Pulse Oximeter Sensor

Our Goal

We hope to quantify the dynamics of an active muscle.

LED spectroscopy provides a way to track fatigue and oxygen trends of an area of interest.

Properties of the Red and IR LED

• Near-IR light has much better penetration into biological tissue than visible light

• NIR light absorption affected by: hemoglobin, myoglobin and cytochrome c oxidase molecules (90%Hb)

• To track specifically muscle activity, it appears that Infrared light will be the most appropriate signal to track

IR signal

We know from Beer-Lambert’s law that the absorption of an LED is a function of each absorbed substance given their:

1. extinction factor2. concentration 3. path length.

Our IR light detects mostly Hb/Mb-deoxygenation in muscle tissue but we cannot account for path length using 1 LED.

Path Length

For a fixed optode (forehead) we could assume path length to be constant.

However, we are interested in quantifying muscle movement within exercise, and our signal includes an initial contraction of muscle (change of path length)

Accounting for path length

The optical path length was accounted for by measuring the absorbance changes obtained during cuff ischemia.

The percent deoxygenation during exercise is then calculated from the maximal deoxygenation of the ischemic response.

A way of normalizing NIRS signals across subjects.

Oxygen saturations

Boushel claims one must use multiple wavelengths to generate algorithms for NIRS.

• Coefficients used at each wavelength deconvolute the overlapping absorption spectra.

• Algorithms are used for either trend monitoring or to assess relative concentrations.

Below are wavelengths and coefficients used in the laboratory at Duke for assessing relative concentrations.

Measurements with Two Wavelengths

Other NIRS applications use two near-IR LEDs (760 and 850nm) and use the difference between the signals to determine relative oxygen saturation. The sum of the signals to provide a reference to blood volume changes.

Red-IR= relative O2

While I am using a Red LED and IR LED,

the data shows plausible oxygen trends in that the

bicep oxygen content desaturates with increased load in

isometric exercise.

Infrared Trends

We changed the experiment set up

Infrared signals have shown a promising fatigue trend in almost all of our samples.

Analyzing IR trends

Static load in some types of activity compromise blood flow to working muscle.

Are our IR decline trends due to decreased blood flow, or is the use of oxygen in the muscle exceeding a continuous supply?

Moving Forward

Ultrasound & IR signal of Nellcor sensor in cooperation. Separate muscle influence and oxygen

Classify experiment activity, static load or active

Use multiple NIRS LEDs?

• Overlapping spectra= trend monitoring/ find relative concentrations of each molecule.