Near infrared spectroscopy, the technique on which the Oxymon and the PortaMon are based, relies mainly on two characteristics of human tissue. First, the relative transparency of tissue to light in the NIR range, and second, the oxygenation-dependent light absorbing characteristics of hemoglobin. By using a number of different wavelengths, the relative changes in hemoglobin concentration can be displayed continuously.
Using the NIRS principle, it becomes possible to monitor:
- In the lab, or even in the field
- Without the need for special infrastructure
- Without specially trained personnel
All cells in all organs of the body have a constant but variable need for oxygen. However, the body stores for oxygen are minimal. A constant and adequate supply of oxygen to the tissues through the circulation is essential. Any interference with tissue oxygenation will lead very rapidly to irreversible damage.
Optical oximetry, and near infrared spectroscopy (NIRS) in particular, is a tool for assessing of the oxygenation status and hemodynamics of various organs, e.g. muscle and brain.
How it started
NIRS started with a paper published by Frans Jöbsis in Science (1977), Jöbsis reported that biological tissues are relatively transparent to light in the near infrared (700-1300 nm).
Therefore, it is possible to transmit enough photons through organs for in situ monitoring. In this near infrared region, hemoglobin - including its two main variants oxyhemoglobin (O2Hb) and deoxyhemoglobin (HHb)- exhibits oxygen-dependent absorption. Hemoglobin is assumed to be the main chromophore in biological tissue that absorbs light in this near infrared region.
If the absorption is known, the Lambert-Beer law can be used to calculate the chromophore's absorption. The Lambert-Beer law is given by:
ODλ is a dimensionless factor known as the optical density of the medium, I0 is the incident light, I the transmitted light, ελ the chromophore's extinction coefficient (in µM-1•cm-1), c is the concentration (in µM) of the chromophore, L the distance (in cm) between light entry and exit points and λ is the wavelength used (in nm).
The Lambert-Beer law is intended to be used in a transparent, non-scattering medium. When it is applied to a scattering medium (Figure 1.1), e.g. biological tissue, a dimensionless pathlength correction factor must be incorporated. This factor, sometimes called the differential pathlength factor (DPF), accounts for the increase in optical pathlength due to scattering in the tissue. The modified Lambert-Beer law for a scattering medium is given by:
where ODλ represents the oxygen-independent optical losses due to scattering and absorption in the tissue. Assuming that ODλ is constant during a NIRS measurement, we can convert the change in optical density into a change in concentration.
This equation is valid for a medium with one chromophore. If more chromophores are involved, we need to measure at least as many wavelengths as there are chromophores present. This results in a set of linear equations. The solution of this set leads to the algorithm used in most NIRS systems. A scattering medium makes it possible to measure the absorption with the near infrared source and detector parallel to each other (Figure 1.1). This offers the opportunity to measure oxygenation in larger tissues, e.g. muscles and brain using NIRS equipment.
Defining the algorithm used by NIRS requires the spectral extinction coefficients of the various chromophores. The spectra of the two main chromophores, O2Hb and HHb.
The sum of O2Hb and HHb is a measure of the total blood volume (tHb) in the tissue. Muscle tissue contains two further chromophores: oxy- and deoxymyoglobin (O2Mb and HMb). In order to distinguish between hemoglobin and myoglobin in muscle tissue, the spectra need to be sufficiently different. Unfortunately this is not the case in the near infrared region of the spectrum. This means, NIRS cannot distinguish if the measured oxygen concentration is carried by hemoglobin or myoglobin. The wavelengths which can distinguish the Hb and Mb are not able to penetrate the tissue deep enough.
Comparison with pulse oximetry
The technique on which near infrared spectroscopy relies is closely analogous to the technique of pulse oximetry.
The main difference is the tissue being sampled. Pulse oximetry calculates the percentage of oxygenated hemoglobin in the arterial blood. NIRS calculates the changes in oxy- and deoxyhemoglobin (and optionally the percentage of oxygenated hemoglobin) in the tissue under investigation (capillaries), which contains both arterial and venous blood.