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Pulse oximetry fundamentals and MCUs

23 Jan 2014  | Jayaraman Kiruthi Vasan

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Once the ratio of ratios is calculated, the SpO2 can be extracted from independently derived, empirical calibration curves based on the diodes being used, generally by means of a lookup table. An approximate formula for calculating oxygen saturation would be:

SpO2 (%) = 110 – 25 X Ratio of Ratios

A pulse oximeter design must do the following internally to achieve accurate measurements:

 • Alternately switch the red and IR LEDs on, measuring the absorbance at each of these wavelengths.
 • Dynamically adjust the LED intensities to achieve optimal results regardless of skin colour, finger thickness, etc.
 • Normalise the differences between the Red and IR LEDs.
 • Digitally amplify the signal output to accurately measure weaker signals caused by low perfusion of blood , numbness of the measurement site, and the like.
 • Digitally filter the signals to remove effects due to baseline wandering and artifacts caused by frequent patient movements.
In order to achieve the above five objectives, the microcontroller design needs to have the following features:

 • An internal ADC – 12bit minimum resolution
 • Two DAC channels – 10bit minimum/12bit preferred.
 • A digitally adjustable gain amplifier – Built-in desired.
 • Sufficient code and data memory for the application.
A typical MCU implementation is depicted by the block diagram in the figure.

Figure: A typical MCU implementation.

In this designT1 through T4 drive the LEDs connected in anti-parallel fashion. T1 and T4 drive the IR LED and T2 and T3 drive the Red LED. Signals IR ENABLE and RED ENABLE switch T1 and T2 on alternatively.

IR INTENSITY and RED INTENSITY are DAC outputs and provide the current intensity control. GAIN ADJ can be a group of SPI signals controlling an external programmable gain amplifier.

During the period each LED is on, the photodiode output provides a measure of the emergent light intensity. The MCU digitizes the IR sample and Red Sample, and then conditions the signals using digital filters implemented through code. Both the digital data then get processed as per the mathematical methods described above, and the MCU estimates and displays the resultant SpO2 value.

A pulse oximeter can also display other parameters, such as the arterial pulse rate (heart beat per minute), and the blood flow pattern, which is called a plethysmograph. The plethysmograph is obtained from either the Red or Infra-red intensity voltage waveform and resembles the blood pressure waveform in shape.

There are many MCUs suitable for creating a pulse oximeter. A partial list is given below. I have also seen commercial pulse oximeters using MCUs such as the 8051, C8051F120, ATMega128, dsPic, and the like.

Pulse oximeters have evolved continuously since their introduction and currently incorporate state-of-the-art digital signal processing. Companies like Masimo and Nellcor have created special algorithms to extract the oxygen saturation data from even those patients who are suffering from severe heart diseases and having blood perfusion issues. Low perfusion performance and Motion Artifact Rejection have become standard features of every pulse oximeter design.

Now that you know how they work, have you ever needed to use a pulse oximeter? Any questions?

About the author
Jayaraman Kiruthi Vasan graduated in electronics and communication engineering from the Institution of Engineers in India. He currently is working with Healthcare Technology Innovation Centre, IIT, Madras as Consultant for Electromechanical Systems, where he is actively involved in medical device development from opportunity identification to market release of the product. With an experience spanning over 26 years, his major focus had been medical devices development, apart from brief stints in product marketing, technical support, and entrepreneurship.

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