During the first few hours of freezing the cardiovascular system must distribute cryoprotectant throughout the body of freeze-tolerant frogs. This study presents initial documentation of the changes in heart rate of wood frogs (Rana sylvatica) during nonlethal freezing. Heart rate was determined by measuring the electrocardiogram of frogs. Within 1 min of the onset of freezing the heart rate nearly doubled to approximately 8.0 beats/min. The heart rate began to slow after the first hour of the freeze, and the heart completely stopped beating near the completion of ice formation approximately 20 h later. Recordings from a single frog revealed that the heart beat resumes within 1 h after thawing and near-normal function is achieved after only a few hours. The release of the latent heat of fusion caused a rise in body temperature (1.7 degrees C) for a few hours and was closely correlated with an increase in the heart rate. However, other factors such as reduction in blood volume, increase in blood viscosity, and progressive hypoxia may prominently influence cardiac function indirectly. Regardless, the heart functions long enough to distribute glucose throughout the body during the first few hours of the freeze.

Objectives: One important function of military audiology is to conduct evaluations of service members (SMs) with hearing loss to ensure they are fit for deployment in dangerous operational environments. The objective of this study was to establish evidence-based auditory fitness-for-duty criteria based on speech-in-noise performance on the 80- and 160-word clinical versions of the Modified Rhyme Test (MRT80 and MRT160). Design: Approximately 2400 SMs with various levels of hearing loss were recruited to complete the MRT80 in conjunction with their annual hearing conservation evaluations. These SMs were also asked to perform one or more operationally-relevant listening tasks based on audio recordings made in highly realistic military training environments. The scores on these tests were compared to determine how well a proposed cutoff criterion for the MRT80 was able to identify individuals who are hard of hearing with an exceptionally high risk of abnormally poor performance on operationally-relevant hearing tasks. Results: The results show that a cutoff criterion that combines the percent correct score on two lists of the MRT80 (i.e. MRT160) with information about the better-ear threshold at 2 kHz is generally able to separate listeners with hearing loss into those who are likely to perform relatively well on operational listening tests and those who are likely to perform poorly on these tasks. This is consistent with current military acquisition standards, which identify the MRT as the preferred test for evaluating speech intelligibility for radios, headsets, and other communication equipment. It is also consistent with prior studies conducted in high-fidelity military simulations which have shown a significant correlation between MRT performance and operational outcomes. Conclusions: The proposed selection criteria, along with the new hearing profile standards that were recently adopted by the US Army, appear to provide an effective evidence-based methodology for identifying those SMs with hearing loss who are most at risk for poor performance on hearing-critical military tasks.

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