Due to the absence in previous history of postflight pulmonary problems, and the normoxic microgravity environment of Skylab, vital capacity was initially proposed as the only functional screening. However, as a result of the magnitude of decreases in cardiac output following the first and second manned Skylab missions (Skylab 2 and 3), it was decided that more thorough pulmonary function screening was necessary for the final and longest duration Skylab mission (Skylab 4).
The nominal composition was inspired oxygen partial pressure equal to 24.1 kPa (188 mm Hg) and inspired nitrogen partial pressure equal to 10.3 kPa (77 mm Hg), at a total pressure equal to 34.4 kPa (258 mm Hg). This atmosphere was planned to provide approximate sea level equivalent alveolar oxygen partial pressure.
Forced vital capacity and its derivatives were measured 5 days preflight, as well as postflight on recovery day and 1 day after recovery for the Skylab 2 mission. During Skylab 3, vital capacity was measured preflight, inflight (during the last 2 weeks of the mission), and 1, 2, 4, and 5 days postflight. Comprehensive pulmonary function screening was accomplished during Skylab 4. The Skylab 4 preflight pulmonary function examinations were conducted 10 days preflight. Vital capacity was measured at 6 day intervals inflight. Complete postflight examinations were conducted on recovery day and 1, 2, and 5 days after recovery. Vital capacity measurements were continued at 11, 17, and 31 days postflight.
Metabolic analyzers were used for all pulmonary function measurements. The Skylab 2 and 3 metabolic analyzers were designed to measure vital capacity and respiratory gas exchange. The Skylab 4 metabolic analyzer was modified to permit residual volume determination by open circuit washout of pulmonary nitrogen during oxygen breathing. A respiratory mass spectrometer monitored nitrogen partial pressure continuously at the subject interface, and an XY plotter provided continuous nitrogen partial pressure as a function of time during residual volume determinations. An analog tape recorder and strip chart were used during all Skylab 4 preflight and postflight testing to provide permanent, synchronous records of nitrogen partial pressure at the mouthpiece, tidal volume, flow rate, and nitrogen washout.
During Skylab 2, one subject's forced expired volume in 1 second (FEV-1) was significantly reduced one day postflight (R+1). Based on the subject's normal forced vital capacity, it is possible that the decreased FEV-1 was due to less than maximal subject effort. No data were obtained postflight for one Skylab 2 subject due to orthostatic intolerance complicated by seasickness.
During Skylab 3 one subject showed no changes in pulmonary function except for a slight increase in vital capacity 5 days post-recovery. Vital capacity of one Skylab 3 subject was slightly higher inflight but normal postflight, relative to preflight values. One Skylab 3 subject exhibited decreased vital capacity inflight but normal vital capacity postflight relative to preflight.
Skylab 4 vital capacities were generally observed to be lower inflight relative to preflight. Vital capacity for one subject remained below preflight levels for the entire inflight period. Nitrogen washout curves for Skylab 4 crewmembers' pulmonary function tests showed no indications of trapping. All washout curves appeared to reflect the anticipated two time constants representing washout of pulmonary and total body nitrogen spaces. One subject had a pronounced vasovagal response following maximal oxygen inspiration during the first closing volume determination on recovery day, and further testing was curtailed until the next day. All other measurements on the first day following recovery, and all measurements on days 2 and 5 postflight were within normal ranges.
Residual volume in one Skylab 4 subject was slightly increased immediately following recovery and on day 2, compared to preflight. Residual volume/total lung capacity percent indicated that these changes were probably insignificant. Ventilatory equivalents of this subject were variable and reflected the irregular respiratory pattern observed during washouts. Although vital capacity for the subject was slightly decreased on the second day following recovery relative to preflight, forced vital capacity was normal. Maximum midexpiratory flow rate (between 25 percent and 75 percent of the forced vital capacity) values for the subject were the highest ever recorded in the investigator's laboratory.
Skylab 4 provided the first opportunity for extensive, noninvasive pulmonary function screening on astronauts before and following an extended zero-g exposure. No physiologically significant quantitative decrement in pulmonary function was shown by any crewman during examinations following this 84-day Earth-orbital mission.
Postflight chest films for all crewmen were compared to preflight films to detect any changes in the pulmonary vessels, parenchyma, or heart size. No significant pulmonary vasculature or parenchymal changes were observed in any instance.
Vital capacity, the only parameter measured preflight, in-flight, and postflight, showed inflight decreases approaching 10 percent in the case of one subject from Skylab 3 and all three crewmembers from Skylab 4. These decreases in vital capacity apparently resulted from one or a combination of the following factors: cephalad shift of the diaphragm in zero-g; body fluid redistribution into the thoracic cavity; or a direct result of decreased ambient pressure.
In summary, the vital capacity changes observed inflight may be partially explained as a response to 34.5 kPa (5 psia) ambient pressure. However, the proportion of vital capacity decreases directly attributable to other factors such as body fluid shifts and a cephalad shift of the diaphragm cannot be determined from the present data. Regardless of the cause(s) of decreased inflight vital capacities, a review of postflight data shows that these changes revert to normal within 2 hours following recovery without significant impact on crew health status.