Impedance pneumography is an indirect method for the measurement of respiration. This measurement is performed by the impedance pneumograph which measures the respiratory volume and rate through the relationship between respiratory depth and thoracic impedance change. Impedance pneumography avoids burdening the subject with spirometers, tubes, and flowmeters; does not hinder respiration and has minimal effect on the physiological state of the subject.
During the measurement of the thoracic impedance change signal and ac excitation is applied to the subject. The choice of the optimum frequency for recording transthoracic impedance changes that accompany respiration is determined by two important factors:
- Excitability of the various tissues between the electrodes.
- Nature of other recordings made at the same time, notably those of bioelectric origin like the ECG.
The excitation frequency used is normally in the range 50-100 kHz with amplitude of the order of one milliampere peak-to-peak and a power of less than one milliwatt. The excitation is too high in frequency and too low in amplitude to stimulate the tissues. The signals of this frequency get attenuated in almost all biopotential amplifiers whose frequency response is limited to well below 10 kHz. In addition, a natural rejection of bioelectric events occurs when frequencies in the tens of kilocycles range are employed for the detection of impedance changes with respiration. For these reasons, a tuned amplifier can be used which has practically zero response for the spectrum of bioelectric events whilst at the same time provides high amplification for the carrier used for impedance pneumography.
The changes in transthoracic impedance during respiration ought to be independent of impedance resulting from the sum of the resting thoracic impedance and the contact impedance between the electrodes and the skin. This is achievable if a constant current is maintained through the subject over a large range of thoracic and electrode-skin impedance, which is likely to be encountered in practice.
Related: Key Considerations in Respiratory Measurements & Instrumentation Involved
The transthoracic impedance is a function of frequency and the type & size of electrodes. There are shortcomings in the use of the impedance pneumograph if absolute measurements of respiration are required. This is because the conversion of impedance change to lung-volume change is a function of electrode function, body size and posture.
