Introduction
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 disturbance of tissue oxygenation will lead to irreversible damage very soon.
Optical oximetry, and Near InfraRed Spectroscopy (NIRS) in specific, is a tool for the assessment 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 is based, relies mainly on two characteristics of human tissue. Firstly, the relative transparency of tissue for light in the NIR range, and secondly, the oxygenation dependent light absorbing characteristics of hemoglobin. By using a number of wavelengths the relative changes in concentrations of the hemoglobin’s can be displayed continuously. Using this principle it becomes possible to monitor:
- Non-invasively.
- In the lab, or even in the field with the PortaMon.
- Continuously.
- Without the need of a special infrastructure.
- Without specially trained personnel.
- Relatively cheap.
Introduction to Near Infrared Spectroscopy theory
NIRS started with an article published by Jöbsis in Science [1977], it is reported that biological tissues have a relatively good transparency for light in the near infrared region (700-1300 nm). Therefore it is possible to transmit sufficient photons through organs for in situ monitoring. In this near infrared region hemoglobin, which can be divided into its main components oxyhemoglobin (O2Hb) and deoxyhemoglobin (HHb), shows oxygen dependent absorption. Hemoglobin is assumed to be the main chromophore in biological tissue, absorbing light in this near infrared region.
If absorption is known, the Lambert-Beer law can be used to calculate the concentration of the chromophore. The Lambert-Beer law is given by:
where ODλ is a dimensionless factor known as the optical density of the medium, I0 is the incident radiation, I the transmitted radiation, ελ the extinction coefficient of the chromophore (µM-1•cm-1), c is the concentration (µM) of the chromophore, L the distance (cm) between light entry and light exit point and λ the wavelength (nm) used.
The Lambert-Beer law is intended to be used in a clear, non-scattering medium. When the law 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 "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 ODR,λ represents the oxygen independent light losses due to scattering in the tissue. Assuming ODR,λ is constant during a NIRS measurement we can convert an optical density change into a concentration change:
This equation is valid for a medium with one chromophore. In the case of more chromophores 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 gives the possibility to measure the absorption with the near infrared light source and detector parallel to each other (Figure 1.2). This creates the opportunity to measure oxygenation in larger tissues e.g. muscles and brain using NIRS equipment.
Spectral extinction coefficients of the chromophores
To define the algorithm used by NIRS, the spectral extinction coefficients of the various chromophores are needed. The spectra for the two main chromophores, O2Hb and HHb, are given in figure 1.3.
The sum of O2Hb and HHb is a measure for the total blood volume (tHb) in the tissue. If muscle tissue is investigated, there are two more chromophores present: oxy- and deoxymyoglobin (O2Mb and HMb). To distinguish between hemoglobin and myoglobin in muscle tissue the spectra need to be sufficiently different. Unfortunately this is not the case in the visible part of the spectrum. This means, NIRS cannot distinguish if the measured oxygen concentration is carried by hemoglobin or myoglobin.
Difference with pulse oximetry
The technique on which near infrared spectroscopy is based, strongly resembles to the technique of pulse oximetry. The main difference is the tissue which is sampled. With pulse oximetry the percentage of oxygenated hemoglobin in the arterial blood is calculated. With NIRS the changes in oxy- and deoxyhemoglobin (and optionally the percentage of oxygenated hemoglobin) in the tissue under investigation (capillaries) is measured, which contains both arterial and venous blood.
