Unchanged cerebrovascular CO2 reactivity and hypercapnic ventilatory response during strict head-down tilt bed rest in a mild hypercapnic environment.

KEY POINTS: Carbon dioxide levels are mildly elevated on the International Space Station and it is unknown whether this chronic exposure causes physiological changes to astronauts. We combined ∼4 mmHg ambient PCO2 with the strict head-down tilt bed rest model of spaceflight and this led to the development of optic disc oedema in one-half of the subjects. We demonstrate no change in arterialized PCO2 , cerebrovascular reactivity to CO2 or the hypercapnic ventilatory response. Our data suggest that the mild hypercapnic environment does not contribute to the development of spaceflight associated neuro-ocular syndrome. ABSTRACT: Chronically elevated carbon dioxide (CO2 ) levels can occur in confined spaces such as the International Space Station. Using the spaceflight analogue 30 days of strict 6° head-down tilt bed rest (HDTBR) in a mild hypercapnic environment ( PCO2 = ∼4 mmHg), we investigated arterialized PCO2 , cerebrovascular reactivity and the hypercapnic ventilatory response in 11 healthy subjects (five females) before, on days 1, 9, 15 and 30 of bed rest (BR), and 6 and 13 days after HDTBR. During all HDTBR time points, arterialized PCO2 was not significantly different from the pre-HDTBR measured in the 6° HDT posture, with a mean (95% confidence interval) increase of 1.2 mmHg (-0.2 to 2.5 mmHg, P = 0.122) on day 30 of HDTBR. Respiratory acidosis was never detected, although a mild metabolic alkalosis developed on day 30 of HDTBR by a mean (95% confidence interval) pH change of 0.032 (0.022-0.043; P < 0.001), which remained elevated by 0.021 (0.011-0.031; P < 0.001) 6 days after HDTBR. Arterialized pH returned to pre-HDTBR levels 13 days after BR with a change of -0.001 (-0.009 to 0.007; P = 0.991). Compared to pre-HDTBR, cerebrovascular reactivity during and after HDTBR did not change. Baseline ventilation, ventilatory recruitment threshold and the slope of the ventilatory response were similar between pre-HDTBR and all other time points. Taken together, these data suggest that the mildly increased ambient PCO2 combined with 30 days of strict 6° HDTBR did not change arterialized PCO2 levels. Therefore, the experimental conditions were not sufficient to elicit a detectable physiological response.

Static monocular visual cues can decrease the vestibular-evoked balance response at low frequencies.

BACKGROUND: The balance system continually integrates and processes diverse sensorimotor cues to maintain upright posture. Yet, little is known about how monocular visual cues may modulate the vestibular control of standing balance. RESEARCH QUESTION: To determine how visual cues, specifically monocular vision, modulate the vestibular-evoked myogenic and whole-body balance response. METHODS: Seventeen (12 female) healthy subjects (age: 24.8 ± 5.3years) were exposed to a random, continuous electrical vestibular stimulation (EVS) signal (±3.5 mA, 0-20 Hz). Subjects stood quietly during four experimental (no vision, non-dominant eye, dominant eye, binocular) conditions. The EVS-medial-lateral ground reaction force (ML GRF) acting on the body and EVS-medial gastrocnemius electromyography (EMG, bilateral) responses were evaluated in the frequency (coherence) and time (cumulant density) domains. RESULTS: Coherence was increased for no vision compared to binocular, dominant eye, and non-dominant eye visual cues, respectively, with the most pronounced increases occurring at lower frequencies. For cumulant density, the EVS-ML GRF medium-latency peak amplitude was increased 45, 26 and 18% with no vision compared to binocular, dominant eye and non-dominant eye visual cues, respectively (p < .05). The EVS-EMG medium-latency peak amplitude during no vision was greater than binocular (p < .05) for both gastrocnemii, but binocular and dominant eye monocular vision was not different (p > .05). The EVS-ML GRF and EVS-EMG (right medial gastrocnemius) medium-latency peak amplitude was greater for non-dominant eye monocular vision compared to binocular vision (p < .05). SIGNIFICANCE: Monocular visual cues, at least for the dominant eye, can depress the vestibular-evoked balance response at low frequencies akin to binocular vision with limited differences exhibited between dominant and non-dominant eye.