Elevated End-Tidal Pco₂ During Long-Duration Spaceflight

BACKGROUND: Elevated ambient Pco₂ in the International Space Station (ISS) has been cited as a potential contributor to the vision impairment intracranial pressure syndrome (VIIP), a significant health risk for astronauts during long-duration space missions. The elevation in ambient Pco₂ is rather modest and normal respiratory compensation could minimize the impact on arterial Pco₂.METHODS: In nine male astronauts, breaths measured prior to a rebreathing maneuver were examined to assess inspired and end-tidal Pco₂ during upright seated preflight and in-flight conditions.RESULTS: Inspired Pco₂ increased from preflight baseline (0.6 ± 0.1 mmHg) to in flight (3.8 ± 0.4 mmHg). End-tidal Pco₂ also increased from preflight baseline (36.0 ± 3.2 mmHg) to in flight (42.1 ± 3.7 mmHg). The difference between end-tidal Pco₂ comparing in flight to preflight (6.1 ± 1.6 mmHg) was greater than the difference between inspired Pco₂ comparing preflight to in flight (3.3 ± 0.5 mmHg).DISCUSSION: The greater increase in end-tidal vs. inspired Pco₂ might reflect alveolar hypoventilation due to differences in ventilatory control with spaceflight. These data suggest that further studies should focus on arterial Pco₂ and acid-base balance to determine if CO₂ dilates cerebral and retinal vessels and might contribute to the incidence of VIIP in astronauts.Hughson RL, Yee NJ, Greaves DK. Elevated end-tidal Pco₂ during long-duration spaceflight. Aerosp Med Hum Perform. 2016; 87(10):894–897.