THE INFLUENCE OF ALTERNATING-CURRENT FREQUENCY ON DOW RELATED ADMITTANCE CHANGES OF BLOOD - A CONCEPT FOR IMPROVEMENT OF IMPEDANCE CARDIOGRAPHY

Impedance cardiography is based on admittance changes induced by volum e changes of the intrathoracic blood vessels, but also by the longitud inal orientation of red blood cells induced by flow. An experiment was set up to separate these two phenomena and to study their frequency d ependence. Admittance changes of flowing blood with variable haematocr it, of a saline solution and of plasma were measured in an in vitro se t-up. Four different alternating current frequencies were used: 100 kH z, 5 MHz, 15 MHz and 20 MHz. The measured admittance appeared to be de pendent on blood Bow: when blood flow increased, admittance in the lon gitudinal direction increased. This increase was stronger for higher h aematocrits, probably due to the longitudinal orientation of the blood cells. At higher frequencies, the orientation effect of the red cells became negligibly small. No frequency or flow dependent admittance ch ange was detected when saline or plasma was used as the perfusate. It is concluded that the orientation effect can be neglected at high freq uencies. Impedance cardiography in this range will give more reliable information about volume changes.