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Motion Artefacts and Signal Extraction Technologies


Much is said about new generation pulse oximeters and immunity to motion artefacts. Whilst this type of technology makes it possible to minimize the impacts of patient movements on the reliability of the pulse oximeter system, it does not make the system immune to inaccurate pulse oximeter sensors. The accuracy of the system is still totally dependant on the accuracy of the sensor in use.

Pulsing of the blood is not the only source of signals that produce a signal in the electrical circuitry of the detector, and these additional signals are unwanted as they can mask the signal of interest. Many of these unwanted signals can be removed by electronic and software filtering techniques. However there are other signals that resemble heart generated pulsatile signals that can not be easily removed by filtering. If one takes an analogy of separating particles by size, it is easy to imagine that different particles of the same size can not be separated in this way. The same problem is encountered when trying to separate signals generated by the pulsing of the blood and signals generated by movements of the patient. Often heart rate and movements of the arms and legs are at similar frequencies and can not readily be separated by conventional signal processing or filtering techniques. These unwanted signals are frequently called motion artefacts.
We encountered this problem in the late 1980s when we designed our first pulse oximeter system. We analysed the signal in terms of frequency using a technique known as Fast Fourier Transform (FFT) analysis. We could not separate heart rate and pedometry signals caused by feet hitting the ground.

In the diagrams the red signal is the signal of interest, whilst the green signal is generated by movements of the patient and is not wanted.

We noticed that if we took a simultaneous signal from the veins we still got the same unwanted signal or noise, but we did not get the heart rate signal as the veins were not pulsatile.

We then subtracted the venous signal from the arterial signal and removed the unwanted noise signal. In May 1990 this became The World's first patent in Adaptive Noise Cancellation in Pulse Oximetry - subsequently sold to Masimo Corporation, California.








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