Survival time prediction in marine environments

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Abstract

One of the most challenging questions regarding aquatic search and rescue or recovery operations is the physiological status of the victims. A Probability of Survival Decision Aid (PSDA) was developed to predict survival time for hypothermia and dehydration during prolonged exposure in marine environments for a wide range of environmental conditions. PSDA calculates the survival time of a victim in the water or floating in an emergency craft as a function of environmental conditions, clothing and human anthropometric parameters. PSDA consists of a human thermoregulatory model and a Graphic User Interface, which manages inputs, run the model and display outputs. PSDA was validated using historical survival data and reports of accidental water immersions. For eight immersion victims of known height and weight, the victims’ predicted survival time was either in close agreement or greater than the observed survival time. Additional physiological data from case histories and/or controlled studies are needed to validate the PSDA and to expand its application to other hazardous situations. PSDA is intended to assist the search and rescue personnel in operation optimal planning.

Highlights

► Prediction of victims’ physiological status is essential to search and rescue operation planning. ► Probability of Survival Decision Aid (PSDA) is a user-friendly support tool. ► PSDA predicts survival time for hypothermia and dehydration in marine environments. ► PSDA was validated using historical survival data and reports of accidental water immersions.

Introduction

Predictions of a survivor’s deteriorating physiological condition and survival time are essential to the Search And Rescue (SAR) planner during the search. The SAR Mission Coordinator uses these predictions to optimize search resources, to contribute to the decision whether to continue or suspend the active search, and to confirm evaluation along with other information. Survival times during accidental immersion are often presented in curves or tables as a function of water temperature, e.g. the classic Molnar survival curve (Molnar, 1946) and the water immersion survival table (http://www.ussartf.org/cold_water_survival.htm, accessed on June 20, 2010). A brief summary of the survival curves and tables is available in chapter 7, Survival Time in Cold Water, in the book Essentials of Sea Survival by Golden and Tipton (2002). As plots and tables are often oversimplified to fit the format, such representations are difficult to use. Using tables to estimate survival time often requires interpolation between discrete values to fit a given set of conditions. Curves provide a continuous analog representation of survival information, but require a graphic interpretation. Furthermore, in addition to the water temperatures, other factors such as clothing worn by victims and anthropometric parameters may significantly influence the survival time, but usually are not taken into account in these tables or curves.

Recently, mathematical models of human thermoregulation suitable for incorporation into software tools have been developed to predict human thermal responses during exposures to cold air or water (Hayward and Eckerson, 1975, Wissler, 1985, Tikuisis and Keefe, 1996, Tikuisis, 1997, Van Dorn, 2000, Xu et al., 2005, Wan and Fan, 2008). Models allow input of more specific information relative to a given incident and thus predict survival times that more accurately reflect the complexity of actual conditions, and the physiological state of victims. However, the mathematical models are often relatively complex and require a certain skill level to run. A user-friendly computer tool, the Cold Exposure Survival Model (CESM), was developed to estimate the survival time (Tikuisis and Keefe, 1996). As indicated by its name, CESM only applies to cold environments where survival is dominated by hypothermia. It does not apply to warmer environments where dehydration will limit survival.

The purpose of this research effort was to adapt a mathematical model of thermoregulation into a user-friendly computer tool, the Probability of Survival Decision Aid (PSDA). It will estimate survival time for both hypothermia and dehydration during prolonged exposure to a wide range of air and water conditions. The interaction of multiple factors determines the likelihood of survival by victims in the water or in an emergency craft. The present modeling effort focuses on the effects of hypothermia and dehydration on survival as determinants of the probability of survival. Other hazards, e.g., shock, swim failure, injury, burns and starvation, cannot be ascertained without in-situ observers or reports and may be unique to a specific event and are beyond the scope of the present modeling effort. In other words, this model assumes that victims survive all unobservable event-specific hazards such as collisions and fires and that survival depends solely on the degree of hypothermia and/or dehydration.

Section snippets

Probability of survival decision aid

PSDA consists of the Six-Cylinder Thermoregulatory Model (SCTM) and a Graphic User Interface (GUI; Fig. 1). The GUI is a critical element of the PSDA, which enables users to easily access the model and enter or select inputs for ten basic parameters: air temperature, water temperature, relative humidity, wind speed, gender, height, weight, percent body fat, immersion state and clothing type. When victim traits are unknown, and thus specific values cannot be entered, the pull-down menus allow

Validation

The SCTM, the basis for the PSDA, has been validated with physiological data collected in the lab in warm and cold air up to 2 h (Xu and Werner, 1997), in immersion in 10 and 15 °C water up to chest and waist levels for approximately 2 h (Castellani et al., 2007), and in immersion in 8 °C water for up to 6 h (Tikuisis et al., 2002, Xu et al., 2005). Comparisons between predicted and measured core temperature (Tc) indicated that the prediction was acceptable and usually fell within the range of

Discussion

The Probability of Survival Decision Aid (PSDA) developed during this study is user-friendly and requires only 10 inputs to estimate survival times. Our validation, based on limited accident case information, showed that the prediction appears to be reasonable. Heights and body weights were available for 8 victims of accidental water immersion cases that did not involve swimming. The predicted survival time for each victim was either in close agreement to, or greater than, the observed survival

Conclusion

A Probability of Survival Decision Aid (PSDA) was developed to predict survival time for hypothermia and dehydration during prolonged exposure in marine environments for a wide range of environmental conditions. PSDA uses only ten inputs of environmental conditions, human physical parameters, immersion state, and clothing to simulate the survival times of victims on or in the water. PSDA was validated using historical survival data, and reports of accidental water immersions. For 8 immersion

Acknowledgments

Approved for public release; distribution is unlimited. The opinions or assertions contained herein are the private views of the author(s) and are not to be construed as official or reflecting the views of Department of the Army or the US Coast Guard, or Department of Defense. Citations of commercial organizations and trade names in this report do not constitute an official Department of the Army or the US Coast Guard, or Department of Defense endorsement or approval of the products or services

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