Localisation of cardiac related impedance changes in the thorax

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Abstract
The existence of variations of normal human thoracic impedance, during the cardiac cycle to high frequency electrical current, is well known. Since the impedance variations within the thorax are synchronous with the electrocardiogram (ECG), they are attributed to cardiac activity. Recent advances in the clinical use of applied potential tomography (APT), or electrical impedance imaging, showed that the APT system gives a good soft-tissue contrast and has good sensitivity to resistivity changes. It is therefore concluded that the origin of thoracic impedance changes related to cardiac activity can be deduced from APT images. The authors' initial studies of ECG gated dynamic APT images of the thorax show that cardiac related thoracic impedance variations originating from different organs can be separated. Sequential APT images of the thorax during the cardiac cycle are presented. The movement of blood from the ventricles to the lungs and the vascular system and back to the ventricles is observable in these images. Limitations of the present APT system for monitoring cardiac related thoracic impedance changes and determining the cardiac parameters are summarised. The time relation between the mechanical activity of the heart and the variations in resistivity distribution within the thorax is discussed.

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