Among patients undergoing noncardiac surgery, peak postoperative hsTnT during the first 3 days after surgery was significantly associated with 30-day mortality. Elevated postoperative hsTnT without an ischemic feature was also associated with 30-day mortality.

In view of the fact that microtubules exhibit strong anisotropic elastic properties, an orthotropic elastic shell model for microtubules is developed to study buckling behavior of microtubules. The predicted critical pressure is found to agree well with recent unexplained experimental data on pressure-induced buckling of microtubules [Needleman, Phys. Rev. Lett. 93, 198104 (2004); Biophys. J. 89, 3410 (2005)] which are lower than that predicted by the isotropic shell model by four orders of magnitude. General buckling behavior of microtubules under axial compression or radial pressure is studied. The results show that the isotropic shell model greatly overestimates the bucking loads of microtubules, except columnlike axially compressed buckling of long microtubules (of length-to-diameter ratio larger than, say, 150). In particular, the present results also offer a plausible explanation for the length dependency of flexibility of microtubules reported in the literature.

Several vibration problems of multiwall carbon nanotubes ͑MWNTs͒ are studied in detail based on a multiple-elastic shell model. According to recent data available in the literature, an updated value of bending stiffness for single-wall carbon nanotubes ͑SWNTs͒ is suggested, which is in a much better agreement with atomistic model for phonon-dispersion relation of SWNTs. For axisymmetric vibrations ͑with circumferential wave number n =0͒, it is found that longitudinal ͑L͒ modes of individual tubes of a MWNT have almost identical frequencies and are usually coupled with each other through Poisson-ratio effect-induced radial ͑R͒ vibrations and interlayer van der Waals interaction. Especially in the transition zone of R-and L modes, the significant Poisson-ratio effect leads to mixed R -L modes with comparable longitudinal and radial displacements. On the other hand, for beamlike vibrations ͑with n =1͒, the present multiple-shell model is found to be in good agreement with the multiple-beam model for almost coaxial bending ͑B͒ modes of large-and smallradius MWNTs and noncoaxial B modes of small-radius MWNTs ͑e.g., of the outermost radius less than 2 nm͒, with relative errors less than 10%. However, for high-order noncoaxial modes of large-radius MWNTs, the relative errors between the two models increase up to 50% in extreme cases due to larger non-beamlike deformation of the cross section while both models give similar overall vibration modes through the entire length of MWNTs. In particular, for lower circumferential wave numbers ͑n =0-10͒, the lowest frequency always corresponds to the minimum half-axial wave number m = 1 for simply supported end conditions. When the wave vector decreases, the lowest frequency decreases and the associated mode shifts from an R mode with larger n to a coaxial B mode with n =1.

This paper examines applicability and limitations of simplified models of elastic cylindrical shells for carbon nanotubes. The simplified models examined here include Donnell equations and simplified Flugge equations characterized by an uncoupled single equation for radial deflection. These simplified elastic shell equations are used to study static buckling and free vibration of carbon nanotubes, with detailed comparison to exact Flugge equations of cylindrical shells. It is shown that all three elastic shell models are in excellent agreement (with relative errors less than 5%) with recent molecular dynamics simulations for radial breathing vibration modes of carbon nanotubes, while reasonable agreements for various buckling problems have been reported previously for Donnell equations. For general cases of buckling and vibration, the results show that the simplified Flugge model, which retains mathematical simplicity of Donnell model, is consistently in better agreement with exact Flugge equations than Donnell model, and has a significantly enlarged range of applicability for carbon nanotubes. In particular, the simplified Flugge model is applicable for carbon nanotubes (with relative errors around 10% or less) in almost all cases of physical interest, including some important cases in which Donnell model results in much larger errors. These results are significant for further application of elastic shell models to carbon nanotubes because simplified shell models, characterized by a single uncoupled equation for radial deflection, are particularly useful for multiwall carbon nanotubes of large number of layers.

SummaryIn Study A, the incidence of arterial oxygen desaturation was studied using pulse oximetry (S a o 2 ) in 100 sedated and 100 nonsedated patients breathing room air who underwent diagnostic upper gastrointestinal endoscopy. Hypoxia (S a o 2 92% or less of at least 15 s duration) occurred in 17% and 6% of sedated patients and nonsedated patients, respectively (p , 0.03). Mild desaturation (S a o 2 94% or less and less than 15 s duration) occurred in 47% of sedated patients compared with 12% of nonsedated patients (p , 0.001). In Study B, the effects of supplementary oxygen therapy and the effects of different pre-oxygenation times on arterial oxygen saturation (S a o 2 ) in sedated patients were studied using pulse oximetry. One hundred and twenty patients who underwent diagnostic upper gastrointestinal endoscopy with intravenous sedation were studied.