Additional Technical Information

The InSpectra™ StO₂ Tissue Oxygenation Monitor works by sending near infrared light through the skin into underlying tissues and taking measurements of the light after it travels through the tissues.

 

When light enters tissues, two things can happen:

1. The light will be reflected off the surface of cells, referred to as scatter,
2. The light will penetrate into the cells and be absorbed by molecules such as hemoglobin and myoglobin.

Due to these two mechanisms, the amount of light returned back to the surface of the skin is approximately 100 million times less than the amount that entered. In addition, the spectrum of the light will have changed because of absorption.

Each scattering event increases the path length (distance the light photons travel). Different tissues have different scatter properties, thus different path lengths.

The spectrum of light is determined by looking at a profile (or graph) of intensity vs. wavelength. When light is absorbed in the tissues, the spectrum of the light changes. For example, as light enters the tissues, its spectrum may look like this:

As the light then leaves the tissues, its spectrum may look like this:

The original light spectrum (as it enters the tissues) is the reference intensity, while the changed light spectrum (as it leaves the tissues) is called the sample intensity. Using the reference and sample intensity values, the absorbance value can now be calculated using a logarithmic equation that assesses the difference between the two values. A large difference between the sample and reference intensities signifies a higher absorbance of light.

After the absorbance values have been determined, the next step is to look at the absorbance spectrum, which is the graph demonstrating absorbance plotted against wavelength. Every molecule that absorbs light has a characteristic absorbance spectrum. Hemoglobin is unique in that its spectrum changes as it becomes oxygenated; deoxygenated hemoglobin and oxygenated hemoglobin each have characteristic absorbance spectra. The differences between the two absorbance spectra can be seen below:

In order to quantify hemoglobin oxygen saturation status at the beginning or baseline tissue condition, the reference intensity measurement for calculating absorbance of light is performed on a material lacking hemoglobin specific absorbance (not the intended measurement site).

In order to determine the oxygenation status of hemoglobin, the spectrometer measures the absorbance at four specific wavelengths: 680nm, 720nm, 760nm, and 800nm. These four wavelengths are specific to the 760nm absorption band that is absent in oxygenated hemoglobin. In addition, these four points relate to either troughs (low points) or crests (high points) on the deoxygenated hemoglobin absorbance spectrum. In other words, these four points will tell the spectrometer the most information about whether the hemoglobin is oxygenated or deoxygenated.

 

With this method, the tissue absorbance measurements are confounded by optical path length and chromophore absorption, thus requiring a second derivative transformation of absorbance measurements. The second derivative transformation removes the linear spectral contributions that optical path length and/or alternate chromophores exhibit within a specified wavelength region (680nm to 800nm) where hemoglobin exhibits significant non-linear absorption as the oxidation state of hemoglobin changes.

This method does not separate out oxyhemoglobin and deoxyhemoglobin concentrations using the pure absorption characteristics of each hemoglobin form. The ratioed second derivative method is empirically correlated to the oxygen state of hemoglobin at all mixtures of oxyhemoglobin and deoxyhemoglobin.

When absorbance values are analyzed, they can be interpreted as absolute or relative values. An absolute absorbance value is determined by comparing the returned light spectrum to original light spectrum sent from the sensor, as was described previously. A relative reference, however, does not take into account the original light spectrum sent. Instead, the returned light spectrum is unknown, and only change over time can be monitored.

Regardless of whether the absorption values are absolute or relative, the method of calculating absorption removes the influence of individual device variances from the final measured value. For example, each device will have slightly different light detection sensitivity, light emission intensity, etc. Using absorbance as a comparison between two values eliminates the possibility of these variances having influence on the measurements.

Aside from individual device variations, other factors influence measurement of light transmission through tissue. In addition to the factors mentioned earlier, absorbance of light through a substance is influenced by the following variables:

With the InSpectra StO₂ Tissue Oxygenation Monitor, the sensor contains both the light emitter and the light detector, and therefore measures the light that is scattered and reflected back to the skin surface. The length of this path cannot be precisely measured. Though the exact length is unknown, the effect of the sensor design on the path of the light has been determined by previous research. On average, the path of the light has a banana-shaped, or an arc-shaped profile. The average depth the light travels into tissue is approximately one-half the distance between the light sent and returned points on the sensor.

Though the approximate path length can be determined, the exact depth cannot. Because the path length is not a precisely measured value, its influence needs to be removed from the absorption calculation. This influence is eliminated by making mathematical manipulations; specifically, taking the second derivative absorbance and scaling it.

Taking the second derivative of an equation is a mathematical function that allows the influence of specific variables to be eliminated. In this case, the variable influence eliminated is wavelength dependent path length. Note: Wavelength dependent path length is a function of tissue scatter properties (size and morphology of cells). Short wavelength light is more effectively scattered and hence has a longer path length.

After taking the second derivative, the result is an absorbance spectrum that is robust to wavelength variations in path length. The second derivative calculation also centers the absorption scale to a common baseline of zero to account for a lack of photometric light calibration. Absorbance scale with reflective mode measurement is arbitrary and can shift above or below zero depending upon gain settings of the photo detectors. Taking the second derivative of measured absorbance results in spectra that the InSpectra StO₂ Tissue Oxygenation Monitor uses to determine the oxygenation status of hemoglobin. After taking the second derivative, the absorption spectra of oxygenated and deoxygenated hemoglobin look approximately like this:



Aside from absorption, light scatter also determines the transmission of light through tissues. Just as the light from automobile headlights is scattered by rain droplets in fog, the light being transmitted through tissues is scattered by cells. The degree of scatter is influenced by the wavelength of the light. Longer wavelengths can travel around and between cells more easily than smaller wavelengths. Smaller wavelengths have a higher chance of colliding with cells, and hence scatter more easily. Therefore, the path that the light makes through the tissue is more tortuous, and thus longer. Because the path is longer, the chances of absorption increase for light at a shorter wavelength. This phenomenon is called wavelength-dependent scattering.

Once the second derivative has been calculated, a last secondary value needs to be determined. Since the number of molecules absorbing the light influences light absorption, a product of both total hemoglobin concentration and average path length, these variables also need to be accommodated for. This is done by dividing the second derivative absorption measurement at 720nm (1st trough) by the second derivative absorbance measurement at 760nm (peak). Because the height of the peak and trough is influenced by total hemoglobin concentration and average path length, making this mathematical division (i.e. scaling) eliminates the influence.

All of these calculations and derivations are built into the InSpectra StO₂ Tissue Oxygenation Monitor. The result is a device capable of measuring tissue hemoglobin oxygen saturation, and producing a value that factors out the influences of path length, sensor spacing, and total hemoglobin concentration.