The influence of alternating current frequency on flow related admittance changes of blood: a concept for improvement of impedance cardiography.

Impedance cardiography is based on admittance changes induced by volume changes of the intrathoracic blood vessels, but also by the longitudinal orientation of red blood cells induced by flow. An experiment was set up to separate these two phenomena and to study their frequency dependence. Admittance changes of flowing blood with variable haematocrit, of a saline solution and of plasma were measured in an in vitro set-up. Four different alternating current frequencies were used: 100 kHz, 5 MHz, 15 MHz and 20 MHz. The measured admittance appeared to be dependent on blood flow: when blood flow increased, admittance in the longitudinal direction increased. This increase was stronger for higher haematocrits, 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 change was detected when saline or plasma was used as the perfusate. It is concluded that the orientation effect can be neglected at high frequencies. Impedance cardiography in this range will give more reliable information about volume changes.

Implications of the dielectrical behaviour of human blood for continuous online measurement of haematocrit.

A study was designed to explore the possibility of detecting the haematocrit of blood by means of admittance measurements. The admittance and phase angle of blood kept in a measuring cell were determined at various frequencies between 60 kHz and 24 MHz. A reliable and accurate estimation of haematocrit was obtained in two ways. First, low-frequency admittance, high-frequency admittance and a factor x, which was the conductive percentage of cell content, were used. Secondly, the maximum phase angle was used. Both methods can be applied to obtain continuous on-line information about haematocrit for blood volume control during haemodialysis.

The desensitizing interaction of hexafluorenium with the cholinergic receptor in the diaphragm of the rat.

1. The interaction of hexafluorenium with acetylcholine, carbachol and suxamethonium with regard to the depolarization of the end-plate of rat diaphragm was studied. The depolarization was measured with the moving meniscus technique of Fatt. The competitive antagonist d-tubocurarine was included in the studies. 2. Hexafluorenium inhibited acetylcholinesterase in the end-plate. 3. The receptors in the end-plate were desensitized by carbachol and suxamethonium. Hexafluorenium enhanced the desensitization by suxamethonium. d-Tubocurarine had no direct influence on desensitization 4. The desensitization of the receptors in rat diaphragm is compatible with a cyclic model of desensitization. 5. The desensitizing interaction of hexafluorenium with the receptors may explain the non-competitive antagonism with depolarizing drugs with regard to the depolarization of the end-plate and the synergism with these drugs with regard to the paralysis of the indirectly stimulated muscle reported earlier. 6. The affinities of carbachol, suxamethonium and d-tubocurarine to the sensitive receptors are 4.9, 5.2 and 6.8, respectively.