Acoustic and Large Eddy Simulation studies of azimuthal modes in annular combustion chambers

The objectives of this paper are the description of azimuthal instability modes found in annular combus- tion chambers using two numerical tools: (1) Large Eddy Simulation (LES) methods and (2) acoustic solv- ers. These strong combustion instabilities are difficult to study experimentally and the present study is based on a LES of a full aeronautical combustion chamber. The LES exhibits a self-excited oscillation at the frequency of the first azimuthal eigenmode. The mesh independence of the LES is verified before ana- lysing the nature of this mode using various indicators over more than 100 cycles: the mode is mostly a pure standing mode but it transitions from time to time to a turning mode because of turbulent fluctu- ations, confirming experimental observations and theoretical results. The correlation between pressure and heat release fluctuations (Rayleigh criterion) is not verified locally but it is satisfied when pressure and heat release are averaged over sectors. LES is also used to check modes predicted by an acoustic Helmholtz solver where the flow is frozen and flames are modelled using a Flame Transfer Function (FTF) as done in most present tools. The results in terms of mode structure compare well confirming that the mode appearing in the LES is the first azimuthal mode of the chamber. Moreover, the acoustic solver provides stability maps suggesting that a reduction of the time delay of the FTF would be enough to sta- bilise the mode. This is confirmed with LES by increasing the flame speed and verifying that this modi- fication leads to a damped mode in a few cycles

Echocardiography during submaximal isometric exercise in children with repaired coarctation of the aorta compared with controls

Objective Patients with repaired coarctation (RCoA) remain at higher risk of cardiac dysfunction, initially often only detected during exercise. In this study, haemodynamics of isometric handgrip (HG) and bicycle ergometry (BE) were compared in patients with RCoA and matched controls (MCs). Methods Case-control study of 19 children with RCoA (mean age 12.9 +/- 2.3 years; mean age of repair 7 months) compared with 20 MC. HG with echocardiography followed by BE was performed in both groups. Results During HG (blood pressure) BP increased from 114 +/- 11/64 +/- 4 mm Hg to 132 +/- 14/79 +/- 7 mm Hg, without significant differences. During HG as well as BE, HR increased less in patients with RCoA. There were no significant differences in (left ventricle) LV dimensions or LV mass. The RCoA group had diastolic dysfunction: both at rest and during HG they had significantly higher transmitral E and A velocities and lower tissue Doppler E' and A' velocities. E/E' was higher, reaching statistical significance during HG (p<0001). Conventional parameters of systolic function (FS and EF) were similar at rest and HG. More sensitive tissue Doppler S' was significantly lower at rest in CoA subjects (5.1 +/- 1.5 cm/s vs 6.5 +/- 1 +/- 1 cm/s; p<0.01), decreasing further during HG by 5% in the CoA group (NS) while unchanged in controls. Conclusions We provide first evidence that HG with echocardiography is feasible, easy and patient-friendly. A decreased systolic (tissue Doppler) and impaired diastolic LV function was measured in the RCoA group, a difference that tended to increase during HG

Different patterns of cerebral and muscular tissue oxygenation 10 years after coarctation repair

The purpose of this study was to assess whether the lower exercise tolerance in children after coarctation repair is associated with alterations in peripheral tissue oxygenation during exercise. A total of 16 children after coarctation repair and 20 healthy control subjects performed an incremental ramp exercise test to exhaustion. Cerebral and locomotor muscle oxygenation were measured by means of near infrared spectroscopy. The responses of cerebral and muscle tissue oxygenation index (cTOI, mTOI), oxygenated (O(2)Hb), and deoxygenated hemoglobin (HHb) as a function of work rate were compared. Correlations between residual continuous wave Doppler gradients at rest, arm-leg blood pressure difference and local oxygenation responses were evaluated. Age, length, and weight was similar in both groups. Patients with aortic coarctation had lower peak power output (Ppeak) (72.3 +/- 20.2% vs. 106 +/- 18.7%, P < 0.001), VO(2)peak/kg (37.3 +/- 9.1 vs. 44.2 +/- 7.6 ml/kg, P = 0.019) and %VO(2)peak/kg (85.7 +/- 21.9% vs. 112.1 +/- 15.5%, P < 0.001). Cerebral O(2)Hb and HHb had a lower increase in patients vs. controls during exercise, with significant differences from 60 to 90% Ppeak (O(2)Hb) and 70% to 100% Ppeak (HHb). Muscle TOI was significantly lower in patients from 10 to 70% Ppeak and muscle HHb was significantly higher in patients vs. controls from 20 to 80% Ppeak. Muscle O(2)Hb was not different between both groups. There was a significant correlation between residual resting blood pressure gradient and Delta muscle HHb/Delta P at 10-20W and 20-30W (r = 0.40, P = 0.039 and r = 0.43, P = 0.034). Children after coarctation repair have different oxygenation responses at muscular and cerebral level. This reflects a different balance between O-2 supply to O-2 demand which might contribute to the reduced exercise tolerance in this patient population

Effect of aortic stiffness versus stenosis on ventriculo-arterial interaction in an experimental model of coarctation repair

Objectives: The aim of this study was to investigate the effect of short- versus long-segment aortic stiffness and stenosis on ventriculo-arterial interaction in a porcine model of coarctation repair. Methods: Short-long aortic stiffness was created by transection/suture [coarctation (CoA) suture, n = 6] and stenting (stent, n = 5) of the proximal descending aorta. Short-long aortic stenosis was achieved by wrapping a prosthetic graft around the aorta to 1/3-circumference reduction, over a segment length of 1 cm (CoA suture stenosis, n = 5) and 4.5 cm (stent stenosis, n = 6). After 3 months, aortic pressure-flow haemodynamics, aortic distensibility by intravascular ultrasound and left ventricular performance by pressure-volume loops were compared to a Sham group (n = 5) at baseline and during dobutamine administration. Results: The aortic impedance increased with 30.3 (12.6%) and 41.3 (20.9%) (P < 0.001) in CoA stenosis and stent stenosis during inotropic response. Impaired haemodynamic aortic compliance was associated with lower aortic distensibility by intravascular ultrasound, specifically in long-segment stenosis. The ventriculo-arterial coupling was disturbed in both groups with stenosis, with blunted contractile response [Sham 140.3 (19.8%), CoA suture 101.3 (14.5%), CoA suture stenosis 75.0 (8.4%), stent 115.5 (12.7%), stent stenosis 55.1 (14.6%), P < 0.001] and increased myocardial stiffness during dobutamine in the long-segment aortic stenosis group [Sham -26.0 (12.9%), CoA suture -27.5 (15.9%), CoA stenosis -9.5 (8.6%), stent -23.4 (4.8%), stent stenosis 19.9 (23.1%), P < 0.001]. Conclusions: This animal study on the sequelae of coarctation repair demonstrated that aortic stiffness had little effect on aortic pressure-flow characteristics in the absence of stenosis. However, the negative chronic effect of stenosis on aortic haemodynamics-especially a longer segment-leads to the rapid impairment of ventriculo-arterial interaction, which is accentuated by inotropy. Therefore, therapeutical management needs to focus on improving aortic remodelling after coarctation repair, preferably by minimizing residual stenosis, even at the cost of inducing aortic stiffness

The pulmonary autograft after the Ross operation : results of 25 year follow-up in a pediatric cohort

Progressive autograft dilation and need for later reoperation remain major concerns of the Ross procedure. The study investigates the clinical outcome after the Ross operation, including a longitudinal analysis of autograft dimensions over 25 years. From November 1991 to April 2019, 137 patients underwent a Ross procedure at the University Hospitals of UCL (Université catholique de Louvain)-Brussels and Ghent. Inclusion criteria were less than or equal to 18 years of age and pulmonary autograft implantation by root replacement. Outcome focused on survival, reoperation rate, and autograft size evolution through linear mixed-model analysis. A Ross or Ross-Konno operation was performed in 110 (80%) and 27 (20%) patients at a median age of 10.4 (interquartile range [IQR], 4.7-14.3) years and 0.5 (IQR, 0.04-5.2) years, respectively. Overall 10-year and 20-year survival was 87% ± 3% and 85% ± 3%, respectively, but was 93% ± 3% for isolated Ross patients. Right ventricular outflow tract-conduit exchange was required in 20.3%, whereas autograft-related reoperation was performed in 14 (10.7%) patients at a median interval of 14 (IQR, 9-16) years, for aortic regurgitation (n = 2) and autograft dilation (n = 12). Autograft z-values increased significantly at the sinus and sinotubular junction (STJ) compared with the annulus (annulus = 0.05 ± 0.38/y, sinus = 0.14 ± 0.25/y, STJ = 0.17 ± 0.34/y; P = .015). The z-value slope for autograft dimensions was significantly steeper for Ross-Konno vs Ross patients (annulus: P = .029; sinus: P < .001; STJ: P = .012), and for children having aortic arch repair (annulus: P = .113, sinus: P = .038; STJ: P = .029). The Ross operation offers children requiring aortic valve replacement an excellent survival perspective, with an acceptable risk of autograft reoperation within the first 25 years. Contrary to the autograft annulus, dilation of the sinus and STJ size is of concern. Closer surveillance of autograft dimensions might be required in patients who underwent a Ross-Konno procedure or aortic arch reconstruction

Description and evaluation of a ventriculo-coronary artery bypass device that provides bi-directional coronary flow

Objective: The objective of this study was to assess acute patency of a new myocardial revascularization device that connects the left ventricular cavity to a coronary artery (termed ventriculo-coronary artery bypass, VCAB) thereby providing proximal and distal blood flow from the site of the anastomosis. Methods: A device made of expanded polytetrafluoroethylene and low density polyethylene was implanted from the base of the left ventricle to the mid left anterior descending coronary artery (LAD) in 11 juvenile domestic pigs using a beating heart approach. Flow rates were measured in the distal LAD before and after implant using ultrasonic flow techniques, and patency was assessed at explant at either 2 or 4 weeks post-implantation. Myocardial perfusion using positron emission tomography (PET) was assessed in a separate set of pigs (n=2) revascularized by VCAB 2 weeks post-implant. Results: Net forward flow distal to the implanted device was 73±15% of native LAD flow. PET demonstrated that the target myocardium was perfused at 85% of that seen in the remote, control myocardium. Device patency rate was 80% (4/5) at 2 weeks in one set of pigs and 83% (5/6) at 4 weeks in a second set of pigs. Histologic analysis showed formation of neointima along the extraventricular segment of the device. Conclusions: This study demonstrates the promise of perfusing ischemic myocardium using a VCAB approach with a device that provides blood flow both proximal and distal to the anastomosis. Patency of the transmyocardial device was encouraging at 2 and 4 weeks and warrants further investigatio

Assessment of liver fibrosis and associated risk factors in HIV-infected individuals using transient elastography and serum biomarkers

Background: Liver fibrosis in human immunodeficiency virus (HIV)-infected individuals is mostly attributable to co-infection with hepatitis B or C. The impact of other risk factors, including prolonged exposure to combined antiretroviral therapy (cART) is poorly understood. Our aim was to determine the prevalence of liver fibrosis and associated risk factors in HIV-infected individuals based on non-invasive fibrosis assessment using transient elastography (TE) and serum biomarkers (Fibrotest [FT]). Methods: In 202 consecutive HIV-infected individuals (159 men; mean age 47 ± 9 years; 35 with hepatitis-C-virus [HCV] co-infection), TE and FT were performed. Repeat TE examinations were conducted 1 and 2 years after study inclusion. Results: Significant liver fibrosis was present in 16% and 29% of patients, respectively, when assessed by TE (≥ 7.1 kPa) and FT (> 0.48). A combination of TE and FT predicted significant fibrosis in 8% of all patients (31% in HIV/HCV co-infected and 3% in HIV mono-infected individuals). Chronic ALT, AST and γ-GT elevation was present in 29%, 20% and 51% of all cART-exposed patients and in 19%, 8% and 45.5% of HIV mono-infected individuals. Overall, factors independently associated with significant fibrosis as assessed by TE (OR, 95% CI) were co-infection with HCV (7.29, 1.95-27.34), chronic AST (6.58, 1.30-33.25) and γ-GT (5.17, 1.56-17.08) elevation and time on dideoxynucleoside therapy (1.01, 1.00-1.02). In 68 HIV mono-infected individuals who had repeat TE examinations, TE values did not differ significantly during a median follow-up time of 24 months (median intra-patient changes at last TE examination relative to baseline: -0.2 kPa, p = 0.20). Conclusions: Chronic elevation of liver enzymes was observed in up to 45.5% of HIV mono-infected patients on cART. However, only a small subset had significant fibrosis as predicted by TE and FT. There was no evidence for fibrosis progression during follow-up TE examinations