Origin of the impedance cardiogram investigated in the dog by exchange transfusion with a stroma-free haemoglobin solution

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Abstract
In an anaesthetized dog an exchange transfusion was carried out with stroma-free haemoglobin solution. The total circulating blood volume was kept constant. The heart-synchronous changes in the thoracic electrical impedance (Z o ) were measured before and after the exchange transfusion.Z o consists of a parallel connection of a tissue impedance (Z t ) and a blood impedance (Z b ). With the aid of this model forZ o the relative variations inZ b (ΔZ b /Z b ) were calculated from the relative variations inZ o (ΔZ o /Z o ). The marked decrease of ΔZ b /Z b during the experiment can only be explained by the fact that apart from the heart-synchronous changes in vascular volume the impedance changes are also caused by flow-dependent changes in the electrical conductivity of blood caused by variations in orientation of the erythrocytes.

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