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.
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
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
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.