Continuous recordings of core body temperature (CBT) are a well-established approach in describing circadian rhythms. Given the discomfort of invasive CBT measurement techniques, the use of skin temperature recordings has been proposed as a surrogate. More recently, we proposed a heat-flux approach (the so-called Double Sensor) for monitoring CBT. Studies investigating the reliability of the heat-flux approach over a 24-hour period, as well as comparisons with skin temperature recordings, are however lacking. The first aim of the study was therefore to compare rectal, skin, and heat-flux temperature recordings for monitoring circadian rhythm. In addition, to assess the optimal placement of sensor probes, we also investigated the effect of different anatomical measurement sites, i.e. sensor probes positioned at the forehead vs. the sternum. Data were collected as part of the Berlin BedRest study (BBR2-2) under controlled, standardized, and thermoneutral conditions. 24-hours temperature data of seven healthy males were collected after 50 days of -6° head-down tilt bed-rest. Mean Pearson correlation coefficients indicated a high association between rectal and forehead temperature recordings (r > 0.80 for skin and Double Sensor). In contrast, only a poor to moderate relationship was observed for sensors positioned at the sternum (r = -0.02 and r = 0.52 for skin and Double Sensor, respectively). Cross-correlation analyses further confirmed the feasibility of the forehead as a preferred monitoring site. The phase difference between forehead Double Sensor and rectal recordings was not statistically different from zero (p = 0.313), and was significantly smaller than the phase difference between forehead skin and rectal temperatures (p = 0.016). These findings were substantiated by cosinor analyses, revealing significant differences for mesor, amplitude, and acrophase between rectal and forehead skin temperature recordings, but not between forehead Double Sensor and rectal temperature measurements. Finally, Bland-Altman analysis indicated narrower limits of agreement for rhythm parameters between rectal and Double Sensor measurements compared to between rectal and skin recordings, irrespective of the measurement site (i.e. forehead, sternum). Based on these data we conclude that (1) Double Sensor recordings are significantly superior to skin temperature measurements for non-invasively assessing the circadian rhythm of rectal temperature, and (2) temperature rhythms from the sternum are less reliable than from the forehead. We suggest that forehead Double Sensor recordings may provide a surrogate for rectal temperature in circadian rhythm research, where constant routine protocols are applied. Future studies will be needed to assess the sensor’s ecological validity outside the laboratory under changing environmental and physiological conditions.

Optimal neuroendocrine responses are essential during hypergravity (+Gz) exposure. Peripheral skin cooling (PSC) may enhance neuroendocrine function, potentially improving +Gz resiliency and influencing leukocyte and hematologic factors. This study investigated whether PSC augments the cumulative +Gz stress index (CGSI) and shifts it toward noradrenergic dependency. Eighteen men underwent a graded +Gz profile in a crossover design, with PSC applied using Arctic Sun cooling pads. Neuroendocrine and blood profiles were assessed pre- and post-+Gz. CGSI did not differ between groups, but serum osmolality increased only in PSC (p = 0.03). In PSC, CGSI correlated with norepinephrine (p < 0.01, r = 0.71) and other markers, suggesting enhanced norepinephrine responsiveness despite similar serum levels. This response may be cardio-protective for space missions and ICU patients. Additionally, baseline serum metanephrine emerged as a potential marker for +Gz resilience, with PSC showing potential leukocytic and hematologic involvement in CGSI.

Introduction: Optimal neuroendocrine responses, particularly noradrenergic responses, are critical during hypergravity (+ Gz) exposure. Peripheral skin cooling (PSC) has the potential to enhance noradrenergic function, which could improve + Gz resiliency and may also affect leukocyte and hematologic factors. We predicted that PSC would augment cumulative + Gz stress index (CGSI) neuroendocrine, leukocyte and hematologic activity compared to controls, and that CGSI would shift to a noradrenergic dependency in PSC compared to controls. Methods 18 men in a cross-over design underwent a graded + Gz profile. PSC was applied using Arctic Sun cooling pads around the thighs with 8°C water infusion. Neuroendocrine parameters and a complete blood profile with differential were obtained before and after + Gz. CGSI was calculated as + Gz * seconds. Pre / post comparisons were made within the groups with paired t-tests and between groups with independent t-tests. Pearson's correlation analysis was performed between CGSI and neuroendocrine/leukocyte/hematological parameters. Results CSGI did not differ between groups. In both groups, all neuroendocrine, leukocyte and hematological factors changed significantly from pre to post + Gz, except for serum osmolality, which increased only in PSC (p = 0.03). In PSC, CGSI correlated with noradrenaline (p < 0.01, r = 0.71), metanephrine (p = 0.02, r = -0.54), neutrophiles (p = 0.03, r = 0.51), and thrombocytes (p = 0.04, r = 0.48), while in controls, CGSI correlated with metanephrine (p = 0.02, r = -0.54). Pre + Gz metanephrine correlated with CGSI in both groups. Discussion PSC did not confer a higher CGSI nor induce a greater neuroendocrine, leukocytic or hematologic response compared to controls. However, the increase in serum osmolality may indicate increased vasopressin activity. The strong correlations between CGSI and norepinephrine in PSC suggest that despite equal average serum levels, the responsiveness of norepinephrine is enhanced. This may serve as an optimal cardio-protective countermeasure for manned space missions and possible ICU patients. The correlations between neutrophiles and thrombocytes in PSC suggest a leukocytic and hematologic involvement in CSGI. In addition, this is the first study to demonstrate that baseline serum metanephrine may serve as a potential neuroendocrine parameter to assess + Gz resilience.