IMPEDANCE PLETHYSMOGRAPHY Dr. Chintan Parmar 2nd Year Resident Dept. Of Physiology Govt. Medical College, Bhavnagar Dt. : 11 / 09 / 11

OUTLINE Introduction History Principle Technique Advantages Applications Limitations

Introduction Impedance PlethysmoGraphy (IPG), is a non - invasive medical test that measures small changes in electrical resistance of the chest, calf or other regions of the body. These measurements reflect blood volume changes, and can indirectly indicate the presence or absence of Cardiac, Venous or Arterial pathology. This procedure provides an alternative to venography or arteriography, which is invasive.

Cardiac For the chest, the technique was developed by NASA to measure the split second impedance changes within the chest, as the heart beats, to calculate both Cardiac output and lung water content. This technique has progressed clinically, often now called BioZ ( Biologic Impedance ), as promoted by the leading manufacturer in the US and allows low cost, non-invasive estimations of Cardiac output and total peripheral resistance. It uses 4 pair of Surface Electrodes.

Arterial For Arteries, blood flow in any area like thigh, calf, arm or forearm can be measured by Impedance Arteriography. Pulsatile flow of arteries will give characteristic Electrical Resistance that can be recorded as a Graph. Along with simultaneous recording of ECG, parameters like PAT, PTT can be measured.

Venous For leg veins, the test measures blood volume in the lower leg due to temporary venous obstruction. This is done by inflating a cuff around the thigh to sufficient pressure to cut off venous flow but not arterial flow, causing the venous blood pressure to rise.

Venous When the cuff is released there is a rapid venous return and a prompt return to the resting blood volume. Cuff Inflation & Deflation will alter the Electrical Resistance of respective region, that will give a characteristic Graph. Delayed emptying of veins in any venous pathology will change the normal response.

History The history extends back to 1932 when Atzler & Lehmann observed changes in the Capacitance between 2 parallel plates kept across the human chest. These changes were observed to be Synchronous with the activity of the Heart. The technique that exists today was first introduced by Jan Nayboer & co – workers in 1940.

History In the year 1978, the first Impedance Plethysmograph System, developed at Electronics Division, BARC. It was taken to Department of Surgery, Seth G.S. Medical College & K.E.M. Hospital and Department of Medicine, Grant Medical College & J.J. Hospital, Mumbai. It was used for the assessment of central and peripheral blood flow in the human body.

History

Principle

Principle In this technique, the ELECTRICAL IMPEDANCE of any part of the body is measured by constant current method & variations in the impedance are recorded as a function of time. Since blood is a good conductor of electricity, the amount of blood in a given body segment is reflected inversely in the electrical impedance of the body segment.

Principle Pulsatile blood volume by heart , that is systemic blood circulation causes proportional decrease in the electrical impedance. Variation in the electrical impedance thus gives adequate information about the blood circulation.

Technique Constant current is passed through the body segment of interest with the help of 2 surface electrodes. Voltage signal devloped along the current path is sensed with the help of another pair of electrodes. The amplitude of the signal sensed is directly propotional to the elecrical impedance of the body segment. Amplification & detection of this signal gives instantaneous electrical impedance Z of the body segment.

Technique

Technique Difference between the instantaneous electrical impedance & initial value of electrical impedance ( Zo ) gives variation in the impedance as a function of time, called the ∆Z(t) waveform. First time derivative of the impedance ( dZ / dt ) is obtained to give the rate of change of impedance. With the help of this dZ / dt, used in different equation, peripheral arterial - venous blood flow & stroke volume can be measured.

Impedance Cardiography

ICG

ICG Left ventricular ejection time (LVET) is a useful measure of ventricular performance and preload.

ICG Stroke volume is then estimated using the Kubicek formula Stroke volume = k p ( L / Zo )² [ LVET ( dZ / dt ) max ] where k is a constant which accounts for variation in body composition based on age, gender, relative fat content, chest circumference; L is the inter - electrode distance; p is the blood specific resistivity computed using hematocrit as [13.5 + (4.29 Hematocrit) ].

Impedance Arteriography

Impedance Arteriography

Impedance Venography

Impedance Venography

Advantages Simple Cheap Effective Portability Repeatability

Applications Research Screening Vascular Diagnosis Continuous Hemodynamic Monitoring - ICU Pharmacological studies - OPD Adjustment of Pacemaker settings

Limitations Colour Doppler Echo Cardiography

Thank You…. Jai Hind..!!!