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ASSESSMENT OF SLEEP AND CIRCADIAN RHYTHM DURING HYPERCAPNIC 6 HEAD-DOWN TILT BED REST

Abstract

K. Christian1, C. Petitti2, K. Ortega-Schwartz1, S. Penrose1, A. Schallerer1, A. Lovering1, S. Laurie3

1University of Oregon, Eugene, OR; 2Sacred Heart Medical Center, Springfield, OR; 3KBRwyle, Houston, TX

PURPOSE: To determine whether the combination of a headward fluid shift induced by 6°head-down tilt bed rest (HDTBR) and chronic exposure to a mild hypercapnic environment of 0.5% CO2for 30-days will alter sleep architecture and circadian rhythm, contributing to spaceflight associated neuro-ocular syndrome (SANS). METHODS: After 14 days of ambulatory normocapnic familiarization with the facility and baseline data collection (BDC), eleven subjects (5 female) were exposed to mild, ambient hypercapnia (0.5% CO2) throughout 30 days of 6°HDTBR at the German Aerospace Center (DLR) :envihab facility. Overnight polysomnography (PSG) was measured using a type-2 sleep monitoring system, and the PSG was scored by a Polysomnographic Technologist to assess sleep duration, staging, latency, and arousals. Circadian rhythm variations were assessed by monitoring 24-hour core body temperature with rectal temperature probes. Arterial PCO2levels were estimated from transcutaneous PCO2monitoring throughout sleep. Following HDTBR, subjects were monitored for an additional 10 days of upright, normocapnic recovery. Measurements were obtained at 3 different time points (one baseline, two HDTBR: HDT4 and HDT17). RESULTS: Compared to baseline, in HDT4 there was a significant decrease in Stage 2 sleep (232.4 ± 52.8 min vs 196.8 ± 39.9 min, mean ± SD, p < .05)paralleled by an increase in Slow Wave Sleep (33.9 ± 29.8 min vs 51.0 ± 30.7 min, p < .05). Compared to baseline, there was a decreased circadian rhythm amplitude in HDT4 (0.40 ± 0.12 °C vs 0.32± 0.10 °C, p < .05). Compared to baseline there was a significant decrease in the change from Pre-Sleep Onset CO2to Peak Sleep CO2at HDT4 (4.2 ± 2.2 mmHg vs 2.6 ± 1.8 mmHg, p < .05), which can be explained by Peak Sleep CO2values, which decreased from baseline at HDT4 (49.3± 3.9 mmHg vs 47.5 ± 4.1 mmHg, p = .06).CONCLUSION: It appears that the head-down tilt condition, in combination with chronic, mild, ambient hypercapnia has minor, but significant effects on sleep architecture and circadian rhythm, which may play a role in symptomology of SANS.

Funding: NASA Human Research Program NNJ14ZSA001N

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