Reduction of motion artifacts using a two-frequency impedance plethysmograph and adaptive filtering

Area of Science:

• Biomedical Engineering
• Physiological Measurement

Background:

• Transthoracic impedance plethysmography is a non-invasive method for monitoring respiration.
• Motion artifacts can significantly degrade the signal quality in impedance measurements.

Purpose of the Study:

• To evaluate the effectiveness of a two-frequency plethysmograph in differentiating respiratory signals from motion artifacts.
• To develop and test an adaptive filter to enhance the signal-to-artifact ratio (SAR) for improved respiratory monitoring.

Main Methods:

• Transthoracic impedance was measured in healthy adults using a two-frequency (57 kHz and 185 kHz) plethysmograph.
• Subjects underwent breathing and motion maneuvers, including simulated obstructive apnea.
• Cross-correlation and amplitude ratios between frequencies were analyzed.
• An adaptive filter was designed and applied to improve SAR.

Main Results:

• High cross-correlation (0.97-1) was observed during normal breathing.
• Minimal cross-correlation (0.81) occurred during simulated obstructive apnea.
• Distinct amplitude ratios between frequencies differentiated breathing from motion.
• The adaptive filter significantly increased SAR: 183% for arm movement, 133% for leg movement, and 34% for apnea.

Conclusions:

• Two-frequency impedance plethysmography can effectively distinguish respiratory signals from motion.
• An adaptive filter substantially improves the signal-to-artifact ratio, enhancing the reliability of impedance-based respiratory monitoring.