Elastic constants and the formation of topological defects in hybrid nematic cells: A Monte Carlo study.

We present a Monte Carlo study of the effects of elastic anisotropy on the topological defects which can be formed in nematic films with hybrid boundary conditions. We simulate the polarized microscopy images and analyze their evolution in uniaxial systems for different values of the Frank elastic constants.

From Point to Filament Defects in Hybrid Nematic Films.

We have studied nematic hybrid films with homeotropic alignment at the top surface and various controlled degrees of in plane ordering, going from a random degenerate organization to a completely uniform alignment along one direction, at the bottom one. We show, by Monte Carlo (MC) computer simulations and experiments on photopatterned films with the bottom support surface fabricated with in-plane order similar to the simulated ones, that the point defects observed in the case of random planar orientations at the bottom tend to arrange along a filament as the surface ordering is sufficiently increased. MC simulations complement the polarized microscopy texture observations allowing to inspect the 3D structure of the defects and examine the role of elastic constants.

A molecular level simulation of a twisted nematic cell.

We have performed a Monte Carlo simulation of a sub-micrometric twisted nematic cell with nearly 106 particles using an off-lattice molecular model of a liquid crystal. This computer experiment is a proof of principle that molecular models can be pushed to the limit of the system sizes addressable with finite element models thus bridging the mesoscopic gap for multiscale modelling while providing a direct molecular level view of the working of the display. This approach, that allows a direct prediction of molecular organisations, properties, and responses of device systems without the requirement of prior estimate or knowledge of material properties (e.g., elastic constants), is particularly important in view of simulating materials and devices for which these quantities are not known. Results for the molecular organisation are discussed, with particular regard to its helical nature in the field-off state.

Effects of short daily hemodialysis and extended standard hemodialysis on blood pressure and cardiac hypertrophy: a comparative study.

BACKGROUND: Causes of hypertension and cardiac hypertrophy in hemodialysis (HD) patients are multiple, but the role of fluid overload appears to be crucial. Short daily HD (sDHD = 2 hr x 6/week) seems to allow reductions in left ventricular mass (LVM) through the reduction of extracellular water (ECW). Better cardiovascular stability during HD can be obtained with short, but more frequent HD sessions, but also by increasing the session length accompanied with a reduction in ultrafiltration (UF)/hr. Regardless of the method, the adequate reduction in extracellular volume should permit better control of hypertension and left ventricular hypertrophy (LVH). This study aimed to compare sDHD with an extended form of standard HD (eSHD = 4.5-5 hr x 3/week) on the reduction of fluid overload, blood pressure (BP) and LVM index (LVMi). PATIENTS AND METHODS: Twenty-four HD patients with hypertension and LVH were enrolled in a prospective non-randomized study. After a 3-month run-in period they were divided in two comparable groups: 12 patients treated with sDHD, and 12 patients treated with eSHD for 6 months. LVMi, 24 hr BP monitoring, ECW, determined with electrical bio-impedance, biochemical correlates and spKT/V were studied at the beginning of the study and 6 months later. RESULTS: The weekly session length was increased in eSHD from 722.9 +/- 7.5 to 877.3 +/- 35.5 min. ECW% was reduced similarly in the two groups (Delta ECW: eSHD = 4.6 +/- 2.4 L; sDHD = 4.1 +/- 2.3 L); 24 hr BP decreased significantly from 157/81 to 137/75 mmHg in eSHD, and from 149/79 to 128/72 mmHg in sDHD. The reduction in systolic BP was similar in the two groups (eSHD = 20.1 +/- 15.3 mmHg, sDHD = 21.2 +/- 16.7 mmHg). Finally, LVMi was similarly reduced (eSHD = 55 +/- 30.3 g/m(2), sDHD = 54.4 +/- 21.3 g/m(2). The number of antihypertensive drugs decreased significantly after ECW% reduction: only 2/10 patients on eSHD and 4/12 patients on sDHD were maintained on therapy (p = ns). Intra-dialysis hypotension episodes did not differentiate between SHD and DHD. The reduction in LVMi was significantly correlated to fluid volume changes when these were measured as phase angle (PA) with bio-impedance (r = -0.43, p < 0.05). CONCLUSIONS: In hypertensive HD patients with LVH, fluid overload is invariably present and its reduction allows the decrease of BP and LVM. These results can be obtained by forcing UF with eSHD and sDHD, but patients maintained on x 3/week schedules need longer dialysis sessions to avoid intra-dialysis symptoms.

Association between brain natriuretic peptide and extracellular water in hemodialysis patients.

BACKGROUND: Brain natriuretic peptide (BNP) is a hormone released by the left ventricle (LV) as a consequence of pressure or volume load. BNP increases in left ventricle hypertrophy (LVH), LV dysfunction, and it can also predict cardiovascular mortality in the general population as well as those undergoing hemodialysis (HD). We investigated the association between BNP and volume load in HD patients. METHODS: We studied 32 HD patients (60 +/- 17.1 years) treated thrice-weekly for at least 6 months. Exclusion criteria were: LV dysfunction, atrial fibrillation, malnutrition. Blood chemistries and BNP were determined on mid-week HD day. Blood pressure (BP) and cardiac diameters were determined on mid-week inter-HD day by using 24-hour ambulatory blood pressure monitoring and echocardiography. Bioimpedance was performed after HD and extracellular water (ECW%), calculated as a percentage of total body water, was considered as the index of volume load. RESULTS: Patients were divided into quartiles of 8 patients depending on the BNP value: 1st qtl BNP < or =45.5 pg/ml (28.4 +/- 10.9 pg/ml), 2nd qtl BNP > 45.5 pg/ml and < or =99.1 pg/ml (60.9 +/- 15.8 pg/ml), 3rd qtl BNP > 99.1 pg/ml and < or =231.8 pg/ml (160.5 +/- 51.8 pg/ml), 4th qtl BNP > 231.8 pg/ml (664.8 +/- 576.6 pg/ml). No inter-quartile differences were reported in age, HD age, body mass index spKt/V, or blood chemistries. As expected patients in the 4th BNP quartile showed the highest values of 24-hour pulse pressure (PP) and LV mass index (LVMi). The study of body composition revealed significant differences in ECW%, which was higher in the 4th quartile when compared to the others (4th q: 50 +/- 9.6%, vs 1st q. 40.1 +/- 2.4%, 2nd q. 41.9 +/- 5%, 3rd q. 42.8 +/- 6.9%). Using multiple stepwise linear regression where BNP was the dependent variable, and PP and ECW% the independent variables, only ECW% maintained statistical significance as a predictor of BNP levels (PP: Beta = 0.86, p = 0.58; ECW%: Beta = 0.64, p < 0.001 p < 0.001). CONCLUSIONS: Few studies have investigated the relationship between plasma BNP and volume load, and direct evidence is lacking. We used bioimpedance and the determination of ECW% to assess volume state in HD patients finding an association between BNP and ECW. The increased synthesis and release of BNP from the LV in HD patients appear to be mainly related to volume stress rather than to pressure load.

Association between extracellular water, left ventricular mass and hypertension in haemodialysis patients.

BACKGROUND: Hypertension and left ventricular hypertrophy (LVH) are present in the majority of patients undergoing haemodialysis (HD). These two pathologies persist after dialysis onset, and pharmacological therapy is often required for adequate control of blood pressure (BP). Although fluid overload is a determinant of hypertension, clinical assessment of this parameter remains difficult and unsatisfactory. Bioimpedance analysis (BIA) spectroscopy and the relative determination of extracellular water (ECW%) may provide a simple and inexpensive tool for investigating fluid overload. We studied 110 patients on thrice-weekly HD to determine whether ECW body content correlates with hypertension and LVH in this patient population. METHODS: Hypertension was determined according to the WHO criteria (office BP >/= 140/90 and/or the use of antihypertensive therapy). Twenty-four hour BP monitoring and echocardiography were performed on midweek inter-HD days. Blood chemistries, dialysis dose (spKt/V) and bioimpedance were analysed on midweek HD days. RESULTS: Hypertension was present in 74.5% of patients. There were no differences for age, spKt/V, haemoglobin, serum creatinine and residual renal function between normotensive and hypertensive patients. Twenty-four hour systolic BP (SBP), 24 h diastolic BP and 24 h pulse pressure were higher in hypertensive patients, in spite of antihypertensive therapy. LVH was present in 61.8% of patients. BIA revealed that ECW% was increased in LVH+ patients (LVH+ = 47.5 +/- 7.9%, LVH- = 42.4 +/- 6.2%, P = 0.01) and in hypertensive patients compared with normotensives (46.5 +/- 7.7% vs 43 +/- 7.2%, P = 0.02). Dry body weights and inter-HD body weight increases did not differ between hypertensive and normotensive patients nor between patients with or without LVH. ECW was correlated with SBP (r = 0.35, P < 0.01) and with left ventricular mass index (LVMi(g/sqm)) (r = 0.49, P < 0.001). A stepwise multiple linear regression model revealed that LVMi(g/sqm) was significantly correlated with ECW%, SBP and male gender (r = 0.65, P < 0.001). CONCLUSIONS: LVH and hypertension are present in a majority of HD patients and they are closely correlated with one another. We found associations between fluid load, measured by BIA and expressed as ECW, and BP and LVM.

Blunted nocturnal blood pressure decrease and left-ventricular mass in hypertensive hemodialysis patients.

Left-ventricular hypertrophy (LVH) represents a frequent complication in hemodialysis (HD) patients. Hypertension is a well-known risk factor of cardiac morbidity which is present in 2 of 3 patients: among them about 60% have a blunted nocturnal decrease of blood pressure (BP). Although some large studies on essential hypertensives have documented that non-dipper patients have a higher number of cardiac events and a higher left ventricle (LV) mass than dipper ones, conflicting results have been reported for dialysis patients. Therefore, the aim of our study was to assess differences in LV mass between dipper and non-dipper hypertensive HD patients. We studied 66 patients with 24-hour ambulatory BP monitoring performed on HD and on inter-HD day. They were classified as dipper when a decrease of at least 10% of nocturnal systolic blood pressure on the inter-HD day was present. Echocardiography and bioimpedance were performed. 29% of the patients were classified as dippers and 71% as non-dippers. The 48-hour systolic and diastolic BP were not significantly different between the two groups (SBP: dipper = 144 +/- 12.9 mm Hg, non-dipper = 149 +/- 17.8 mm Hg; DBP: dipper = 80 +/- 9.9 mm Hg, non-dipper = 81 +/- 10.6 mm Hg). LV mass index (LVMi) did not differ between the two groups (dipper = 143.1 +/- 40.7 g/m(2); non-dipper = 159.4 +/- 46.3 g/m(2)). No differences were reported between dipper and non-dipper patients regarding extracellular water distribution (ECW: 48.1 +/- 7.7 vs. 49.8 +/- 10.8%). SBP night/day ratio and 48-hour SBP were not correlated to LVMi. A strong correlation was reported between ECW% and LVMi (r = 0.53, p < 0.001). In conclusion, 2 of 3 hypertensive HD patients are non-dipper, and this condition does not seem to be associated with significant differences in 48-hour blood pressure and LV mass. Volume overload appears to be the main independent determinant of LVH in these patients.