3 Pulse Oximetry for the Measurement of Oxygen Saturation …
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the relative motion between the probe and the patient is arrested even when the patient
moves. By suitable processing of the PPG signals, the effect of motion artefact can
be reduced on the pulse oximetry readings. A simple solution to reduce error due to
motion artefact is to find and display the average of several SpO 2 readings.
3.11 Motion Artifact Reduction in PPG Signals
In pulse oximetry, computation of SpO 2 requires red and IR PPG signals. To obtain a
PPG signal, a sensor with two sources (one in the red wavelength and the other in the
IR wavelength) and a photo detector is placed on the body. If the PPG sensor is of the
transmission type, then the sources are kept on one side of an extremity such as finger
or toe and the detector on the opposite side (the finger or toe is sandwiched between
the sources and the detector). On the other hand, in reflective type PPG sensors, both
the sources and the detector are kept on the same plane. In both cases the magnitude
of the PPG obtained is dictated by the amount of light from the source that is coupled
to the body and the amount of light (transmitted or reflected) emanating from the
body, coupled to the detector. In order to ensure maximum light is coupled to the
body part, both the sources and the detector must be in close contact with the body.
To ensure close contact, the sources and the detector need to be pressed against the
body. However, applying pressure on the body causes:
(i) an increase in the temperature of the region underneath the sensor resulting in
pain and sweating and hence discomfort to the patient.
(ii) Pressure constricts the blood vessels, leading to reduced blood perfusion,
resulting in a reduction in the PPG signal.
Hence a PPG sensor in any pulse oximeter is designed to exert just bare minimum
pressure required to make a contact between the body and the sensor. Any movement by a patient connected to a pulse oximeter results in variations in the contact
between the sensor and the patient’s body. Variations in the contact corrupt the red
and IR PPG signals obtained during such periods of movement with motion artefacts.
SpO 2 computed using the motion artefact corrupted PPG signals would be erroneous.
Most pulse oximeters manufacturers solve this problem by simply suppressing the
output (with or without indication on the front panel of the oximeter) during such
periods. Some pulse oximeters simply display the last valid SpO 2 reading available
during such periods of movements. Even this simple technique of suppression during
periods, where the computation of SpO 2 is not possible, requires that the oximeters
are made capable of recognizing such periods. In other words, most of the processing
in present day pulse oximeters is geared towards artefact recognition rather than artefact reduction. In order to obtain uninterrupted readings from a pulse oximeter, the
red and IR PPG signals must be processed with a view to remove or reduce motion
artefacts, if any, present in the PPG signals. Several methods have been proposed to
reduce the influence of motion artefacts from corrupted PPG signals. A popular and
common technique employed for the reduction of the effect of the motion artefact in
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the relative motion between the probe and the patient is arrested even when the patient
moves. By suitable processing of the PPG signals, the effect of motion artefact can
be reduced on the pulse oximetry readings. A simple solution to reduce error due to
motion artefact is to find and display the average of several SpO 2 readings.
3.11 Motion Artifact Reduction in PPG Signals
In pulse oximetry, computation of SpO 2 requires red and IR PPG signals. To obtain a
PPG signal, a sensor with two sources (one in the red wavelength and the other in the
IR wavelength) and a photo detector is placed on the body. If the PPG sensor is of the
transmission type, then the sources are kept on one side of an extremity such as finger
or toe and the detector on the opposite side (the finger or toe is sandwiched between
the sources and the detector). On the other hand, in reflective type PPG sensors, both
the sources and the detector are kept on the same plane. In both cases the magnitude
of the PPG obtained is dictated by the amount of light from the source that is coupled
to the body and the amount of light (transmitted or reflected) emanating from the
body, coupled to the detector. In order to ensure maximum light is coupled to the
body part, both the sources and the detector must be in close contact with the body.
To ensure close contact, the sources and the detector need to be pressed against the
body. However, applying pressure on the body causes:
(i) an increase in the temperature of the region underneath the sensor resulting in
pain and sweating and hence discomfort to the patient.
(ii) Pressure constricts the blood vessels, leading to reduced blood perfusion,
resulting in a reduction in the PPG signal.
Hence a PPG sensor in any pulse oximeter is designed to exert just bare minimum
pressure required to make a contact between the body and the sensor. Any movement by a patient connected to a pulse oximeter results in variations in the contact
between the sensor and the patient’s body. Variations in the contact corrupt the red
and IR PPG signals obtained during such periods of movement with motion artefacts.
SpO 2 computed using the motion artefact corrupted PPG signals would be erroneous.
Most pulse oximeters manufacturers solve this problem by simply suppressing the
output (with or without indication on the front panel of the oximeter) during such
periods. Some pulse oximeters simply display the last valid SpO 2 reading available
during such periods of movements. Even this simple technique of suppression during
periods, where the computation of SpO 2 is not possible, requires that the oximeters
are made capable of recognizing such periods. In other words, most of the processing
in present day pulse oximeters is geared towards artefact recognition rather than artefact reduction. In order to obtain uninterrupted readings from a pulse oximeter, the
red and IR PPG signals must be processed with a view to remove or reduce motion
artefacts, if any, present in the PPG signals. Several methods have been proposed to
reduce the influence of motion artefacts from corrupted PPG signals. A popular and
common technique employed for the reduction of the effect of the motion artefact in