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Astronauts who are exposed to the microgravity environment of a spacecraft for extended periods experience a range of physical effects, including bone loss, muscle loss, and cardiovascular deconditioning. For shorter missions, or longer-duration missions into low earth orbit (e.g., to the International Space Station) where a return to Earth is relatively quick, the risks to the crew are more manageable. During longer missions out of low earth orbit (e.g., to Mars), however, crew members must transition from spending many months in a weightless environment to functioning in the gravitational field of a planetary surface. For these crew members, the risks associated with bone and muscle loss are much more severe. NASA invests considerable research efforts into developing countermeasures to the deleterious effects of long-term exposure to microgravity.

Just as important as these countermeasures is the instrumentation needed to monitor the crew’s fitness level. This instrumentation must be reliable, compact, and rugged. NASA developed the Portable Unit for Metabolic Analysis (PUMA) to monitor a crew’s cardiovascular fitness level during long-duration spaceflight. PUMA measures critical aspects of an individual’s physiological state including minute ventilation (the average rate of exhaled gas flow), oxygen uptake (the rate of oxygen consumption), and carbon dioxide output (rate of carbon dioxide production) in a relatively small and rugged package.

Monitoring a patient’s physiological state, however, is not only critical in long-duration spaceflight. Terrestrial applications of physiological monitoring are widely varied. Metabolic monitors are routinely used in the training of elite athletes (i.e., measuring maximal oxygen uptake, VO2, max). Clinical applications include monitoring patients with long-term cardiovascular disease. For persons in extreme physical environments, such as soldiers in combat or firefighters, physiological monitoring can mean the difference between life and death. All of these terrestrial applications can potentially benefit from the technology developed to monitor an astronaut’s physiological health.

How PUMA works

Figure 1 shows the third-generation prototype PUMA hardware that consists of headgear that contains the gas sensors on the left side and the flow sensor on the right side. The mask is tethered optically and electrically to an avionics box containing a small onboard processor and additional electronics and optics for the sensors.

 NASA’s PUMA provides valuable patient data, Fig. 1

The critical sensors in PUMA are the gas sensors for oxygen and carbon dioxide and the flow sensor. The oxygen sensor was developed at NASA GRC. Sinusoidally modulated, blue light from a laser diode excites a Ruthenium-based dye at the end of an optical fiber. The dye fluoresces orange light that is phase-shifted relative to the excitation light, and the degree of phase shift is proportional to the oxygen concentration.

The carbon dioxide sensor is an infrared absorption technique that uses several infrared (IR) light-emitting diodes (LEDs) focused on a thermoelectrically cooled detector approximately 1cm away. The LEDs emit light in the range of 4.3µm, exactly where carbon dioxide has an extremely strong and unique absorption cross-section.

Other sensors in PUMA include a flow sensor, commercial pressure transducer, thermistor, and heart rate monitor. The flow sensor is a commercial ultrasonic sensor custom modified to accurately measure the large gas flows typical in exercise tests. The PUMA computer controls and acquires data from all sensors at 10 Hz, performs rudimentary calculations, and transmits sensor signal data wirelessly to a remote computer via Bluetooth.

 NASA’s PUMA provides valuable patient data, Fig. 2

By placing the essential sensors close to the mouth and sampling at 10 Hz, PUMA overcomes timing, sample dilution, and slow sampling issues present on nearly all commercial metabolic carts and portable metabolic units. Figure 2 shows raw sensor data from PUMA for approximately 20 seconds. Using these data (along with the other sensors), it is a relatively simple matter to compute the minute ventilation, oxygen uptake, and carbon dioxide output on a breath-by-breath basis.

 NASA’s PUMA provides valuable patient data, Ve equation

 NASA’s PUMA provides valuable patient data, VO2 Equation

 NASA’s PUMA provides valuable patient data, VCO2 Equation

In these expressions, V(t) represents the volumetric flow rate; XO2 and XCO2 the mole fractions of oxygen and carbon dioxide, respectively; and tbi, tbe, and tee represent the beginning of an inhalation, the beginning of an exhalation and the end of an exhalation, respectively. PUMA measures both inspiratory and expiratory data so that the equations above make no a priori assumptions regarding the composition of the ambient air.

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