A NASA sponsored validation study of the PPFS was conducted in 2009 (EORS_PPFS), prior to its delivery to the ISS. The general findings of that study were that the PPFS yielded metabolic gas analysis values fairly comparable to a well-validated clinical system, and where statistically significant differences did occur, they were not large enough to be of clinical relevance. However, the reproducibility of the PPFS remained untested. In addition, the validation findings suggested that expired carbon dioxide (FECO2) when measured with the PPFS may differ from that measured by the clinical reference system. Because the validation study design precluded simultaneous measurements of either expired oxygen (FEO2) or FECO2 from both devices, this finding was speculative.
The purpose of the current study was to examine the reliability and intra-subject repeatability of PPFS obtained metabolic gas analysis and cardiovascular data during exercise tests which contained both steady-state submaximal stages and maximal exertion levels. In addition, a portion of the current study was designed to obtain PPFS and reference system values of FEO2 and FECO2 in a manner which would allow direct comparison of the data.
The cycle ergometer used for all testing was a LODE Excalibur Sport (Groningen, NL). Prior to the exercise test sessions, the subjects completed an overnight fast and consumed a standardized dietary supplement (Ensure®) approximately 2 hours prior to testing which was performed in the morning, typically between 0830 and 1000. The Ensure® was given to insure that none of the subjects consumed either an extremely high fat or high carbohydrate meal prior to their tests as dietary extremes have been demonstrated to effect metabolic gas analysis results, particularly the respiratory exchange ratio (RER=VCO2/VO2). None of the subjects performed heavy exercise in the 24 hours prior to the testing sessions and refrained from caffeine consumption for 12 hours prior to testing.
Subjects performed an initial peak cycle test protocol identical to that used by NASA to determine pre-flight VO2max of astronauts prior to participating in an ISS mission. Subjects with a body mass of > 65 kg were to cycle for 3 min at 50, 100 and 150 Watts (W) followed by stepwise increases of 25 W/min until peak exertion was reached. If the subject’s body mass was < 65 kg, the protocol required cycling for 3 minutes at 50, 75, and 100 W followed by stepwise increases of 25 W/min until peak exertion was attained. The peak exertion point was defined by volitional exhaustion (i.e. the subject indicated that he/she could no longer continue) or, despite encouragement; the subject could not maintain a pedal cadence at or near 75 revolutions per min (rpm). During this initial test a ParvoMedics TrueOne© (PM) system was used to perform metabolic gas analysis. For each subject, the VO2 and work rate data from this test was used to prescribe the protocol for PPFS reliability exercise testing.
To obtain data regarding the reproducibility of PPFS metabolic gas measurements during exercise, each subject performed three repeated trials. The cycle exercise testing protocol, identical to that used for the current ISS VO2max study consisted of 3, 5-minute stages designed to elicit 25%, 50%, and 75% of the individual’s previously determined VO2max. These stages were followed by stepwise increases of 25W/min work rate until subjects reached volitional exhaustion. These tests were conducted within one month after the initial test, and were separated by at least a week in order to minimize the potential effects of residual soreness or fatigue. No more than 3 weeks elapsed between trials within a subject. Each subject was instructed not to substantially vary their physical activities between trials.
For all the reliability trials, prior to the start of exercise the subjects rested quietly for 5 minutes in the seated position on the ergometer while resting measurements of heart rate (HR) and rhythm and blood pressure (BP) were taken. HR and rhythm were measured electrocardiographically using the PPFS, which has an ECG incorporated into the device. HR and metabolic expired gas were measured continuously throughout the exercise test protocol. During the last minute of seated rest, and the last minute of each of the first 3 exercise stages, a non-invasive measurement of cardiac output was obtained.
The PPFS appears in practically all respects to yield highly reliable metabolic gas analysis data. The lower reliability of RER measurements are commonly reported in the literature and likely are not a function of the PPFS device per se. Further examination of PPFS CO2 versus PM CO2 data is warranted to precisely quantify this relationship. The PPFS should provide accurate data when utilized to study the adaptations and responses of humans to exposure to the space flight environment.
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