Characterization of the Nasal Cannula as a Carbon Dioxide Washout Measurement Technique in the Mark III Space Suit

A space suit must provide adequate carbon dioxide (CO2) washout inside the helmet to prevent symptoms of hypercapnia. In the past, an oronasal mask has been used to measure the inspired air of suited subjects to determine a space suit’s CO2 washout capability during ground-based tests. Although sufficient for super-ambient pressure testing of space suits, the oronasal mask fails to meet several human factors and operational criterion needed for future sub-ambient pressure testing (e.g., compatibility with a Valsalva device). This paper describes the evaluation of a nasal cannula as a device for measuring inspired air within a space suit. Eight test subjects were tasked with walking on a treadmill or operating an arm ergometer to achieve target metabolic rates of 1000, 2000, and 3000 British thermal units per hour (BTU/hr) at helmet ventilation flow rates of 2, 4, and 6 actual cubic feet per minute (ACFM). The test points at lower metabolic rates were conducted twice, with subjects instructed to breathe either through the nose only or however they felt comfortable. Comparing nasal cannula test data to historical oronasal mask data shows that the nasal cannula provides more statistically consistent data across test subjects than the oronasal mask, regardless of the breathing style. The data also shows that inhaling/exhaling through only the nose provides a lower sample variance than an unrestricted breathing style. However, it may not be realistic to require nose-only breathing for future CO2 washout evaluations because test subjects cannot breathe through their nose continuously at high metabolic rates. The test subjects in this study also provided feedback to human factors criteria, reporting that the nasal cannula is comfortable and can be used with a Valsalva device.