J. Kysar1, K. Christian1, T. Olson1, S. Laurie2, A. Lovering1

1University of Oregon, Eugene, OR; 2KBRwyle, NASA Johnson Space Center, NASA, Houston, TX

PURPOSE: To investigate whether prolonged hypercapnic bed rest increases ventilatory chemosensitivity to carbon dioxide (CO2). METHODS: Eleven (5 female) healthy subjects were studied prior to, during, and after 30 days of 6° head-down tilt bed rest combined with 0.5% ambient CO2. Ventilation (VE) and end-tidal CO2(PETCO2) were measured on a breath-by-breath basis (BreezeSuite, MGC Diagnostics) to determine ventilatory chemosensitivity as quantified by the ventilatory recruitment threshold (VRT) and sensitivity (VES). Subjects were studied in the 6° head-down tilt position at all time points. After 10 min of quiet breathing, subjects were coached to hyperventilate for 1 min with the target of maintaining PETCO2~20-25 mmHg. Then, a two-way valve was switched to a 6L rebreathing bag containing 100% O2where subjects were instructed to resume normal breathing until PETCO2reached 60 mmHg, subjects depleted the bag volume, or signaled to return to breathing room air. Two, 2-way mixed analyses of variance were used to analyze the effect of time point and sex on VRT and VES. Pairwise comparisons were conducted using a Bonferroni correction factor, and significance was set at p < .05. Descriptive statistics are reported as means ± standard deviations. RESULTS: There was a significant effect of time point on VRT such that even after 13 days of recovery from hypercapnic bed rest, mean onset of the VRT occurred at a lower PETCO2(M= 45.8 ± 3.7 mmHg) compared to the control time point (M= 50.2 ± 5.3 mmHg) F(6, 54) = 11.81, p < .001. Additionally, there was a significant effect of sex on VRT such that females (M= 45.6 ± 1.3 mmHg) had a lower VRT when compared to males (M= 50.5 ± 1.2 mmHg) F(1, 9) = 8.28, p < .05. The VES was significantly decreased after 30 days of hypercapnic bed rest (M= 1.17 ± 0.71 L/min/mm Hg) compared to the control time point (M= 1.62 ± 0.77 L/min/mmHg) F(1, 9) = 8.38, p < .05, yet was no longer blunted after 13 days of recovery (M= 1.47 ± 1.03 L/min/mmHg) F(1, 9) = .61, p > .05. CONCLUSION: Intriguingly, we found that VRT onset occurred at a lower PETCO2even after 13 days of recovery, while VES was blunted at 30 days, yet was restored to resting levels during the recovery period. These data suggest that exposure to chronic, mild increases in ambient CO2during head-down tilt bed rest may augment ventilatory chemosensitivity in as few as 30 days.

Support: NASA Human Research Program NNJ14ZSA001N

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