Impedance Plethysmography gives an indirect assessment of central and peripheral blood flow non-invasively. Since blood is a good conductor of electricity, any change in blood volume in any part of the body is reflected in its electrical impedance. Thus a record of gross electrical impedance of the body segment, change in the impedance as a function of time and rate of change of impedance gives an indirect estimate of the central as well as peripheral blood flow. 
Change in the blood volume V in a body segment of volume V and electrical impedance Z is related to the change in the impedance Z by the Nyboer's equation This method has been observed to be 96% sensitive and 98% specific for the diagnosis of arterial occlusive disease and greater than 85% sensitive for the diagnosis of deep vein therombosis and valvular diseases of the heart. 

It has the following application areas:

Diagnosis of peripheral vascular disease: Aortic and arterial occlusive disease is detected with a sensitivity of 96% and specificity of 98%. Deep vein thrombosis is detected with a sensitivity of 85% and specificity of 83%. Severity of primary varicosity of the veins can also be assessed.

Monitoring of coronary artery disease: Cardiac output and left ventricular function can be monitored on-line in intensive care units in patients with coronary artery disease.

Assessment  of  valvular  and congenital heart disease: Aortic regurgitating fraction and mitral regurgitating fraction can be assessed in patients  with valvular disorders. Associated tricuspid- reguritation can also be detected. Pulmonary  hypertension  can  be  assessed  in  patients  with  valvular and congenital cardiac disorders.

Assessment  of  fluid retention: Fluid retention in any part of the body can be assessed from the  value  of  gross  electrical  impedance  of  the  body segment.  This  is  applicable  in patients with pulmonary cedema, nephrotic syndrome, lymphangitis and deep vein thrombosis.

Post  therapeutic  evaluation: This technique offers an excellent procedure for post therapeutic evaluation of all the conditions described above. Since the method is simple and noninvasive, it can be repeated any number of times on a given patient without causing any harm or discomfort.

The instrument employs constant current method for measurement of electrical impedance of the body segment. A sinusoidal current of frequency 50 kHz and constant amplitude 4 mA is passed through the extreme ends of the body with the help of two surface electrodes, called the current electrodes.

Voltage signal developed across any two points along the current path is sensed with the help of another pair of electrodes, called the voltage electrodes. The signal thus obtained is processed to yield gross electrical impedance of the body segment (Zo), changes in the impedance as a function of time Z and first time derivative of the impedance (dZ/dt). Phase locked signal averaging of these signals using a microcomputer (Intel 8085 based) gives average plethysmographic waveform with a signal to noise ratio of 30 dB and minimized respiratory artifacts. The average plethysmographic waveform can be recorded on a strip chart recorder.