Filtering techniques for the removal of ventilator artefact in oesophageal pulse oximetry

  • K. Shafqat
  • D. P. Jones
  • R. M. Langford
  • P. A. Kyriacou
Technical Note


The oesophagus has been shown to be a reliable site for monitoring blood oxygen saturation (SpO2). However, the photoplethysmographic (PPG) signals from the lower oesophagus are frequently contaminated by a ventilator artefact making the estimation of SpO2 impossible. A 776th order finite impulse response (FIR) filter and a 695th order interpolated finite impulse response (IFIR) filter were implemented to suppress the artefact. Both filters attenuated the ventilator artefact satisfactorily without distorting the morphology of the PPG when processing recorded data from ten cardiopulmonary bypass patients. The IFIR filter was the better since it conformed more closely to the desired filter specifications and allowed real-time processing. The average improvements in signal-to-noise ratio (SNR) achieved by the FIR and IFIR filters for the fundamental component of the red PPG signals with respect to the fundamental component of the artefact were 57.96 and 60.60 dB, respectively. The corresponding average improvements achieved by the FIR and IFIR filters for the infrared PPG signals were 54.83 and 60.96 dB, respectively. Both filters were also compared with their equivalent tenth order Butterworth filters. The average SNR improvements for the FIR and IFIR filters were significantly higher than those for the Butterworth filters.


Pulse oximetry Photoplethysmography Oesophagus Artefact Filtering 


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Copyright information

© International Federation for Medical and Biological Engineering 2006

Authors and Affiliations

  • K. Shafqat
    • 1
  • D. P. Jones
    • 3
  • R. M. Langford
    • 2
  • P. A. Kyriacou
    • 1
  1. 1.School of Engineering and Mathematical SciencesCity UniversityLondonUK
  2. 2.Anaesthetic Laboratory, St. Bartholomew’s HospitalBarts and The London NHS TrustLondonUK
  3. 3.Medical Electronics & Physics, Department of Engineering, Queen MaryUniversity of LondonLondonUK

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