Question

Design an impedance plesmythography device that measures changes in electrical resistance across the chest with respiration. Compare the impedance plesmythography device with the pneumatic compression belt-triggering type.

Answer 1

- In Four electrode impedence plethysmography, current is injected through two outer electrode, and voltage is measured through inner two electrodes

- Amplification and demodulation yield Z+Delta Z

- A balancing voltage Vb is applied to produce desired delta Z.

- In the automatic reset system, when saturation of Vo occurs, the comparator commands the sample and hold to sample Z+Delta Z and hold it as Vb.

- This resets the input to the final amplifier and Vo zero.

- Further changes in Delta Z causes changes in Vo without saturation