IMPEDANCE PLETHYSMOGRAPH

Typical Number in Hospital: 1

Cost Bands: 4

References: 3

The electrical impedance between two electrodes placed on the skin surface will vary in response to volume changes in the tissue between them. electrodes placed at two points down the leg will record electrical impedance which will vary in time with arterial pulsations, and similarly, electrodes on the chest can be used to sense changes in resistance caused by different blood volumes in the chest at various parts of the cardiac cycle. Changes in volume due to breathing can also be detected.

The resistance is normally sensed using alternating currents between 1 and 100 kHz, with a constant current drive of (say) 100 [u]A. The impedance measurements may be made by two- or four- electrode techniques. The four-electrode technique partially eliminates electrode-skin impedance problems.

Electrical impedance plethysmography has a number of theoretical problems in application, but its most important areas of potential use (cardiac output estimation and peripheral blood flow) are alternatives to invasive and dangerous procedures. It seems likely therefore that the development of the subject will continue until the technique is sufficiently reliable to replace some of these other techniques.

Such devices might be found in the cardiology department or in the vascular surgery unit.