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AbstractMany animals follow annual cycles wherein physiology and behavior change seasonally. Hibernating mammals undergo one of the most drastic seasonal alterations of physiology and behavior, the timing of which can have significant fitness consequences. The environmental cues regulating these profound phenotypic changes will heavily influence whether hibernators acclimate and ultimately adapt to climate change. Hence, identifying the cues and proximate mechanisms responsible for hibernation termination timing is critical. Northern Idaho ground squirrels (Urocitellus brunneus)-a rare, endemic species threatened with extinction-exhibit substantial variation in hibernation termination phenology, but it is unclear what causes this variation. We attached geolocators to free-ranging squirrels to test the hypothesis that squirrels assess surface conditions in spring before deciding whether to terminate seasonal heterothermy or reenter torpor. Northern Idaho ground squirrels frequently reentered torpor following a brief initial emergence from hibernacula and were more likely to do so earlier in spring or when challenged by residual snowpack. Female squirrels reentered torpor when confronted with relatively shallow snowpack upon emergence, whereas male squirrels reentered torpor in response to deeper spring snowpack. This novel behavior was previously assumed to be physiologically constrained in male ground squirrels by testosterone production required for spermatogenesis and activated by the circannual clock. Assessing surface conditions to decide when to terminate hibernation may help buffer these threatened squirrels against climate change. Documenting the extent to which other hibernators can facultatively alter emergence timing by reentering torpor after emergence will help identify which species are most likely to persist under climate change.

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Cardiovascular disease is the leading cause of morbidity and mortality in maintenance hemodialysis (MHD) patients. The Transonic (TRS; Transonic Systems, Ithaca, NY) device is frequently used for determination of cardiac output (CO) by an indicator dilution technique. The Task Force Monitor (TFM; CN Systems, Graz, Austria) has gained attention as noninvasive tool for continuous beat-to-beat assessment of cardiovascular variables, including CO by impedance cardiography. Despite its use in cardiology and intensive care settings, the TFM has yet not been validated in dialysis patients. This study compares CO measurements in 12 MHD patients by TFM and TRS. Bland-Altman and regression analysis were used. CO was measured simultaneously by TRS and TFM. Average CO was 5.4 L/min by TRS and 5.0 L/min by TFM, respectively. Bland-Altman analysis revealed no significant systematic differences between the two methods (mean difference: 0.4 L/min; SD: 0.6; p > 0.05). Linear regression analysis showed significant correlation between both techniques (r = 0.802, p = 0.002). The SD of mean individual CO values was 1.1 L/min with TRS and 0.8 L/min with TFM, respectively.CO measured by TFM and TRS does not differ significantly, thus making the TFM an attractive noninvasive tool for the continuous beat-to-beat assessment of CO in MHD patients.

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The aim of the study was to investigate whether body mass index (BMI) influences the estimation of extracellular volume (ECV) in hemodialysis (HD) patients when using segmental bioimpedance analysis (SBIA) compared to wrist-to-ankle bioimpedance analysis (WBIA) during HD with ultrafiltration (UF). Twenty five HD patients (M:F 19:6,) were studied, and further subdivided into two groups of patients, one group with a high BMI (25 kg m-2) and the other with a low BMI (<25 kg m-2). Segmental (arm, trunk, leg) and wrist-to-ankle bioimpedance measurements on each patient were performed using a modified Xitron 4000B system (Xitron Technologies, San Diego, CA). No differences in extracellular resistance (R(E), ohms) between wrist-to-ankle (R(W)) and sum of segments (R(S)) were noted for either the high BMI (489.2+/-82 ohm versus 491.6+/-82 ohm, p=ns) or low BMI groups (560.8+/-77 ohm versus 557.5+/-75 ohm, p=ns). UF volume (UFV, liters) did not differ significantly between the groups (4.0+/-0.9 L versus 3.3+/-1.0 L, p=ns), but change in ECV (DeltaECV) differed not only between methods: WBIA versus SBIA in the high BMI group (2.74+/-1.1 L versus 3.64+/-1.4 L, p<0.001) and in the low BMI group (1.86+/-0.9 L versus 2.91+/-1.0 L, p<0.05) but also between the high and lower BMI groups with WBIA (2.74+/-1.1 L versus 1.86+/-0.9 L, p<0.01). However, there was no significant difference in SBIA between BMI groups. This study suggests that the segmental bioimpedance approach may more accurately reflect changes in ECV during HD with UF than whole body impedance measurements.

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