Capabilities and limitations of a new thermal finite volume model for the evaluation of laser-induced thermo-mechanical retinal damage.

Many experimental studies focus on the physical damage mechanisms of short-term exposure to laser radiation. In the nanosecond (ns) pulse range, damage in the Retinal Pigment Epithelium (RPE) will most likely occur at threshold levels due to bubble formation at the surface of the absorbing melanosome. The energy uptake of the melanosomes is one key aspect in modeling the bubble formation and damage thresholds. This work presents a thermal finite volume model for the investigation of rising temperatures and the temperature distribution of irradiated melanosomes. The model takes the different geometries and thermal properties of melanosomes into account, such as the heat capacity and thermal conductivity of the heterogeneous absorbing melanosomes and the surrounding tissue. This is the first time the size and shape variations on the melanosomes' thermal behavior are considered. The calculations illustrate the effect of the geometry on the maximum surface temperature of the irradiated melanosome and the impact on the bubble formation threshold. A comparison between the calculated bubble formation thresholds and the RPE cell damage thresholds within a pulse range of 3 to 5000 ns leads to a mean deviation of µ=22mJ/cm2 with a standard deviation of σ=21mJ/cm2. The best results are achieved between the simulation and RPE cell damage thresholds for pulse durations close to the thermal confinement time of individual melanosomes.

The Platinum 5 min in TCCC: Analysis of Junctional and Extremity Hemorrhage Scenarios with a Mathematical Model.

Introduction: To achieve the aim of zero preventable deaths on the battlefield a deeper understanding of uncontrolled hemorrhage from junctional or proximal extremity sources is mandatory. While tourniquet application to the extremities has drastically reduced morbidity and mortality, there is still room for improvement regarding the timing of tourniquet placement as the available evidence clearly points out a tight correlation between timing of tourniquet application and outcome. To save as many lives as possible the "point of no return" regarding the circulatory breakdown due to hemorrhage, colloquially addressed as platinum 5 min, needs to be determined. As clinical analysis or controlled studies are difficult, if not impossible, and animal experiments cannot be translated to bleeding in humans, we present a mathematical modeling approach. The key assumption of the model is that hemodynamics in the early phase of massive hemorrhage are determined by the cardiac function, the passive physical properties of the vascular system, that is, compliances etc., as humoral compensatory mechanisms kick in at a later point in time, and the baroceptor reflex, which constitutes the immediate response to volume loss. Materials and Methods: A lumped mathematical model based on differential equations describing three distinct arterial and two venous compartments, the heart and the baroceptor mechanism is developed. With this model, different patterns of blood loss (%) and duration of bleeding (s) are simulated: 10%/30 and 60 s, 20%/30 and 60 s, 30%/30, 60 and 120 s, and 35%/30, 120 and 180 s. These bleeding patterns are chosen such that they resemble clinically scenarios following junctional and proximal extremity injuries. Results: Three hemodynamic patterns can be distinguished. The system stabilizes on a lower blood pressure level (10%/30 and 60 s, 20%/30 and 60 s), the system formally stabilizes on a very low level, which is physiologically not reasonable (30%/30, 60 and 120 s), the system irreversibly breaks down with no signs of restabilization (35%/30, 120 and 180 s). Conclusion: Thus the immediacy of intervention in terms of application of a tourniquet is clearly emphasized by the simulation, that is, the window of opportunity for a life-saving intervention, especially in a combat setting, is significantly smaller than the symbolic "platinum five minutes" might suggest. With respect to the 3-min window of opportunity identified in the simulations the effective application of these devices in a TCCC setting appears questionable. Given these observations, further research and development into solutions that allow the timely identification of a junctional bleeding problem and application of compression is necessary.