Fir Filter Design for Removal of Ventilator Artefact in Oesophageal Photoplethysmographic Signals

The oesophagus has been found to be a reliable monitoring site for blood oxygen saturation (SpO2) in anaesthetised patients. Despite it being a very well perfused organ, it was not possible to estimate SpO2 in the lower to deep oesophagus due to movement artefact caused by the mechanical ventilator. This limitation made the measurements more difficult since the probe had to be placed carefully at a depth where the magnitude of the ventilator artefact was less than 30% of the oesophageal photoplethysmographic (PPG) amplitude. To overcome this limitation, two filters, a 384 order FIR Equiripple linear-phase filter and a 10 order Butterworth bandpass filter, were implemented and compared. The Equiripple filter performed better than the Butterworth filter in terms of attenuation and phase characteristics. This Equiripple filter achieved an attenuation of about 80 dB in the stopbands which significantly reduced the ventilator artefact without changing the morphology of the PPG signal. Such a filter should allow the monitoring of SpO2 within the whole length of the oesophagus.