Analysing exhaled air with photoacoustics to provide near-instant data on health conditions

In cooperation with NASA, ESA has developed a new device that is now part of the ISS Pulmonary Function System (PFS). Developed by the Danish company INNOVISION SA, the new device analyses exhaled gas from astronauts' lungs and provides near-instant data on the state of crew health. It also has important medical uses here on Earth. Normal air contains 21% oxygen and only a trace of carbon dioxide. A healthy body takes up oxygen and excretes carbon dioxide: at rest, the exhaled air is about 16% oxygen and 1% carbon dioxide. If the subject is exercising, the ratio of carbon dioxide to oxygen increases. It is a good measure of metabolic efficiency. And it can also reveal the 'deconditioning' effects of lengthy exposure to microgravity. Breath analysis has been an important medical and research tool for many years. Until now, though, it depended on a device called a mass spectrometer, which is highly sensitive but requires highly skilled operators and is notoriously difficult to calibrate accurately. The PFS uses an entirely different approach: photoacoustics. The method depends on the fact that different gases absorb infrared energy at different and very precise wavelengths. The new device divides the exhaled breath sample into several small test cells. Each of these is fitted with a window filter that admits only the exact wavelength that matches a specific gas. Then the test cells are exposed to an infrared source, beamed through a spinning 'chopper wheel' - a disk with holes cut in it. When the hole is in front of the window, the infrared energy heats and expands the gas. When the hole has passed, the gas contracts again. A microphone picks up the pulses. Their amplitude - their 'loudness' - gives an accurate measure of how much of a given gas is present in the sample. Six different gases can be measured simultaneously, with several measurements being made within a single breath. And as well as telling scientists a great deal about their subject's lung function, the data also reveals much about blood flow and the entire cardio-vascular system. It is almost as accurate as a mass spectrometer, but it is much easier to uses. The PFS can really be used in the field." Or the hospital ward, which is why this new device is attracting serious interest from the medical profession. At present, doctors seeking to measure heart output from seriously ill patients often have to insert catheters into arteries - an invasive technique that is always difficult and can be dangerous. Now the same data can be obtained with nothing more onerous than a puff of breath.