Utilisation of an operative difficulty grading scale for laparoscopic cholecystectomy

Background A reliable system for grading operative difficulty of laparoscopic cholecystectomy would standardise description of findings and reporting of outcomes. The aim of this study was to validate a difficulty grading system (Nassar scale), testing its applicability and consistency in two large prospective datasets. Methods Patient and disease-related variables and 30-day outcomes were identified in two prospective cholecystectomy databases: the multi-centre prospective cohort of 8820 patients from the recent CholeS Study and the single-surgeon series containing 4089 patients. Operative data and patient outcomes were correlated with Nassar operative difficultly scale, using Kendall’s tau for dichotomous variables, or Jonckheere–Terpstra tests for continuous variables. A ROC curve analysis was performed, to quantify the predictive accuracy of the scale for each outcome, with continuous outcomes dichotomised, prior to analysis. Results A higher operative difficulty grade was consistently associated with worse outcomes for the patients in both the reference and CholeS cohorts. The median length of stay increased from 0 to 4 days, and the 30-day complication rate from 7.6 to 24.4% as the difficulty grade increased from 1 to 4/5 (both p < 0.001). In the CholeS cohort, a higher difficulty grade was found to be most strongly associated with conversion to open and 30-day mortality (AUROC = 0.903, 0.822, respectively). On multivariable analysis, the Nassar operative difficultly scale was found to be a significant independent predictor of operative duration, conversion to open surgery, 30-day complications and 30-day reintervention (all p < 0.001). Conclusion We have shown that an operative difficulty scale can standardise the description of operative findings by multiple grades of surgeons to facilitate audit, training assessment and research. It provides a tool for reporting operative findings, disease severity and technical difficulty and can be utilised in future research to reliably compare outcomes according to case mix and intra-operative difficulty

Solution of Navier-Stokes equations on nonstaggered grid at all speeds

The present author recently devised a pressure correction algorithm for solution of incompressible Navier-Stokes equations on a nonstaggered grid [6]. This algorithm introduced the notion of smoothing pressure correction to overcome the problem of checkerboard prediction of pressure. In this article, the algorithm is extended to prediction of compressible flows with and without shocks. The predictions show that the algorithm yields results that compare extremely favorably with previous ones [2] obtained using a staggered grid. Accurate shock capturing on coarse grids, however, requires use of total variation diminishing (TVD) discretization of the covective terms coupled with measures for stabilization of the iteration process

Complete pressure correction algorithm for solution of incompressible Navier-Stokes equations on a nonstaggered grid

When Navier-Stokes equations for incompressible flow are solved on a nonstaggered grid, Be problem of checkerboard prediction of pressure is encountered. So far, this problem has been cured either by evaluating the cell face velocities by the momentum interpolated principle [1] or by evaluating an effective pressure gradient in the nodal momentum equations [2]. In this article it is shown that not only are these practices unnecessary, they can lead to spurious results when the true pressure variation departs considerably from linearity. What is required instead is afresh derivation of the pressure correction equation appropriate for a nonstaggered grid. The pressure correction determined from this equation comprises two components: a mass-conserving component and a smoothing component. The former corresponds to the pressure correction predicted by a staggered grid procedure, whereas the latter simply accounts for the difference between the point value of the pressure and the cell-averaged value of the pressure. The new pressure correction equation facilitates (in a significant way) computer coding of programs written for three-dimensional geometries employing body-fitted curvilinear coordinate grids

A STRONG ENTHALPY FORMULATION FOR THE STEFAN PROBLEM

This paper demonstrates how the condition of constancy with respect to time of the phase-change interface temperature can be incorporated to arrive at a 'strong' enthalpy formulation. The finite-difference solutions obtained with this formulation show that the problem of 'waviness' of the temperature histories encountered with the 'weak' formulation is now removed and accurate solutions are obtained even with a coarse grid irrespective of the time step. The formulation derived requires no 'book-keeping' of the phase-change node, and allows line-by-line integration of the finite-difference equations

Numerical prediction of laminar flow and heat transfer in a tube with twisted-tape insert: Effects of property variations and buoyancy

In a recent experiment with laminar flow of Servotherm oil in a horizontal tube containing a twisted tape, the Nusselt number is correlated with Rayleigh number in addition to the forced convection parameters [1]. For Servotherm oil, density and viscosity vary appreciably with temperature. The measured Nusselt numbers were found to he higher than those predicted by the earlier isothermal correlation. The purpose of this: study is to investigate if this difference arises because of property variations or because of buoyancy. Numerical predictions can assist in this inquiery. Based on numerical predictions, appropriate correlations are derived and the results compared with experimental correlations

Fluid dynamical view of pressure checkerboarding problem and smoothing pressure correction on meshes with colocated variables

This paper presents derivation of the pressure correction equation appropriate for colocated grids within the framework of the SIMPLE algorithm. It is shown that checkerboard prediction of pressure can be prevented by employing algebraic smoothing pressure correction [Numer. Heat Transfer, Part B 29 (1996) 441] that is very simple to implement on both the structured as well as unstructured grids. The ability of the smoothing correction (which is shown to be independent of transformations of the system of coordinates) in providing the necessary dissipation is explained and the connection of the former with requirements of the Stokes's laws is established. (C) 200

Heat and mass transfer analysis of a clay-pot refrigerator

The simple clay-pot refrigerator is ideally suited for preserving vegetarian foods in hot and dry climates. The refrigerator works on the evaporative cooling principle. In this paper, steady-state performance of the refrigerator is analysed using Reynolds flow model of convective heat/mass transfer. For the assumed respiratory cooling load, the preservation temperature is predicted under a variety of ambient temperatures and relative humidities. (C) 2012 Elsevier Ltd. All rights reserved

A NOVEL ENTHALPY FORMULATION FOR MULTIDIMENSIONAL SOLIDIFICATION AND MELTING OF A PURE SUBSTANCE

This paper presents a new Finite-difference formulation of the multidimensional phase change problems involving unique phase change temperature. The solutions obtained with this formulation show that the problem of ''waviness'' of the temperature histories encountered with the conventional enthalpy formulation is now removed. The formulation derived provides a simple method for ''local'' tracking of the interface using the enthalpy variable in a novel way. During the solution of the finite-difference equations, the present formulation obviates the need for ''book-keeping'' of the phase-change nodes, and hence allows solution of the equations by tridiagonal matrix algorithm. It is argued that the benefits of enthalpy formulation can be extended to phase-change problems involving convection by solving the equations of motion on non-staggered grid