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Tuesday, 19 December 2017

Impedance pneumography

It is an indirect method used for the measurement of respiratory rate. It is comparatively simpler method because it does not require any placement of mask on the face, fixing of tubes etc. In this method we place only an electrode over the thorax region of patient.

We know that the impedance of the skin of thoracic region changes during respiration depending on the depth and rate of respiration. In this method we are actually measuring the change of impedance of the skin using typical circuits. The motion artifacts can be minimized using a self adhesive type electrode.

Block diagram of impedance pneumograph measurement technique is shown. Here a low voltage ac signal is applied to the chest of patient through surface electrodes. High value fixed resistors connected in series with each electrode create a constant ac current source. Usually the current through the patient’s chest is very small.

Advantages of impedance pneumograph:

1. Artifacts can be easily recognized
2. Electrically safe
3. Equipment is easy to use
4. The ability to obtain ECG from same electrode makes it useful during surgical anesthesia.
5. It is comfortable to the patient

The equivalent circuit of measurement technique is shown below.

The voltage drop across the resistance represents the patient’s thoracic impedance

E0 = I(R + ΔR), where E0 is the output voltage in Volts.
I = Current through the chest in Ampere.
R = Chest Impedance without respiration in Ohms.
ΔR = Change in chest impedance caused by respiration in Ohms.

The signal E0 is amplified by the ac amplifier and applied to a synchronous AM detector. Amplitude variations in E0 are caused due to change in resistance (ΔR) which changes the respiration waveform. A LPF is used after the synchronous detector to remove carrier signal. DC amplifier after LPF is used to increase the output waveform up to a level as required by the display device.

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