6. Impedance plethysmography .pptx

impedance plethysmography

1. Impedance plethysmography

2. Impedance Plethysmography(IPG) This method also named Rheography . It allows to measure arterial and venous blood volume changes in nearly each body segment (arms, legs, head, …) It works non-invasively and continuously and is suitable to be used for arterial and venous vascular diagnosis. The IPG is based on the measurement of the electrical impedance (resistance) of a selected body segment.

3. INTRODUCTION The IPG is based on the measurement of the electrical impedance (resistance) of a selected body segment. In comparison to other tissue, such as, muscle or bone, blood has a much lower impedance. Therefore, blood volume variations in a body segment correspond with measurable changes of the electrical impedance whereby an increase of the blood volume results in a lower impedance.

4. MEASUREMENT PRINCIPLE IPG relies on the fact that blood vessels expand and contract with each heartbeat, leading to changes in blood volume. These changes in blood volume cause alterations in the  electrical impedance  of the arteries. By applying a weak electrical current and measuring the resulting voltage through electrodes attached to the skin, IPG captures these impedance variations.

5. BLOCK DIAGRAM

6. ELECTRODES For the measurement of the electrical impedance usually 4 electrodes are applied to the body surface approximately in a line. The 2 outer electrodes (usually called current electrodes) are used to pass a very low and constant alternating current (1.5 mA, 86 kHz) through the body segment. The 2 inner electrodes (usually called measuring electrodes) are placed between the 2 current electrodes and measure the voltage which is caused when the current flows through the body segment. This voltage corresponds with the impedance of the body segment which changes depending on venous and arterial blood volume variations.

7. ELECTRODE PLACEMENT

8. EXPLANATION BLOCK DIAGRAM  shows a schematic outline of blood flow measurement using the electrical impedance in a segment of a limb. A constant  AC current  of 0.1–10 mA and 20–200 kHz is supplied to the outer electrodes, and the voltage is measured at the inner electrodes. The impedance,  Z , of the limb segment is defined as the ratio of the voltage across the segment and the supplied current .  

9. The reciprocal of Z is the admittance, Y. Measurement of blood volume change in the limb by electrical impedance is based on the parallel conductor model

10. PARALLEL CONDUCTOR MODEL

11. Consider a limb segment of length  L , volume  V , and tissue resistivity  ρ . The cross-sectional area,  A , is assumed to be constant along the segment axis such that  A  =  V / L . When additional blood of volume  V b  and resistivity  ρ b  is added to this segment, the parallel conductor model postulates that the blood is distributed uniformly in the segment forming a parallel conductor of length  L , cross-sectional area  A b  =  V b / L , and resistivity  ρ b . DERIVATION AND EXPRESSION

12. and the impedance,  Z , composed of parallel conductors is expressed by

13. This equation implies that the blood volume added to the segment can be estimated by the change in impedance due to the increase in blood volume as long as the blood resistivity is known. A similar equation can be derived from the admittance change Δ Y  as In the actual human limb, the validity of the parallel conductor model when applied to the venous occlusion method was confirmed.

14. BLOOD FLOW IN ARTERIES 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.

15. Impedance Arteriography pulse transit time (PTT)  - represents the time between two pulses measured at two locations Pulse Arrival Time(PAT) is the time it takes for the pulse to travel from the heart to a peripheral artery

16. BLOOD VOLUME IN V ENOUS 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. 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.

17. IMPEDANCE VENOGRAPHY

18. IPG

19. ADVANTAGES Noninvasiveness : IPG is a noninvasive technique, meaning it doesn’t require any surgical incisions or invasive procedures. Deep Artery Measurement : Unlike some other methods, IPG allows the observation of a pulse in every artery, regardless of its depth.  Miniaturization Potential :  This simplicity enables the development of miniaturized devices, which can be portable and convenient for various applications

20. ADVANTAGES

21. DRAWBACK   The  drawback  is found to be in the motion artefacts caused by the slight movements of the hand, thus decreasing accuracy and also that a regulator/potentiometer is required for signal adjustment.

22. CLINICAL APPLICATIONS IPG is commonly used for peripheral vascular studies, assessing blood flow and volume changes. It provides valuable information about venous return, arterial pulsations, and overall vascular health. Researchers and clinicians utilize EIP to study conditions such as deep vein thrombosis, venous insufficiency, and peripheral artery disease.

23. APPLICATIONS