Mathematical model of the risk of drug error during anaesthesia: the influence of drug choices, injection routes, operation duration and fatigue.

The incidence of an anaesthetic drug error can be directly observed in large trials. In an alternative approach, we developed a probabilistic mathematical model in which the anaesthetist is modelled as a 'fallible entity' who makes repeated drug administration choices during an operation. This fallibility was factored in the model as an initial 'intrinsic error rate'. The choices faced included: dose; timing of administration; and the routes available for injection (e.g. venous, arterial, epidural, etc.). Additionally, we modelled the effect of fatigue as a factor that magnifies the cumulative error rate. For an initial intrinsic error rate of 1 in 1000 (which from first principles we consider a reasonable estimate), our model predicted a cumulative probability of error over a ~12 h operation of ~10%; that is, 1 in 10 operations this long results in some drug error. This is similar to the rate reported by large observational trials. Serious errors constitute a small fraction of all errors; our model predicts a Poisson distribution for the uncommon serious errors, also consistent with independent observations. Even modest assumptions for the development of fatigue had a dramatic and adverse impact on the cumulative error rate. The practice implications of our modelling include: exercising caution or avoiding starting work if under par; added vigilance in unfamiliar environments; keeping anaesthetic recipes simple; and recognising that operation durations > 5-6 h constitute a time of exaggerated risk. These implications are testable predictions in observational trials. If validated, our model could serve as a potential research tool to investigate the impact of safety interventions on the rate of intrinsic error using simulation.

Surface ordering in dilute dihexadecyl dimethyl ammonium bromide solutions at the air-water interface.

At elevated temperatures and in dilute solution, we have observed lamellar surface ordering at the air-water interface of dihexadecyl dimethylammonium bromide, DHDAB, in the presence of electrolyte. With increasing temperature, the onset in ordering is observed between 35 and 40 degrees C. At 40 degrees C, there is an abrupt change in the lamellar spacing, from approximately 33 to approximately 40 A. Furthermore, in the presence of the cosurfactant benzyl alcohol, the ordering occurs at a lower temperature, between 20 and 25 degrees C. The change in lamellar spacing with temperature is attributed to a surface-induced transition, similar to the Lbeta to Lalpha phase transition observed in bulk lamellar dispersions.

Tracking the evolution of interatomic potentials with high resolution inelastic neutron spectroscopy.

High resolution inelastic neutron scattering has been used to characterize the low temperature rotational dynamics of methane ( CH4) films on MgO(100) surfaces as a function of layer thickness. At low temperatures the films grow layer-by-layer similar to the (100) planes of bulk solid CH4. We observe a crossover in the CH4 tunneling spectrum from one characteristic of the 2D solid at monolayer coverages into one that is bulklike (i.e., 3D) when the film is six layers thick. These findings place significant constraints on the microscopic models used to describe CH4-CH4 and CH4-MgO interactions.