Impedance plethysmography; a physical analysis for quantitative blood flow studies

Plethysmography, or volume recording, plays an important part in the evaluation of circulation in human patients. There are several techniques used: indirect mechanical methods involving fluid displacement; indirect photoelectric methods involving transmission and/or reflection of light; indirect electrical methods Involving the correlation of impedance changes with blood volume changes. The impedance plethysmograph is evaluated in this paper with respect to carrier frequency and electrode characteristics. A comparison is made between measured results and values predicted on the basis of physical analyses of tissue impedance. It is shown that volume variations may be reproduced by direct air volume displacement measurements and by impedance measurements at frequencies of 100, and 175 kilo-cycles per second. Special techniques and calculations are demonstrated whereby the impedance plethysmograph may be used for quantitative measurement of blood flow rates, pulse volumes, elasticity of vessels and circulation times.