Movement artefact rejection in impedance pneumography
Impedance pneumography is a non-invasive and a very convenient technique for monitoring breathing. However, a major drawback of this technique is that it is impossible to monitor breathing due to large artefacts introduced by the body movements. The aim of this project was to develop a technique for reducing these 'movement artefacts'. In the first stage of the project, experimental and theoretical studies were carried out to identify an 'optimum' electrode placement that would maximise the 'sensitivity' of measured thoracic impedance to lung resistivity changes. This maximum sensitivity was obtained when the drive and the receive electrode pairs were placed in two different horizontal planes. This sensitivity was also found to increase with increase in electrode spacing. In the second stage, the optimum electrode placement was used to record thoracic impedance during movements. Movement artefacts occurred only when the electrodes moved from their initial location along with the skin, during movements. Taking into consideration these observations, a strategy was decided for placing 4 electrodes in one plane so that movement artefacts could be reduced by combining the two independent measurements. Further studies showed that movement artefacts could be reduced using a strategic 6- electrode placement in three dimensions. It was also possible to detect obstructive apnoea, as the amplitude of the breathing signal was higher than that due to obstructive apnoea and this difference was statistically significant. In these studies, the main cause of movement artefacts was identified as the movement of electrodes with the skin. A significant reduction in movement artefacts was obtained using the 6-electrode placement. This advantage of the 6-electrode placement proposed in this project, can be of great use in clinical applications such as apnoea monitoring in neonates. Further studies can be carried out to determine an optimum frequency of injected current to achieve reduction in residual movement artefacts.