Electrical impedance plethysmography; a physical and physiologic approach to peripheral vascular study.

The quantity of blood measured by electrical impedance plethysmography is defined by its resistive effect in parallel to the resistance of other tissue of the segment. By substitution of this parallel resistive value, together with data relative to the resistivity of blood and the length of the segment in the formula for the volume of an electrical conductor, we are able to derive the volume of the pulse in cubic centimeters. It follows that the volume displaced from the venous reservoir and the rate of refilling of the venous reservoir of an extremity may also be determined quantitatively. N UMEROUS attempts and methods have been evolved to measure pathologic and functional vascular change in the extremities. Little attention, however, has been given to the quantitative changes recordable by various methods of electrical plethysmogra-phy,'-5 which until recently have been difficult to formulate. The current presentation deals principally with the calculation and interpretation of volemic changes in the extremities as they appear measurable by electrical conductivity. The electrical conductivity method gives a physical measure of the ionic conduction of a given body segment in contrast with electronic conduction characteristic of metallic substances. An attempt will be made to restate and formulate the laws of electrical conduction as they appear applicable to changes within a body segment which is being studied by the passage of a radio frequency current. As has been shown elsewhere,6 transient and static values of electrical conductivity are associated,erans Administration, who assumes no responsibility for the opinions expressed or conclusions drawn by the author. 811 respectively, with dynamic and balanced conditions of arteriovenous blood volume differences within a given segment. METHOD A differential electrical impedance bridge operating at 150 to 200 kilocycles, as described elsewhere ,6 is used as the basic instrument for measurement of small and large variations in volume of the extremities. In this study, our measurements on human extremities are made with aluminum foil strips 1 cm. wide, mounted on a gummed tape 2 cm. wide. After wetting the foil with a concentrated salt paste (non-drying), the electrodes are applied firmly without preliminary massage or washing. This tends to avoid local circulatory reactions which may modify our measurements. The electrodes are usually applied circumferentially and held loosely by a clip lead to avoid constriction (fig. 1). There appears to be an inherent advantage in a four electrode technic as shown later. The two outer electrodes (I, …