Bioelectric tissue impedance measurements to determine or infer biological information have a long history dating back to before the turn of the century. .BEI measurements are commonly used in apnea monitoring,especially for infants,and in the detection of venous thrombus. Many papers report the use of the BEI technique for peripheral blood flow, cardiac stroke volume, and body composition. Commercial equipment is available for these latter three applications, although the reliability, validity, and accuracy of these applications have been questioned and, therefore, have not received widespread acceptance in the medical community.
Most single frequency BEI measurements are in the range of 50 to 100 kHz (at these frequencies no significant electrical shock hazard exists) using currents from 0.5 to 4 mA RMS. Currents at these levels
If the BEI( bioelectric impedance) is measured across the thorax,a variation of approximately 1 to 2 ohm /l of lung volume change is observed, which increases with inspiration. The most common position of the electrodes for respiratory measurements is on each side of the thorax along the midaxillary line. The largest signal is generally obtained at the level of the xiphsternal joint although a more linear signal is obtained higher up near the axilla . The problems encountered with the quantitative use of BEI for respiration volume are movement artifacts and the change in the response depending on whether diaphragmatic or intercostal muscles are used. For most applications, the most serious problem is body movement and positional changes artifacts which can cause impedance changes significantly larger than the change caused by respiration. The determination of apnea or whether respiration has stopped in infants is one of the most widely used applications of BEI. For convenience and due to the lack of space on the thorax of infants,only two electrodes are used. These are placed at the mid-thoracic level along the midaxillary line and are also used to obtain the ECG. No effort is usually made to quantitate the volume change. Filtering is used to reduce movement artifacts and automatic gain controls and adaptive threshold detection is used in the breath detection circuits. Due to movement artifacts, the normal breath detection rate in infants is not highly reliable. When respiration stops,body movement ceases which eliminates the movement artifacts and then apnea can be detected. Ventation detection problems can occur if the airway is obstructed and the infant makes inspiratory movement efforts or cardiac-induced impedance changes are interpreted as a respiratory signal. Figure below shows a typical impedance measurement during an apneic period.
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| The four-electrode impedance measurement technique and the associated instrumentation |
Most single frequency BEI measurements are in the range of 50 to 100 kHz (at these frequencies no significant electrical shock hazard exists) using currents from 0.5 to 4 mA RMS. Currents at these levels
are usually necessary to obtain a good signal-to-noise ratio when recording the small pulsatile changes that are in the range of 0.1 to 1% of the total impedance.The use of higher frequencies creates instrumentation design problems due to stray capacity. BEI measurements in the 50-100 kHz range have typical skin impedance values 2–10 times the value of the underlying body tissue of interest depending on electrode area.
If the BEI( bioelectric impedance) is measured across the thorax,a variation of approximately 1 to 2 ohm /l of lung volume change is observed, which increases with inspiration. The most common position of the electrodes for respiratory measurements is on each side of the thorax along the midaxillary line. The largest signal is generally obtained at the level of the xiphsternal joint although a more linear signal is obtained higher up near the axilla . The problems encountered with the quantitative use of BEI for respiration volume are movement artifacts and the change in the response depending on whether diaphragmatic or intercostal muscles are used. For most applications, the most serious problem is body movement and positional changes artifacts which can cause impedance changes significantly larger than the change caused by respiration. The determination of apnea or whether respiration has stopped in infants is one of the most widely used applications of BEI. For convenience and due to the lack of space on the thorax of infants,only two electrodes are used. These are placed at the mid-thoracic level along the midaxillary line and are also used to obtain the ECG. No effort is usually made to quantitate the volume change. Filtering is used to reduce movement artifacts and automatic gain controls and adaptive threshold detection is used in the breath detection circuits. Due to movement artifacts, the normal breath detection rate in infants is not highly reliable. When respiration stops,body movement ceases which eliminates the movement artifacts and then apnea can be detected. Ventation detection problems can occur if the airway is obstructed and the infant makes inspiratory movement efforts or cardiac-induced impedance changes are interpreted as a respiratory signal. Figure below shows a typical impedance measurement during an apneic period.
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| Example of BEI respiration signal and ECG |
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