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SUMMARYVariations of transthoracic electrical impedance and their relations with physiological events within the thorax were studied by using electrodes applying constant sinusoidal current at 55kHz on bilateral chest walls of anesthetized dogs during breath holding. The level of the impedance lowered during decrease of oxygen and elevated during ether gas evaporation in the lungs with steady pulmonary blood flow. The impedance decreased when concentrated saline was selectively infused into the pulmonary vasculature but not into the left heart and the great vessels. The impedance levels also were related inversely with the change of blood flow and resultant blood volume in the lung when the pulmonary blood flow was decreased as during occlusion of the venous return to the right heart, pulmonary trunk and unilateral pulmonary artery. The impedance levels on the bilateral chest walls varied independently by manipulations mentioned above which was employed in the individual lung of each hemithorax. These results clearly suggest that the current flux applied on the chest wall well penetrates through the chest cage and passes preferentially through the lung parenchyma, the conductivity of which depends on the changes in the amount of electrically conductive or resistive materials associated with physiological events within the lungs.

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