Regimes of chemical reaction waves initiated by nonuniform initial conditions for detailed chemical reaction models.

Regimes of chemical reaction wave propagation initiated by initial temperature nonuniformity in gaseous mixtures, whose chemistry is governed by chain-branching kinetics, are studied using a multispecies transport model and a detailed chemical model. Possible regimes of reaction wave propagation are identified for stoichiometric hydrogen-oxygen and hydrogen-air mixtures in a wide range of initial pressures and temperature levels, depending on the initial non-uniformity steepness. The limits of the regimes of reaction wave propagation depend upon the values of the spontaneous wave speed and the characteristic velocities of the problem. It is shown that one-step kinetics cannot reproduce either quantitative neither qualitative features of the ignition process in real gaseous mixtures because the difference between the induction time and the time when the exothermic reaction begins significantly affects the ignition, evolution, and coupling of the spontaneous reaction wave and the pressure wave, especially at lower temperatures. We show that all the regimes initiated by the temperature gradient occur for much shallower temperature gradients than predicted by a one-step model. The difference is very large for lower initial pressures and for slowly reacting mixtures. In this way the paper provides an answer to questions, important in practice, about the ignition energy, its distribution, and the scale of the initial nonuniformity required for ignition in one or another regime of combustion wave propagation.

The influence of the Rashba spin-orbit coupling on the two-dimensional magnetoexcitons.

The influence of the Rashba spin-orbit coupling (RSOC) on the two-dimensional (2D) electrons and holes in a strong perpendicular magnetic field leads to different results for the Landau quantization in different spin projections. In the Landau gauge the unidimensional wave vector describing the free motion in one in-plane direction is the same for both spin projections, whereas the numbers of Landau quantization levels are different. For an electron in an s-type conduction band they differ by one, as was established earlier by Rashba (1960 Fiz. Tverd. Tela 2 1224), whereas for heavy holes in a p-type valence band influenced by the 2D symmetry of the layer they differ by three. The shifts and the rearrangements of the 2D hole Landau quantization levels on the energy scale are much larger in comparison with the case of conduction electron Landau levels. This is due to the strong influence of the magnetic field on the RSOC parameter. At sufficiently large values of this parameter the shifts and rearrangements are comparable with the hole cyclotron energy. There are two lowest spin-split Landau levels for electrons as well as four lowest ones for holes in the case of small RSOC parameters. They give rise to eight lowest energy bands of the 2D magnetoexcitons, as well as of the band-to-band quantum transitions. It is shown that three of them are dipole-active, three are quadrupole-active and two are forbidden. The optical orientation under the influence of circularly polarized light leads to optical alignment of the magnetoexcitons with different orbital momentum projections in the direction of the external magnetic field.

Hydrogen-oxygen flame acceleration and transition to detonation in channels with no-slip walls for a detailed chemical reaction model.

The features of flame acceleration in channels with wall friction and the deflagration to detonation transition (DDT) are investigated theoretically and using high resolution numerical simulations of two-dimensional reactive Navier-Stokes equations, including the effects of viscosity, thermal conduction, molecular diffusion, and a detailed chemical reaction mechanism for hydrogen-oxygen gaseous mixture. It is shown that in a wide channel, from the beginning, the flame velocity increases exponentially for a short time and then flame acceleration decreases, ending up with the abrupt increase of the combustion wave velocity and the actual transition to detonation. In a thin channel with a width smaller than the critical value, the exponential increase of the flame velocity is not bounded and ends up with the transition to detonation. The transition to detonation occurs due to the pressure pulse, which is formed at the tip of the accelerating flame. The amplitude of the pressure pulse grows exponentially due to a positive feedback coupling between the pressure pulse and the heat released in the reaction. Finally, large amplitude pressure pulse steepens into a strong shock coupled with the reaction zone forming the overdriven detonation. The evolution from a temperature gradient to a detonation via the Zeldovich gradient mechanism and its applicability to the deflagration-to-detonation transition is investigated for combustible materials whose chemistry is governed by chain-branching kinetics. The results of the high resolution simulations are fully consistent with experimental observations of the flame acceleration and DDT.

Intra-Landau-level excitations of the two-dimensional electron-hole liquid.

The intra-Landau-level excitations of the two-dimensional electron-hole liquid are characterized by two branches of the energy spectrum. The acoustical plasmon branch with in-phase oscillations of electrons and holes has a linear dispersion law in the range of small wavevectors, with a velocity which does not depend on the magnetic field strength, and monotonically increases with saturation at higher values of the wavevectors. The optical plasmon branch with oscillations of electrons and holes in opposite phases has a quadratic dependence in the range of long wavelength, a weak roton-type behaviour at the intermediary values of the wavevectors and monotonically increases with saturation similar to the case of the acoustical branch. The influence of the supplementary in-plane electric field leads to the drift of the charged particles in the crossed electric and magnetic fields and to the energy spectrum as in the reference frame, where the e-h system is moving with the drift velocity. A perturbation theory using the Green function method is developed on the basis of a small parameter v(2)(1-v(2)), where v(2) is the filling factor and (1-v(2)) displays the phase space filling effect.

Colonoscopy: why are general surgeons being excluded?

BACKGROUND: The role of surgeons as endoscopists has been extensively debated in the literature, with conflicting studies published regarding the safety and efficacy of surgeons performing colonoscopies. A multitude of medical federations and societies have set various standards for granting endoscopy privileges, many with a bias against general surgeons [1, 3]. We reviewed the colonoscopy experience at our institution to evaluate differences between gastroenterologists (GI) and general (GS) and colorectal surgeons (CRS) in procedure times and complication and cecal intubation rates. METHODS: Between January 2000 and July 2002, 5237 colonoscopies were performed at our institution. The data for procedure times, completion, and complication rates were collected in a prospective database. Complications were defined as perforation, bleeding, and postpolypectomy syndrome. Incomplete colonoscopies due to colitis, poor bowel preparation, or tumor obstruction were excluded. Chi-squared test was used to compare complication and cecal intubation rates between the three groups. Median procedure times were compared using the Kruskall-Wallis and Dunn's pairwise tests. A significant p-value was defined as <0.05. RESULTS: No differences in the complication rate was noted between the three groups: GI (0.12%), CRS (0.15%), and GS (0.11%) ( p = 0.99). There was a trend toward a lower incomplete colonoscopy rate in the GS group compared to CRS and GI: 0.32% vs 0.84% and 0.36%, respectively ( p = 0.07). The median colonoscopy times for GS (29 min), however, were shorter than for GI (34 min, p < 0.001) or CRS (31 min, p < 0.001). CONCLUSION: General surgeons perform colonoscopies expeditiously, with as low a morbidity rate and as high a completion rate as their gastroenterology or colorectal surgery colleagues. As the results of this study confirm, general surgeons should not be excluded from endoscopy suites.

Ondansetron versus placebo for prophylaxis of nausea and vomiting in patients undergoing ambulatory laparoscopic cholecystectomy.

BACKGROUND: Postoperative nausea and vomiting is a common problem in patients undergoing laparoscopic cholecystectomy (LC). The purpose of this study was to evaluate the efficacy of ondansetron given at the induction of anesthesia in patients scheduled for ambulatory LC. METHODS: A total of 84 patients undergoing ambulatory LC were enrolled in a randomized, prospective, double-blinded study in which the subjects received either placebo or 4 mg ondansetron intravenously at induction of anesthesia. A nausea scoring system was employed utilizing a 5-point linear scale, with 1 point given for no nausea and a maximum of 5 points for an episode of emesis. Each patient received a total of four scores postoperatively. RESULTS: The patients receiving placebo had significantly more episodes of nausea (53 versus 32; P <0.009) and emesis (11 versus 2; P <0.02), higher mean total nausea scores (7.2 versus 5.4; P <0.006), and need for additional postoperative antiemetics (23 versus 14; P <0.05) than those receiving ondansetron. CONCLUSIONS: In patients undergoing ambulatory laparoscopic cholecystectomy, ondansetron at induction was highly effective in decreasing postoperative nausea and vomiting and should become the standard.

Nonlinear equation for curved nonstationary flames and flame stability.

A time-dependent nonlinear equation for a nonstationary curved flame front of an arbitrary expansion coefficient is derived under the assumptions of a small but finite flame thickness and weak nonlinearity. On the basis of the derived equation, stability of two-dimensional curved stationary flames propagating in tubes with ideally adiabatic and slip walls is studied. The stability analysis shows that curved stationary flames become unstable for sufficiently wide tubes. The obtained stability limits are in a good agreement with the results of numerical simulations of flame dynamics and with semiqualitative stability analysis of curved stationary flames. Possible outcomes of the obtained instability at the nonlinear stage are discussed. The instability may result in extra wrinkles at a flame front close to the stability limits and in self-turbulization of the flame far from the limits. The self-turbulization can also be interpreted as a fractal structure. The fractal dimension of a flame front and velocity of a self-turbulized flame are evaluated.

Residency training in advanced laparoscopic surgery: how are we doing?

Despite the phenomenal growth of laparoscopic surgery since 1987, surgical residency training programs are failing to train residents in advanced laparoscopic procedures. Data were obtained from the operative registry maintained by the Department of General Surgery, Naval Medical Center San Diego and included all operations performed during the course of a 5-year general surgery residency by residents graduating in 1992 (group 1) and those graduating in 1996 (group 2). Each of the group 1 residents averaged 18 laparoscopic cholecystectomies and 68 open cholecystectomies, whereas the group 2 residents averaged 95 laparoscopic cholecystectomies and 18 open cholecystectomies. Group 1 residents averaged <1 advanced laparoscopic procedure during their residency, whereas group 2 residents averaged 46. Although there was no significant difference in total open cases between groups 1 and 2 (1,233 vs 1,197 respectively; p > 0.99), group 2 performed a significantly greater proportion of laparoscopic cases (12% vs 1%; p < 0.001). The teaching of advanced laparoscopic procedures can and should be incorporated into the surgical residency.

Incidental laparoscopic appendectomy for acute right lower quadrant abdominal pain. Its time has come.

BACKGROUND: Removing the normal appendix when operating for suspected acute appendicitis is the standard of care. The use of laparoscopy should not alter this practice. METHODS: Retrospective review of 72 patients found to have grossly normal appendices while undergoing laparoscopy for suspected appendicitis. Twenty-eight patients underwent diagnostic laparoscopy (DL) alone while 44 patients underwent diagnostic laparoscopy with incidental laparoscopic appendectomy (ILA). RESULTS: There was no difference in length of hospitalization (DL = 44 h, ILA = 43 h, p = 0.49) or morbidity (DL = 11%, ILA = 5%, p = 0.37). One patient required appendectomy 11 days after diagnostic laparoscopy for recurrent acute right lower quadrant abdominal pain. Five percent of resected appendices (2/44) demonstrated acute inflammation upon pathologic review. CONCLUSIONS: Laparoscopic removal of the normal appendix produces no added morbidity or increase in length of hospitalization as compared to diagnostic laparoscopy. It demonstrates cost effectiveness by preventing missed and future appendicitis. Incidental laparoscopic appendectomy is the preferred treatment option.

Routine cholangiography reduces sequelae of common bile duct injuries.

BACKGROUND: An effort was made to determine whether a policy of routine cholangiography affects the incidence, morbidity, and cost of bile duct injuries. METHODS: A retrospective review of consecutive 3,242 laparoscopic cholecystectomies was performed. Most patients had routine intraoperative cholangiography. RESULTS: There were 12 bile duct injuries (0.37%). All injuries were Bismuth levels 1 and 2. Eleven of 12 injuries were recognized intraoperatively. Ten were repaired primarily and one required hepaticojejunostomy. All repairs were successful. Average hospital charges were $26,669. One of 12 patients had delayed recognition of a bile duct injury and underwent primary repair over a T-tube on postoperative day 7. Hospital charges were $43,957. CONCLUSION: Routine cholangiography did not appear to decrease the absolute incidence of bile duct injuries compared to previously published reports. Injury severity, morbidity, late sequelae, and costs were reduced by a policy of routine cholangiography.

Cost-effective management of complicated choledocholithiasis: laparoscopic transcystic duct exploration or endoscopic sphincterotomy.

BACKGROUND: In the United States of America, approximately 700,000 patients undergo laparoscopic cholecystectomy (LC) each year and at least 10 percent of these patients will have common bile duct stones (CBDS). The purpose of this study was to evaluate patients with choledocholithiasis and compare the economic and clinical results obtained by LC with endoscopic sphincterotomy (ES) with those of LC with laparoscopic transcystic common bile duct exploration (LTCBDE). STUDY DESIGN: From June 1991 to September 1994 patients undergoing LC plus LTCBDE and those undergoing LC plus ES at a single institution were compared where cost data were available. Of the 76 patients with choledocholithiasis, 59 patients underwent LC plus LTCBDE (group 1) and 17 patients underwent LC plus ES (group 2). A subset of group 1 patients undergoing urgent LC plus LTCBDE (group 3) for cholecystitis, cholangitis, or pancreatitis plus laparoscopy were examined separately. RESULTS: Laparoscopic cholecystectomy plus LTCBDE, whether including all-comers (group 1) or just urgent cases (group 3), was associated with a significantly decreased length of hospital stay (6.1 and 6.9 days, respectively, compared with group 2, 12.4 days) (p < 0.001). The morbidity of patients in group 1 was also markedly lower than for patients in group 2; 12 percent compared with 41 percent, respectively. Patients in group 1 had a significantly decreased cost of hospitalization (+13,151), when compared with patients in group 2 (+18,712) (p = 0.05). This difference is even more pronounced when professional fee reimbursement is considered. The cost of treatment for patients in group 1 was +14,732 compared with +21,125 for patients in group 2 (p < 0.05). The total hospital cost for patients in group 3 was only +13,564 compared with +18,712 for patients in group 2. When professional reimbursement was considered, the cost was +15,150 for patients in group 3 compared with +21,125 for patients in group 2. CONCLUSIONS: Patients undergoing LC plus LTCBDE for CBDS, whether urgently or electively, have markedly decreased morbidity rates, length of hospital stay, and costs when compared with patients undergoing LC plus ES.

Laparoscopic colectomy vs traditional colectomy for diverticulitis. Outcome and costs.

BACKGROUND: The aim of this study was to evaluate the outcome of patients undergoing laparoscopic colectomy for diverticulitis. METHODS: Fourteen consecutive patients undergoing laparoscopic sigmoid colectomy (LSC) for diverticulitis were evaluated. Medical records from a control group of 14 matched patients undergoing traditional open sigmoid colectomy (OSC) for diverticulitis were reviewed for comparison. RESULTS: Mean age, operative time, morbidity, and mortality of the LSC and OSC groups were not significantly different. However, the mean estimated blood loss (171cc vs 321cc), days to p.o. liquids (2.9 vs 6.1), and postoperative stay (6.3 vs 9.2 days) were all significantly less in the LSC patients. Although the mean operating room charges were greater in the LSC patients ($10,589 vs $8,207) the mean total hospital charges ($29,981 vs $36,745) and costs ($11,528 vs $13,426) were markedly less. CONCLUSIONS: Compared with OSC for diverticulitis, LSC results in a more rapid return of bowel function and shortened hospital stay. Despite the greater operating room charges of LSC, the total hospital charges and costs are lessened.

Management of choledocholithiasis during pregnancy: a new protocol in the laparoscopic era.

The management of symptomatic cholelithiasis during pregnancy remains a serious and difficult problem. This condition may be further complicated by the presence of common bile duct stones (CBDS). Two cases of CBDS in pregnancy are presented. During the period from August 1990 to June 1994, 1127 consecutive patients underwent LC by a single surgical team. Two (0.2%) of these patients were pregnant and both were found to have choledocholithiasis. One patient's stones were lavaged into the duodenum via the cystic duct and the other patient had transcystic choledochoscopy and transcystic duct tube drainage. The patients were discharged from the hospital on postoperative days 4 and 6, respectively. There were no complications and both successfully carried their pregnancies to term. Laparoscopic cholecystectomy (LC) with transcystic common bile duct exploration (TCDE) was advantageous in both patients. Neither patient required an abdominal incision or choledochotomy. Laparoscopic TCDE with or without cystic tube drainage is a viable option in the management of CBDS in the pregnant patient.

Single-dose cefotetan or cefoxitin versus multiple-dose cefoxitin as prophylaxis in patients undergoing appendectomy for acute nonperforated appendicitis.

OBJECTIVE: A prospective, double-blind study was performed to examine the effects of prophylactic cefotetan and cefoxitin in postoperative wound infection for patients with nonperforated acute appendicitis. METHODS: One hundred thirty-six of 179 patients with a clinical diagnosis of appendicitis were evaluated and divided into three groups: group 1 received 2 g cefotetan preoperatively, group 2 received 2 g cefoxitin preoperatively, and group 3 received 2 g cefoxitin preoperatively followed by three postoperative doses. RESULTS: The overall wound infection rate was 4.6 percent. Group 2 (single-dose cefoxitin) had a significantly higher wound infection rate (11.1 percent) than group 1 (single-dose cefotetan) (zero percent) or group 3 (multiple-dose cefoxitin) (1.9 percent). CONCLUSIONS: Single-dose cefotetan and multiple-dose cefoxitin are equally effective. However, because of the greater convenience and markedly decreased cost, single-dose cefotetan is the prophylaxis of choice in appendectomy for nonperforated appendicitis.