All cells in all organs of the body have a constant but variable need for oxygen. However the body stores for oxygen are minimal. So a constant and adequate supply of oxygen to the tissues through the circulation is essential. Any interference with tissue oxygenation will lead cery 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.
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-
NIRS started with a paper published by Frans Jöbsis in Science , Jöbsis reported that biological tissues are relatively transparent to light in the near infrared (700-
If the absorption is known, the Lambert-
where ODR,λ represents the oxygen-
The technique on which near infrared spectroscopy relies is closely analogous to the technique of pulse oximetry. The main difference is in the tissue being sampled. Pulse oximetry calculates the percentage of oxygenated hemoglobin in the arterial blood. NIRS calculates the changes in oxy-
where ODλ is a dimensionless factor known as the optical density of the medium, I0 is the incident light, I the transmitted light, e? the chromophore's extinction coefficient (in µM-
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 1.2). 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, are shown in figure 1.3.
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-