Circuitry and working principles of Pulse Oximeter

Measurement Principles:

A Pulse Oximeter is a non-invasive device for measuring the percentage of arterial blood (or hemoglobin) that is saturated with oxygen. This device can also measure heart rate.

The principle of operation of a Pulse Oximeter is based on measuring the absorption of red and near-infrared light that passes through a patient's finger or ear lobe by utilizing light sensors. Oxy-hemoglobin absorbs infrared wavelength (800-940 nm) and deoxy-hemoglobin absorbs visible RED wavelength (600-700 nm). 

LEDs are used as the light source and are sequentially pulsed at a fast rate. During a heartbeat, blood volume increases and the photodetector is used to calculate the absorption of oxy- and deoxy-hemoglobin.

• At the measuring site there are constant light absorbers that are always present like skin, tissue, venous blood, and the arterial blood. However, with each heartbeat the heart contracts and there is a surge of arterial blood, which momentarily increases arterial blood volume at the measuring site. This results in more light absorption during the surge. 
• If light signals received at the photodetector are looked at 'as a waveform', there should be peaks with each heartbeat and troughs between heartbeats. 

When light absorption at the trough (which includs all the constant absorbers) is subtracted from the light absorption at the peak, the absorption characteristics due to added volume of blood which is arterial is obtained. Since peaks occur with each heartbeat or pulse, the term "pulse oximetry" was coined.

• After the transmitted red (R) and infrared (IR) signals pass through the measuring site and are received at the photodetector, the R/IR ratio is calculated. The R/IR is compared to a "look-up" table (made up of empirical formulas) that convert the ratio to an SpO2 value.
• Typically a R/IR ratio of 0.5 equates to approximately 100% SpO2, a ratio of 1.0 to approximately 82% SpO2, while a ratio of 2.0 equates to 0% SpO2.

Circuit Design:

Oxygen in the blood is measured by alternating the on-times of a red LED with a 660-mm wavelength and an NIR (near-infrared) LED with a 940-mm wavelength, taking the ratio between the intensities from a photodiode, and comparing that ratio with an SpO2 look-up table in the microcontroller.

Sequence of LED operation:

• The red LED is on for 50 µsec
• both LEDs are off for 450 µsec
• NIR LED is on for 50 µsec
• then both LEDs are off for 450 µsec

The system repeats this cycle continuously.

1. The transimpedance amplifier, A1, converts the photodiode current generated by the LEDs to a voltage at the output. 
2. The signal then travels through a bandpass filter and gain stage to the 12-bit ADC. 
3. The signal also travels through a lowpass filter to regulate the driver power to the LEDs. 
4. The microcontroller acquires the signals from the 12-bit ADC, computes the ratio of the red- and NIR-LED signals, and compares the results with a look-up table. 
5. The LCD shows a percentage of oxygenated hemoglobin versus nonoxygenated hemoglobin and the heart rate.

References:

1. Al-Shaikh, Baha, and Simon Stacey. Essentials Of Anaesthetic Equipment. Edinburgh: Churchill Livingstone, Elsevier, 2013. Print.
2. Medical Instruments Applications Guide. Texas Instruments, Medical Instruments Applications Guide, 2016. Web. 5 Jan. 2016.
3. Baker, Bonnie. "Best Of Baker's Best - Amplifiers Ebook - TI.Com". Ti.com. N.p., 2016. Web. 5 Jan. 2016.
4. Oximetry.org,. "Pulse Oximetry". N.p., 2016. Web. 5 Jan. 2016.