Technical note Reduction of motion artefacts during paediatric/infant apnoea monitoring M. J. Mayotte J.G. Webster W.J. Tompkins Department of Electrical Engineering, University of Wisconsin, 1415 Johnson Drive, Madison WI 53706, USA Keywords--Apnoea, Electrodes, Impedance, Monitors Med. & Biol. Eng. & Comput., 1996, 34, 93-96 1 Introduction APNOEA IS generally agreed to be the cessation of airflow for more than 20 s (POTSlC, 1989; GUILLEMINAULT et al., 1975), and it has been reported to occur in 30-95% of premature infants during sleep (BOUTERLnVE-YOUNG and SMITH, 1950). It has been proposed that this may be due to 'a failure to respond to the metabolic and mechanical stress imposed by the first experience with nasopharyngitis, or a dysfunction of the autonomic nervous system due to a transient anatomic or functional defect of the neuron or a temporary deficiency of a metabolite or neurotransmitter' (KELLY and SHANNON, 1979). Such vulnerabilities decrease with age (KELLY et al., 1978). Owing to the possible connection between sleep apnoea and Sudden Infant Death Syndrome (SIDS), infants believed to be at risk of SIDS are monitored for apnoeic episodes. As instances of bradycardia are common in central apnoea ('WARBURTON et aL, 1977; KULKARa, 1991; SAHAKIAN et aL, 1985), a combination of impedance pneumography and electrocardiography is the most commonly used acquisition system for apnoea detection (SAHAKL~ et al., 1985). Impedance pneumography, however, is prone to several problems including susceptibility to false breath detections due to the subject's motion artefacts, difficulty in detecting obstructive apnoea, changes in intrathoracic blood volume and venous return (which cause changes in the thoracic impedance), and cardiogenic artifacts (WARBURTON et al., 1977; NnacDc, 1987; UPTON et aL, 1990; JEFFERY et al., 1981). Movement artefacts, cardiogenic artefacts or breathing efforts during obstructive apnoea can cause fluctuations in the chest wall impedance that may resemble ventilatory fluctua- tions, and thus may be mistaken for ventilation. Although some attention has focused on breath-detection algorithms, much less attention has focused on the types of electrodes. Most electrodes used for ventilation monitoring were designed for ECG monitoring. Thus determining which factors are beneficial for ventilation monitoring may improve electrode design for apnoea monitoring, thereby improving impedance pneumography. Previous studies have shown that electrodes with good stability, strong adhesion, low face-to-face impedance, low transthoracic-plus-electrode impedance and a large effective Correspondence should be addressed to John G. Webster. First received 12 July 1994 and in final form 31 May 1995 9 IFMBE: 1996 area are optimal for impedance monitoring. In addition, a previous study (M.AYOTTE et al., 1994) indicated that a large area surrounding the conductive disk may also be optimal. The focus of this study is to investigate this suggestion. We have evaluated three electrode types used with infant apnoea monitors by comparing their signal-to-motion artefacts ratio (SAR) on adult subjects with and without a 5 x 5 cm piece of *tape in order to evaluate the effect of increasing the area surrounding the conductive disk. Our reasons for using adult subjects rather than infants are as follows. (i) It would be extremely difficult to obtain a significant sample of infants to be subjected to a study involving such a large number of electrode types and trials. (ii) It would be difficult or uncomfortable for the infants to perform the required protocol. (iii) It is both necessary and important that the motion protocols are performed in a repetitive fashion in order for the results to be valid. (iv) Our preliminary studies have indicated that, although different body geometries may have differing SAR magnitudes, the overall results on an individual subject have the same basic trends when comparing the different electrode types. This study shows that some of the electrode characteristics themselves have an effect on the SAR, independent of the body geometry. Although paediatric patients were not subjects in this study for the above reasons, it is likely that the general results (i.e. SAR improvement with an increased adhesive area surrounding the conductive electrode disk) of this study are applicable for paediatric subjects. The magnitude of SAR improvement, however, cannot be inferred from the adult data presented in this work. O<c c a b Fig. 1 o \\ , Oc--c c d (a) subject's position before movement; (b) subject with stretched out arms on both sides of head in preparation for movement; (c) subject during one arm movement; (d) subject's position at the end of one arm movement * 3M Transpore is a registered trademark. Medical & Biological Engineering & Computing January 1996 93