Continuity theorems for the M/M/1/nM/M/1/n queueing system

In this paper continuity theorems are established for the number of losses during a busy period of the $M/M/1/n$ queue. We consider an $M/GI/1/n$ queueing system where the service time probability distribution, slightly different in a certain sense from the exponential distribution, is approximated by that exponential distribution. Continuity theorems are obtained in the form of one or two-sided stochastic inequalities. The paper shows how the bounds of these inequalities are changed if further assumptions, associated with specific properties of the service time distribution (precisely described in the paper), are made. Specifically, some parametric families of service time distributions are discussed, and the paper establishes uniform estimates (given for all possible values of the parameter) and local estimates (where the parameter is fixed and takes only the given value). The analysis of the paper is based on the level crossing approach and some characterization properties of the exponential distribution.Comment: Final revision; will be published as i

Prefrontal cortex activation and young driver behaviour: a fNIRS study

Road traffic accidents consistently show a significant over-representation for young, novice and particularly male drivers. This research examines the prefrontal cortex activation of young drivers and the changes in activation associated with manipulations of mental workload and inhibitory control. It also considers the explanation that a lack of prefrontal cortex maturation is a contributing factor to the higher accident risk in this young driver population. The prefrontal cortex is associated with a number of factors including mental workload and inhibitory control, both of which are also related to road traffic accidents. This experiment used functional near infrared spectroscopy to measure prefrontal cortex activity during five simulated driving tasks: one following task and four overtaking tasks at varying traffic densities which aimed to dissociate workload and inhibitory control. Age, experience and gender were controlled for throughout the experiment. The results showed that younger drivers had reduced prefrontal cortex activity compared to older drivers. When both mental workload and inhibitory control increased prefrontal cortex activity also increased, however when inhibitory control alone increased there were no changes in activity. Along with an increase in activity during overtaking manoeuvres, these results suggest that prefrontal cortex activation is more indicative of workload in the current task. There were no differences in the number of overtakes completed by younger and older drivers but males overtook significantly more than females. We conclude that prefrontal cortex activity is associated with the mental workload required for overtaking. We additionally suggest that the reduced activation in younger drivers may be related to a lack of prefrontal maturation which could contribute to the increased crash risk seen in this population

Numerical Investigation of Turbulent Combustion Noise Using a Hybrid LES/CAA Approach

The acoustic response of round turbulent flames is investigated by a hybrid large-eddy simulation / computational aeroacoustics (LES/CAA) with respect to the burner geometry and inflow turbulence distribution. The first study is conducted to extend the findings for a rectangular slot burner studied by Schlimpert et al. to round burners. The results show that the round flame is more stable than the rectangular slot flame due to the additional expansion direction and the initial curvature. Consequently, smaller flame front perturbations and less flame pockets are generated at the flame tip reducing the noise emission in this region. Compared to the round burner, the additional length scale of the rectangular slot burner leads to a medium frequency region in the heat release spectrum with a stronger roll-off behaviour and a lower peak frequency in the acoustic spectrum. In the second study, the hybrid approach is used to investigate the impact of the inflow turbulence on the flame’s acoustic emission. It is evident that various turbulence distributions lead to different turbulent kinetic energy spectra in the low frequency region of the cold jet. These differences also result in changes of the flame motion and significantly influence the acoustic spectra in the low frequency regime. Furthermore, a non-reactive LES computation including the burner’s plenum revealed significant differences in the turbulent kinetic energy spectrum compared to the LES results of the reduced domain

The efficacy of apneic oxygenation during intubation using a prototype of an oxygenation laryngoscope - a technical simulation

Abstract Background Recently, a non-commercial oxygenation laryngoscope was able to maintain apneic oxygenation during simulated intubation efforts. Since that prototype was 3 mm wider than a standard Macintosh laryngoscope blade, the intubation performance of this device may differ from standard blades. A new prototype of an oxygenation laryngoscope was developed, consisting of a standard-size Macintosh blade and a fixed oxygen supply line to the side. Actually, it is unclear at which point of this blade the oxygen supply line should end to facilitate the best possible oxygen supply for apneic oxygenation. Methods In this simulation study using a standardized human airway manikin, the efficacy of apneic oxygenation by oxygen insufflation using standard and modified Macintosh blades was compared: a standard Macintosh blade without oxygen supply line as control, one with an additional oxygen supply line ending proximal near the handle, one with the line ending at the middle of the blade, and one with the line ending near the tip. A preoxygenated test lung was connected to an oximeter with a flow rate of 200ml/min, simulating oxygen consumption of a male adult, and to the trachea of an anatomically correctly shaped airway manikin. Apneic oxygenation was performed and oxygen content was measured over a 20-minutes observation period. Experiments were repeated five times for each laryngoscope blade. Results Oxygen percentage in the test lung dropped from 100 ± 0% at the start of the experiment to 53 ± 1.5% in the room air control group (p < 0.001 compared to all other groups), and to 74 ± 2.5% in the proximal oxygen line group, whereas oxygen percentage remained at 100% in both the medium and distal oxygen line groups (p = 1 between these groups; p < 0.001 between all other groups). Conclusions In this simulation study with a preoxygenated airway manikin, the use of a modified Macintosh laryngoscope blade with oxygen line attached at the tip or at the middle were able to maintain apneic oxygenation without measurable drop of oxygen content over 20 min. Proximal placement of the oxygen supply line still showed an advantage against room air, however it did not completely prevent room air from entering the airway. Trial registration Not applicable

Identification of the Optimal Position of a Nasal Oxygen Cannula for Apneic Oxygenation: A Technical Simulation

Background: In a cannot-ventilate-cannot-intubate situation, careful preoxygenation with high FiO2 allowing subsequent apneic oxygenation can be life-saving. The best position for an oxygen supply line within the human airway at which oxygen insufflation is more effective than standard preoxygenation with a face mask is unknown. Methods: In this experimental study, we compared the effectiveness of preoxygenation by placing an oxygen cannula at the nose entrance, through the nose at the soft palatine, or at the base of the tongue; as a control we used ambient air. We connected a fully preoxygenated test lung on one side to an oximeter with a flow rate of 200 mL/min simulating the oxygen consumption of a normal adult on the other side of the trachea of an anatomically correctly shaped airway manikin over a 20 min observation period five times for each cannula placement in a random order. Results: The oxygen percentage in the test lung dropped from 100% in all groups to 53 &plusmn; 1% in the ambient air control group, to 87 &plusmn; 2% in the nasal cannula group, and to 96 &plusmn; 2% in the soft palatine group; it remained at 99 &plusmn; 1% in the base of the tongue group (p = 0.003 for the soft palatine vs. base of the tongue and p &lt; 0.001 for all other groups). Conclusions: When simulating apneic oxygenation in a preoxygenated manikin, oxygen insufflation at the base of the tongue kept the oxygen percentage at baseline values of 99%, demonstrating a complete block of ambient air flowing into the airway of the manikin. Oxygen insufflation at the soft palatine or insufflation via a nasal cannula were less effective regarding this effect

Impact of the injector lateral offset on the dynamics of a lean premixed flame and the thermoacoustic stability of a burner

The response of a laminar lean premixed flame to excitations based on its position in the combustion chamber and the stability of a burner are numerically investigated. A finite-volume large-eddy simulation method is used to solve the compressible Navier-Stokes equations and a combined G-equation progress variable approach is used to model the flame. Various injector positions in the combustion chamber are investigated in a computational setup with an acoustically non-reflecting outflow boundary condition to analyze the impact on the response of the flame to an external excitation. Due to the changes in the flow field the shape of the flames depends on the displacement offset of the injector. Consequently, the instantaneous, local distribution of the heat release rate fluctuations which is caused by wrinkles on the flame surface is determined by the position of the injector. The heat release rate fluctuations are the dominant source of sound in the investigated configurations. Due to the discrepancies in the local heat release rate fluctuations the flame response to an excitation depends on the lateral offset of the injector. The overall trend of the integrated heat release is the same in most configurations, however, the phase is significantly altered. Since the phase angle of the response of the flame to an excitation determines the stability of a burner, self-excited instabilities can be avoided by adjusting the position of the injector. This is demonstrated using a modified computational domain with an acoustically reflecting outflow boundary condition, which causes the burner to have an acoustic quarter-wave eigenmode. Based on the results of the previous analysis of the flame responses, two injector positions in the combustion chamber are chosen such that the response of the flames is mutually phase-shifted by approximately pi. Therefore, it is expected that one of the configurations will lead to a stable burner while the other one exhibits a self-excited instability. The results show that the injector position determines the stability of the burner configuration for the investigated flames

Therapeutic hypothermia in 2015. Influence of the TTM study on the intensive care procedure after cardiac arrest

In the 1960s, Peter Safar et al. postulated the benefit of postcardiac arrest hypothermia after successful cardiopulmonary resuscitation (CPR). However, therapeutic hypothermia postCPR did not become a standard procedure until the first few years of the new millennium. Various noninvasive and invasive cooling methods are available. Generally, more invasive cooling methods are more effective-but also tend to involve more complications. Furthermore, invasive measures need more time and thus may be instituted late in the postCPR process, delaying the cooling efforts in the initial phase. There is ongoing controversy about when best to commence cooling. Recent studies of initial out-of-hospital cooling did not show any benefit for the patients compared to starting cooling in the hospital. The exact target temperature is the subject of multiple ongoing discussions. A recent study showed no disadvantage of cooling to 36 aintegral compared to 33 aintegral, which is in the widely accepted standard target temperature range of 32-34 aintegral. Nevertheless, cooling to 32-34 aintegral according to the 2010 guidelines is still the accepted standard procedure unless and until new studies generate more evidence. The European Resuscitation Council has given advance notice of a statement on the optimal target temperature in the near future. Finally, large registry studies have demonstrated the benefit of combining postCPR hypothermia with early percutaneous cardiac interventions (PCI) in acute coronary syndromes, which are often a cause of cardiac arrest. Transport of patients after CPR to specialized postcardiac arrest centres with the possibility of acute PCI and cooling, comparable to the transfer of multiple trauma patients to trauma centres, may be beneficial

Efficiency of different flows for apneic oxygenation when using high flow nasal oxygen application - a technical simulation

Background Preoxygenation and application of apneic oxygenation are standard to prevent patients from desaturation e.g. during emergency intubation. The time before desaturation occurs can be prolonged by applying high flow oxygen into the airway. Aim of this study was to scientifically assess the flow that is necessary to avoid nitrogen entering the airway of a manikin model during application of pure oxygen via high flow nasal oxygen. Methods We measured oxygen content over a 20-min observation period for each method in a preoxygenated test lung applied to a human manikin, allowing either room air entering the airway in control group, or applying pure oxygen via high flow nasal oxygen at flows of 10, 20, 40, 60 and 80 L/min via nasal cannula in the other groups. Our formal hypothesis was that there would be no difference in oxygen fraction decrease between the groups. Results Oxygen content in the test lung dropped from 97 +/- 1% at baseline in all groups to 43 +/- 1% in the control group (p < 0.001 compared to all other groups), to 92 +/- 1% in the 10 L/min group, 92 +/- 1% in the 20 L/min group, 90 +/- 1% in the 40 L/min group, 89 +/- 0% in the 60 L/min group and 87 +/- 0% in the 80 L/min group. Apart from comparisons 10 l/ min vs. 20 L/min group (p = .715) and 10/L/min vs. 40 L/min group (p = .018), p was < 0.009 for all other comparisons. Conclusions Simulating apneic oxygenation in a preoxygenated manikin connected to a test lung over 20 min by applying high flow nasal oxygen resulted in the highest oxygen content at a flow of 10 L/min; higher flows resulted in slightly decreased oxygen percentages in the test lung