Thermodynamic characterization of the pretrasition of unilamellar dipalmitoyl-phosphatidylcholine vesicles

High-sensitivity differential scanning calorimetry of small unilamellar vesicles (SUV) made by sonication of dipalmitoylphosphatidylcholine (DPPC) reveals that the gel-liquid crystalline transition at 37 C is preceded by a pretransition at 28 C that is relatively slow (t 1/2≃2–4 min) and has an enthalpy change of ca. 0.2 kcal/mol. On incubation at 4 C, these SUV fuse spontaneously into large unilamellar vesicles (LUV). LUV also exhibit both a pretransition and a main-phase transition. The temperature of the main transition (T_m) and the enthalpy change of both the pretransition and main transition of these fused vesicles are similar to those of large multilamellar vesicles (MLV). The enthalpy change associated with the transition at 28 C decreases in SUV in a manner directly correlated to the decrease in the apparent enthalpy change of the 37 C main transition, indicating that the smaller (low temperature) transition is indeed a pretransition that is an inherent property of SUV. Therefore, unilamellar vesicles of DPPC appear to exhibit a pretransition at a temperature that varies from 28 C for the small vesicles to 35 C for the much larger vesicles.     

Cardiac surgery for grown-up congenital heart patients: survey of 307 consecutive operations from 1991 to 1994

The cardiac surgery performed from 1991 to 1994 in a unit dedicated specifically for grown-up congenital heart (GUCH) patients was reviewed to determine the frequency of various procedures, incidence of first and reoperations, early mortality, and its determinants. The 295 patients, aged 16 to 77 years (31 +/- 13), had 307 operations. First operations (n = 128, 42%) were most commonly for closure of atrial septal defect (n = 40), aortic valve replacement (n = 31) or repair of aortic coarctation (n = 14). Reoperations were more frequent (n = 179, 58%) and divided among first corrective repair (n = 49), reoperation after corrective repair (n = 115), and further palliation (n = 15). First corrective surgery was mainly for aortic valve disease (n = 17), Fallot (n = 7), and lesions needing a Fontan procedure (n = 5). Reoperations after corrective repair were needed for aortic valve disease (n = 43), right-sided conduit (n = 30), or recoarctation (n = 11). Early mortality was influenced by presence of central cyanosis (9 of 49, 18% in cyanotic patients; 12 of 258, 5% in acyanotic; p <0.001), increased number of previous operations (0 = 4%, 1 = 7%, 2 = 11%, >2 = 13%; p = 0.003), and increasing age of patients. Cyanotic patients had more serious postoperative complications: pleural and pericardial effusions, severe bleeding, renal insufficiency, and sepsis, and their hospital stay was longer compared with acyanotic patients (20 +/- 17 vs 11 +/- 8 days; p <0.001). In GUCH patients, reoperations cause the largest demand on cardiac surgical services. Increased survival of patients with complex cardiovascular malformations brings difficult challenges not only to cardiologists but also to cardiovascular surgeons. There is a need to provide continued highly specialized care. Resources, patients, and funding should be concentrated in a few designated centers.     

Elastic constants of porous silver compacts after acid assisted consolidation at room temperature

Acid assisted consolidation (AAC) is a technique that provides elevatedlevels of cohesiveness to silver powder compacts without recourse to a high temperature treatment. Ultrasonic techniques were used to measure the elastic constants of high purity silver compacts as a function of several processing variables. The elastic moduli of untreated (NT) samples were found to be significantly lower than those of samples that had undergone AAC and compacted at the same pressure. Post compaction sintering increases the elastic constants of both AAC and NT samples. The results indicate that the elastic constants are dependent not only on the density that was attained but also on the processing route that was followed. The elastic constant of a porous metal, M, can be expressed as M = M ₀ g, where M ₀ is the elastic modulus of the bulk metal, g is a geometrical factor that reflects the interparticle contact area and is a quality factor that depends on the nature of the interparticle interfaces. The results suggest that sound wave velocity is a parameter more appropriate than density for predicting the elastic moduli of porous metallic compacts.