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This simulation study compared two procedures to enable an adaptive test to select items in correspondence with a content blueprint. Trait level estimates obtained from testlet-based and constrained adaptive tests administered to 10,000 simulated examinees under two trait distributions and three item pool sizes were compared to the trait level estimates obtained from traditional adaptive tests in terms of mean absolute error, bias, and information. Results indicate that using constrained adaptive testing requires an increase of 5% to 11% in test length over the traditional adaptive test to reach the same error level and, using testlets requires an increase of 43% to 104% in test length over the traditional adaptive test. Given these results, the use of constrained computerized adaptive testing is recommended for situations in which an adaptive test must adhere to particular content specifications.

Suboptimal hemodynamic performance, tissue calcification, and limitation in long-term durability have been encountered clinically after aortic valve replacement with currently available bioprostheses. It is believed that some of these problems may be caused, directly or indirectly, by the stents of the bioprostheses. To address these deficiencies, the authors undertook the development of the Edwards Prima Stentless Bioprosthesis. This study was designed to evaluate the hemodynamic performance of the Edwards Prima Stentless Bioprosthesis in a pulse duplicator system. The stented Carpentier-Edwards Porcine Bioprosthesis (Baxter Healthcare Corp., Irvine, CA), which has been used in United States clinics for more than 10 years, was used as a control device. The flow fields in the vicinity of the test bioprostheses were inspected with color Doppler flow mapping. The transvalvular pressure gradients were measured invasively with a catheter and calculated with the Doppler determined velocity using a simplified Bernoulli equation. Additionally, the leakage volumes were determined with an electromagnetic flowmeter. In the Doppler flow mapping study, during systole, a central flow was observed distal to the stentless and stented bioprostheses. The central flow distal to the stentless bioprosthesis was broader than that observed distal to its stented counterpart. During diastole, no regurgitation was detected by color Doppler flow mapping in either the stentless or stented groups. The Doppler determined transvalvular pressure gradients correlated well with those measured by catheter (r = 0.990). Moreover, it was learned that the transvalvular pressure gradients of the stentless bioprosthesis were less than those of its stented counterpart, especially for the smaller sizes.(ABSTRACT TRUNCATED AT 250 WORDS)

Concerns with currently available bioprostheses are calcification, long-term durability, and functional and hemodynamic performance. It has been well known that these concerns are all more or less related to the fixatives, glutaraldehyde or formaldehyde, used in preserving bioprostheses. To address these concerns, we undertook the development of a porcine bioprosthesis fixed with an epoxy compound. It was discovered that the porcine leaflets fixed with the epoxy compound appeared more natural than those preserved with glutaraldehyde. The performance of this newly developed epoxy compound bioprosthesis (three samples) was evaluated in a juvenile sheep model. The results were compared to those of its glutaraldehyde counterpart (three samples). Two-dimensional echocardiographic inspection of the valvular leaflet motion indicated that the epoxy compound leaflets were more pliable than their glutaraldehyde counterparts. In addition, the epoxy compound valve appeared to open more widely than the glutaraldehyde valve. Color Doppler flow mapping demonstrated that the blood flow distal to the epoxy compound valve was slightly broader than that observed distal to the glutaraldehyde valve. Moreover, at retrieval, less calcium and pannus ingrowth were observed in the epoxy compound valve than its glutaraldehyde counterpart. The results of this preliminary evaluation indicated that the performance of this newly developed epoxy compound valve was at least equivalent to its glutaraldehyde counterpart, if not better.

Currently available aortic bioprosthetic heart valves are all relatively suboptimal due to the constraints placed on leaflets by stenting. The constraints on the stenting mechanism may directly or indirectly cause turbulence in the orifice neighborhood, be nidi of tissue calcification, and impart transleaflet stress differentials. To address all these deficiencies, we undertook the development of a stentless porcine aortic bioprosthesis (SPAB). This study evaluates the hemodynamic performance and durability of this design in special chambers with near physiologic compliance. The results are compared to those shown by a stented porcine aortic bioprosthesis. The pressure measurements under steady flow conditions showed that the hemodynamic performance (including pressure gradient and effective orifice area) of SPAB is superior to that of its stented counterpart, especially in the smaller sizes. In addition, it is expected that this stentless design should minimize the mechanical stress to which the leaflets are subjected throughout the cardiac cycle, thus enhancing the durability of the bioprosthesis. In our accelerated durability study for up to 2 equivalent years, no valve failure has been observed. More in vitro studies under pulsatile flow conditions, including color Doppler flow visualization, are currently being conducted.

The acceleration of13C nmr relaxation rates by oxygen or by nitroxide radical was determined for representative perfluorohexane-heptane mixtures. In the 90:10 (v/v) solution, oxygen is found to affect perfluorohexane, but on the other hand, nitroxide affects the alkane the more strongly. In 15∶:85 solutions, little differentiation was noticeable although the data are less secure. In the former solution, domains are believed to exist, which are similar to the pure solvent in ability to dissolve paramagnetic relaxants. Force field calculations on the shape of fluorocarbon molecules are reviewed.

Two subarctic fens (one nutrient-poor, one nutrient-rich) were sampled from October, 1984 to July, 1985 near Schefferville, northern Quebec. The changes in concentations of chemicals (pH, conductance, Ca, Mg, Na, K, Fe, dissolved organic carbon, P, NH4+-N and N03−-N) dissolved in the peat water were identified during the freeze-thaw cycle. The highest chemical concentrations were in the winter (associated with the ice formation process), followed by the spring-summer, then fall. Four main processes influenced the concentration of dissolved chemicals in subarctic fens: 1) Snowmelt diluted the peat water; 2) Freezing of the peat increased concentrations of dissolved nutrients and other chemicals, believed to originate from biological sources; 3) Further increases in concentrations over the winter were caused by the incorporation of peat water, which migrated into the frozen peat; 4) Thawing of peat influenced the water chemistry by combining the release of the above 3 processes, along with biotic utilization. The freeze-thaw cycle in the subarctic fens appeared to increase the availability of important nutrients (such as phosphorus) during the spring.

The high-resolution mass spectra of two stable enol tautomers, 4,6-diethanoyl-1-methyl-5-phenyl-3-cyclohex-ene-1,3-diol (2) and 4-ethanoyl-5-hydroxy-2-(1-hydroxyethylidene)-5-methyl-3-phenylcyclohexanone (3), and the corresponding diketone tautomer, 2,4-diethanoyl-5-hydroxy-5-methyl-3-phenylcyclohexanone (1), are reported. Collisional activation of the molecular ions and major ionic fragments is used to assign decomposition pathways and to compare ion structures. Ionized 1 and 2 do not interconvert by a keto-enol tautomerism, but isomerize to a common intermediate. This isomerization probably involves opening of the cyclohexanone ring and transfer of the enolizable hydrogen, and is followed by elimination of water. The radical cation of 3 does not interconvert with [1]+˙ and [2]+˙ Instead, [3]+˙ dissociates by loss of an acetyl radical and eliminates water via a higher-energy pathway. As a result, the mass spectra of 1 and 2 are similar, whereas the mass spectrum of 3 is unique.

Two isomeric pairs of cis- and trans-1-cyclohexyl-2-phenyl-3-benzoylaziridines have been synthesized: (1) with a nitrogen-15 labelled nitrogen, and (2) with carbon-13 labelled ring carbons. The carbon-13 to X (where X=nitrogen-15, carbon-13 or hydrogen-I) spin-spin coupling constants were measured and interpreted in terms of stereoelectronic effects. X-ray crystallographic data (earlier determined for cisand trans-1-cyclohexyl-2-phenyl-3-(p-toluyl)aziridines)1 appear in good agreement with the NMR data. Bonding is discussed for the three-ring itself (NMR studies) and for its substituents (X-ray studies). It is concluded that stereochemical interaction of the Van der Waals type is an important determinant of aziridine bond length. Three-ring to carbonyl hyperconjugation is correlated with stereoelectronic interactions in the trans isomer.

This report summarizes the the problems with declining performance in the introductory courses, analysis of their causes, and some suggestions for overcoming them.