G. Hahn et al., CHANGES IN THE THORACIC IMPEDANCE DISTRIBUTION UNDER DIFFERENT VENTILATORY CONDITIONS, Physiological measurement, 16, 1995, pp. 161-173
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.