CHANGES IN THE THORACIC IMPEDANCE DISTRIBUTION UNDER DIFFERENT VENTILATORY CONDITIONS

The present study was performed with the aim of checking the suitabili ty of EIT in imaging regional thoracic impedance variations during lun g ventilation under predefined conditions and to compare EIT with esta blished reference techniques. A new technique of functional EIT imagin g designed to visualize physiologically relevant information from the sequentially registered series of thoracic impedance distributions. wa s introduced. Experiments were performed on five spontaneously breathi ng healthy subjects and on 12 anaesthetized supine pigs. 16 electrodes were placed around the thorax and consecutive transthoracic impedance distributions were measured at a rate of 1 Hz (Sheffield; APT. system mark I, IBEES, Sheffield, UK). Several voluntary breathing manoeuvres were performed. in human subjects and the tracings of local impedance were compared with standard spirometry. In animal experiments EIT was applied during artificial ventilation at different ventilation rates and during stepwise passive emptying and filling of either one or both lungs while the respiratory muscles were relaxed. Further, selective blockade of lung regions resulting in regionally reduced ventilation w as performed and the capability of EIT to follow and differentiate loc al ventilatory disturbances was checked by reference techniques (x-ray and staining methods). The experiments revealed an overall agreement between the spirometric and impedance data in all breathing patterns p erformed. A linear relationship between changes of the air content of the lungs and the regional thoracic impedance was shown (intraindividu al correlation coefficient range, 0.986-0.999; n = 12 animals). The fu nctional images of the impedance distribution across the thorax reprod uced adequately the typical anatomical characteristics of the pig and the human thorax. The spatial resolution of EIT functional images was sufficient to differentiate lung areas corresponding to approximately 20 ml tissue volume. EIT with the additional evaluation procedure of f unctional imaging was shown to be a suitable and reliable method of im aging different ventilatory conditions with the potential to become a useful tool for monitoring respiratory function.