear stability of these new solutions is investigated, uncovering new results for the SHBL and the ASBL. Previously, each of these flows had been described with its own formalism and coordinate system, such that the solutions could not be transformed into each other. Using a new compound formalism, we are able to show that the ASBL is the physical limit of the SHBL with wall suction when the chordwise velocity component vanishes while the homogeneous sweep velocity is maintained. A corresponding non-dimensionalization is proposed, which allows conversion of the new Reynolds number definition to the classical ones. Linear stability analysis for the new class of solutions reveals a compound neutral surface which contains the classical neutral curves of the SHBL and the ASBL. It is shown that the linearly most unstable Görtler–Hämmerlin modes of the SHBL smoothly transform into Tollmien–Schlichting modes as the chordwise velocity vanishes. These results are useful for transition prediction of the attachment-line instability, especially concerning the use of suction to stabilize boundary layers of swept-wing aircraft.

Noninvasive pressure difference mapping derived from 4D flow MRI in patients with unrepaired and repaired aortic coarctation.

PURPOSE To develop a method for computing and visualizing pressure differences derived from time-resolved velocity-encoded three-dimensional phase-contrast magnetic resonance imaging (4D flow MRI) and to compare pressure difference maps of patients with unrepaired and repaired aortic coarctation to young healthy volunteers. METHODS 4D flow MRI data of four patients with aortic coarctation either before or after repair (mean age 17 years, age range 3-28, one female, three males) and four young healthy volunteers without history of cardiovascular disease (mean age 24 years, age range 20-27, one female, three males) was acquired using a 1.5-T clinical MR scanner. Image analysis was performed with in-house developed image processing software. Relative pressures were computed based on the Navier-Stokes equation. RESULTS A standardized method for intuitive visualization of pressure difference maps was developed and successfully applied to all included patients and volunteers. Young healthy volunteers exhibited smooth and regular distribution of relative pressures in the thoracic aorta at mid systole with very similar distribution in all analyzed volunteers. Patients demonstrated disturbed pressures compared to volunteers. Changes included a pressure drop at the aortic isthmus in all patients, increased relative pressures in the aortic arch in patients with residual narrowing after repair, and increased relative pressures in the descending aorta in a patient after patch aortoplasty. CONCLUSIONS Pressure difference maps derived from 4D flow MRI can depict alterations of spatial pressure distribution in patients with repaired and unrepaired aortic coarctation. The technique might allow identifying pathophysiological conditions underlying complications after aortic coarctation repair.

Noninvasive 4D pressure difference mapping derived from 4D flow MRI in patients with repaired aortic coarctation: comparison with young healthy volunteers

To assess spatial and temporal pressure characteristics in patients with repaired aortic coarctation compared to young healthy volunteers using time-resolved velocity-encoded three-dimensional phase-contrast magnetic resonance imaging (4D flow MRI) and derived 4D pressure difference maps. After in vitro validation against invasive catheterization as gold standard, 4D flow MRI of the thoracic aorta was performed at 1.5T in 13 consecutive patients after aortic coarctation repair without recoarctation and 13 healthy volunteers. Using in-house developed processing software, 4D pressure difference maps were computed based on the Navier-Stokes equation. Pressure difference amplitudes, maximum slope of pressure amplitudes and spatial pressure range at mid systole were retrospectively measured by three readers, and twice by one reader to assess inter- and intraobserver agreement. In vitro, pressure differences derived from 4D flow MRI showed excellent agreement to invasive catheter measurements. In vivo, pressure difference amplitudes, maximum slope of pressure difference amplitudes and spatial pressure range at mid systole were significantly increased in patients compared to volunteers in the aortic arch, the proximal descending and the distal descending thoracic aorta (p < 0.05). Greatest differences occurred in the proximal descending aorta with values of the three parameters for patients versus volunteers being 19.7 ± 7.5 versus 10.0 ± 2.0 (p < 0.001), 10.9 ± 10.4 versus 1.9 ± 0.4 (p = 0.002), and 8.7 ± 6.3 versus 1.6 ± 0.9 (p < 0.001). Inter- and intraobserver agreements were excellent (p < 0.001). Noninvasive 4D pressure difference mapping derived from 4D flow MRI enables detection of altered intraluminal aortic pressures and showed significant spatial and temporal changes in patients with repaired aortic coarctation.

FEM/FD Immersed Boundary FSI Simulations

Immersed boundary simulations have been under development for physiological flows, allowing for elegant handling of fluid-structure interaction modelling with large deformations due to retained domain-specific meshing. We couple a structural system in Lagrangian representation that is formulated in a weak form with a Navier-Stokes system discretized through a finite differences scheme. We build upon a proven highly scalable imcompressible flow solver that we extend to handle FSI. We aim at applying our method to investigating the hemodynamics of Aortic Valves. The code is going to be extended to conform to the new hybrid-node supercomputers.

The shock index and the simplified PESI for identification of low-risk patients with acute pulmonary embolism

We compared the test characteristics of the shock index (SI) and the simplified pulmonary embolism severity index (sPESI) for predicting 30-day outcomes in a cohort of 1,206 patients with objectively confirmed pulmonary embolism (PE). The primary outcome of the study was all-cause mortality. The secondary outcome was nonfatal symptomatic recurrent venous thromboembolism (VTE) or nonfatal major bleeding. Overall, 119 (9.9%) out of 1,206 patients died (95% CI 8.2-11.5%) during the first month of follow-up. The sPESI classified fewer patients as low-risk (369 (31%) out of 1,206 patients, 95% CI 28-33%) compared to the SI (1,024 (85%) out of 1,206 patients, 95% CI 83-87%) (p<0.001). Low-risk patients based on the sPESI had a lower 30-day mortality than those based on the SI (1.6% (95% CI 0.3-2.9%) versus 8.3% (95% CI 6.6-10.0%)), while the 30-day rate of nonfatal recurrent VTE or major bleeding was similar (2.2% (95%CI 0.7-3.6%) versus 3.3% (95%CI 2.2-4.4%)). The net reclassification improvement with the sPESI was 13.4% (p = 0.07). The integrated discrimination improvement was estimated as 1.8% (p<0.001). The sPESI quantified the prognosis of patients with PE better than the SI.

Cardiac troponin testing and the simplified Pulmonary Embolism Severity Index. The SWIss Venous ThromboEmbolism Registry (SWIVTER)

A low simplified Pulmonary Embolism Severity Index (sPESI), defined as age ≤80 years and absence of systemic hypotension, tachycardia, hypoxia, cancer, heart failure, and lung disease, identifies low-risk patients with acute pulmonary embolism (PE). It is unknown whether cardiac troponin testing improves the prediction of clinical outcomes if the sPESI is not low. In the prospective Swiss Venous Thromboembolism Registry, 369 patients with acute PE and a troponin test (conventional troponin T or I, highly sensitive troponin T) were enrolled from 18 hospitals. A positive test result was defined as a troponin level above the manufacturers assay threshold. Among the 106 (29%) patients with low sPESI, the rate of mortality or PE recurrence at 30 days was 1.0%. Among the 263 (71%) patients with high sPESI, 177 (67%) were troponin-negative and 86 (33%) troponin-positive; the rate of mortality or PE recurrence at 30 days was 4.6% vs. 12.8% (p=0.015), respectively. Overall, risk assessment with a troponin test (hazard ratio [HR] 3.39, 95% confidence interval [CI] 1.38-8.37; p=0.008) maintained its prognostic value for mortality or PE recurrence when adjusted for sPESI (HR 5.80, 95%CI 0.76-44.10; p=0.09). The combination of sPESI with a troponin test resulted in a greater area under the receiver-operating characteristic curve (HR 0.72, 95% CI 0.63-0.81) than sPESI alone (HR 0.63, 95% CI 0.57-0.68) (p=0.023). In conclusion, although cardiac troponin testing may not be required in patients with a low sPESI, it adds prognostic value for early death and recurrence for patients with a high sPESI.

A comparison of the original and simplified Pulmonary Embolism Severity Index

The Pulmonary Embolism Severity Index (PESI) is a validated clinical prognostic model for patients with pulmonary embolism (PE). Recently, a simplified version of the PESI was developed. We sought to compare the prognostic performance of the original and simplified PESI. Using data from 15,531 patients with PE, we compared the proportions of patients classified as low versus higher risk between the original and simplified PESI and estimated 30-day mortality within each risk group. To assess the models' accuracy to predict mortality, we calculated sensitivity, specificity, and predictive values and likelihood ratios for low- versus higher-risk patients. We also compared the models' discriminative power by calculating the area under the receiver-operating characteristic curve. The overall 30-day mortality was 9.3%. The original PESI classified a significantly greater proportion of patients as low-risk than the simplified PESI (40.9% vs. 36.8%; p<0.001). Low-risk patients based on the original and simplified PESI had a mortality of 2.3% and 2.7%, respectively. The original and simplified PESI had similar sensitivities (90% vs. 89%), negative predictive values (98% vs. 97%), and negative likelihood ratios (0.23 vs. 0.28) for predicting mortality. The original PESI had a significantly greater discriminatory power than the simplified PESI (area under the ROC curve 0.78 [95% CI: 0.77-0.79] vs. 0.72 [95% CI: 0.71-0.74]; p<0.001). In conclusion, even though the simplified PESI accurately identified patients at low-risk of adverse outcomes, the original PESI classified a higher proportion of patients as low-risk and had a greater discriminatory power than the simplified PESI.

A strategy combining imaging and laboratory biomarkers in comparison with a simplified clinical score for risk stratification of patients with acute pulmonary embolism

This study aimed to assess the performance of two prognostic models-the European Society of Cardiology (ESC) model and the simplified Pulmonary Embolism Severity Index (sPESI)-in predicting short-term mortality in patients with pulmonary embolism (PE).

Social cognition as a mediator variable between neurocognition and functional outcome in schizophrenia: empirical review and new results by structural equation modeling

Cognitive impairments are currently regarded as important determinants of functional domains and are promising treatment goals in schizophrenia. Nevertheless, the exact nature of the interdependent relationship between neurocognition and social cognition as well as the relative contribution of each of these factors to adequate functioning remains unclear. The purpose of this article is to systematically review the findings and methodology of studies that have investigated social cognition as a mediator variable between neurocognitive performance and functional outcome in schizophrenia. Moreover, we carried out a study to evaluate this mediation hypothesis by the means of structural equation modeling in a large sample of 148 schizophrenia patients. The review comprised 15 studies. All but one study provided evidence for the mediating role of social cognition both in cross-sectional and in longitudinal designs. Other variables like motivation and social competence additionally mediated the relationship between social cognition and functional outcome. The mean effect size of the indirect effect was 0.20. However, social cognitive domains were differentially effective mediators. On average, 25% of the variance in functional outcome could be explained in the mediation model. The results of our own statistical analysis are in line with these conclusions: Social cognition mediated a significant indirect relationship between neurocognition and functional outcome. These results suggest that research should focus on differential mediation pathways. Future studies should also consider the interaction with other prognostic factors, additional mediators, and moderators in order to increase the predictive power and to target those factors relevant for optimizing therapy effects.