Absorption spectroscopy in newborn baby lungs

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Any kind of non-invasive optical measurement of structures deep inside the human body is challenging, and absorption spectroscopy is no exception. But despite the apparent difficulties, a research team from Lund University want to use spectroscopy to measure the concentration of gases in the lungs of newborn babes. They particularly want to do this for premature babies, because they are often born with respiratory problems. At the present, their lungs are monitored with blood tests and x-rays, and there are obvious limits to how regularly these can be performed. If non-invasive optical sensing could give some indication of how well the newborns’ lungs are functioning, it could be an attractive way of providing continual monitoring during treatment.

The idea of absorption spectroscopy is to measure the degree to which a material absorbs different wavelengths of light. This provides useful information because different molecular structures have different wavelength-dependent absorption profiles. The measurement can be made either by illuminating with a range of wavelengths simultaneously, and passing the returning light to a spectrally resolved detector, or by illuminating with one wavelength at a time, allowing a single point detector to be used. Either way, it can be difficult to detect low concentrations of something deep within highly scattering tissue, because the light that is collected will have scattered lots of times off different structures.

In their paper published in Paediatric Research1, the team used a technique called ‘GASMAS’ – Gas in Scattering Media Absorption Spectroscopy. As the name suggests, it’s a way of measuring the concentration of gases even when they are enclosed inside solid materials such as the lung. It works because gases have very sharp absorption peaks at specific wavelengths, whereas the sounding solid material has much broader peaks. It’s therefore possible, at least in principle, to pick out the sharper peaks above the background. The strength of the absorption peak is then related to the product of the concentration of the gas and the path length of gas that the light has travelled through. Unfortunately, GASMAS is quite difficult in practice, as it amounts to trying to see a small signal among a large amount of noise.

To help overcome this problem, the team used a special technique called digital lock-in detection. This involves sweeping the laser wavelength at two different frequencies. The first is a slow modulation, at 5 Hz, which is the wavelength sweep needed to perform spectroscopy. Super-imposed on this is a much higher frequency modulation, at around 10 kHz. The detected signal is band-passed around the second harmonic of this higher frequency, reducing noise that comes from random fluctuations of the laser power. This approach also allowed the authors to use two lasers, one with a wavelength to the main absorption peak of oxygen, and one to water vapour. By modulating the two lasers at different frequencies, the two signals could be distinguished and the two measurements made simultaneously.

The laser light is delivered to the patient via fibre optics embedded in a probe. This probe is placed on the chest of the baby, at some point along a line between the nipple and the collar bone. A second probe, containing a photodetector, is placed under the armpit and measures the light transmitted through the lung. The illumination probe contains a second photodetector which is used to correct for power fluctuations of the laser. Some clever processing then allows the spectra to be converted into an estimate of the concentration of the two gases.

In an initial study involving 30 healthy newborns, the team were able to detect both oxygen and water vapour with a high degree of confidence. The approach worked consistently across a range of baby weights (3-4 kg), for both sexes and using either the right or the left lung. However, it’s not yet clear if the concentration measurements were quantitatively accurate, and certainly the ratio of the water vapour to oxygen signal was not as expected. But it also has to be remembered that these results were on full-term babies, and so the technique may be even more effective with premature babies. In any case, this looks like an exciting new potential clinical application of spectroscopy.

References

  1. Svanberg, Emilie Krite, et al. “Diode laser spectroscopy for non-invasive monitoring of oxygen in the lungs of newborn infants.” Pediatric research (2015).

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