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Technical aspects of impedance plethysmography.
Correspondence Address:
This paper describes the basic methods for measurement of body impedance, electrodes and their configuration, and the measuring instrument with its limitations. A microcomputer assisted impedance plethysmograph system, developed at BARC and different lead configurations for impedance plethysmographic investigation are also described. Typical impedance plethysmographic waveforms recorded from a normal subject and measurement of their amplitude and various time intervals are illustrated.
The methods employed for the measurement of electrical impedance of the body segment are 1) constant current method and 2) bridge method. In constant current method a sinusoidal current of constant amplitude is passed through the body segment and the voltage signal developed along the current path is sensed and processed to yield the impedance of the body segment. In bridge method the body impedance forms one arm of the wheat-stone bridge and is compared with resistance and capacitor of known values connected in series forming another arm of the bridge. Though this method is more accurate, it is lengthy and therefore, constant current method is preferred in clinical environment. The frequency of the sinusoidal current, generally employed for measurement of body impedance, is kept between 20 kilo Hertz and 100 kilo Hertz. This is because at high frequencies, above 1 mega Hertz, biological tissues have resistive and dielectric properties and at even higher frequencies they behave as dielectrics[2]. The lower limit is placed at 20 kHz so that impedance measurement does not interfere with the frequency range of ECG, EEG and EMG signals, and also to prevent stimulation of muscles and nerves with current intensities upto a few milliamperes[2]. The amplitude of the current employed in impedance plethysmographic measurement is chosen between 1 mA and 10 mA. The passage of this current through the body in the frequency range of 20 kHz to 100 kHz is considered completely harmless as the perception threshold at these frequencies is 30 mA and 200 mA respectively[4]. Furthermore passage of thousand fold higher current in dogs in this frequency range has failed to produce any discernible change in their electrocardiograms.
Impedance plethysmography usually employs band electrodes applied on the body surface in the form of loop around the body segment or spot electrodes similar to those used in electrocardiography. When band electrodes are used, current density distribution no more remains uniform as the current lines get converged at the electrode-body interface. Baker [2] has shown that band electrodes, however, provide a uniform current distribution in the central portion of the body segment[2]. The potential difference sensed with the help of another pair of electrodes from the central portion of the body segment therefore gives the impedance of the body segment between sensing electrodes with minimum error. This configuration of four electrodes, namely tetrapolar, is preferred over the use of only two electrodes (bipolar) due to uniform current density distribution in the central region of the body segment in the former. However, in situations where placing four band electrodes is not practically feasible, bipolar configuration can be employed. Spot electrodes have been shown to give even more non-uniform current distribution[2] and are, therefore, avoided.
[Figure - 1] shows block diagram of an impedance plethysmograph (IPG) system developed at Electronics Division of Bhabha Atomic Research Centre. The special features of this system are calibration [5] for dZ/dt and on-line ensemble averaging of IPG signal. It is comprised of a basic IPG unit, an interface unit and a signal processor unit as described below. The basic IPG unit is comprised of a sinusoidal oscillator operating at a frequency of 50 KHz. The output of the oscillator is fed directly to voltage to current converter in patient mode and through an analog multiplier in calibration mode. The second input of analog multiplier is fed with a triangular waveform (amplitude 100 mV) superimposed on 9.9 volts DC signa1[5] as shown in the figure. The voltage to current converter outputs sinusoidal current to the current electrodes (I1, I2) in patient mode and to a resistance network in calibration mode. The amplitude of this current is constant (6 mA) in patient mode and varies between 5.97 mA to 6.03 mA in a triangular wave pattern in calibration mode. The value of calibration resistance Ro is kept as 25 ohms. The voltage signal sensed with the help of voltage electrodes (V1, V2) is fed to an amplifier in patient mode. In calibration mode the signal developed across Ro is fed to the amplifier. The amplified signal is detected using a precision rectifier for obtaining the voltage output which is proportional to the instantaneous electrical impedance of the body segment (Z-signal). The Z-signal is fed to a differentiator for obtaining dZ/dt signal. It is also fed to a difference amplifier, the other input of which is the basal impedance (Zo) obtained from signal processor unit. Thus, the output of the difference amplifier is the increment-Z (t). The output of the differentiator (dZ/dt) and that of the difference amplifier is connected to an amplifier followed with a buffer through mode selection switch as shown in the figure. The Z signal and the buffered output [dZ/dt or increment-Z(t)] is connected to the interface unit for time-locking and multiplexing of these signals. The amplified ECG signal obtained from ECG amplifier is also given to the interface unit for R-wave detection. The interface unit is comprised of an R-wave detector circuit, R-pulse generator and the multiplexer. In patient mode the output of R-wave detector changes its state on occurrence of the R-wave in the ECG signal. In calibration mode this circuit is triggered by the falling edge of the square wave from the 1 Hz waveform generator. This circuit does not give a false positive or false negative output as long as the signal to noise ratio of ECG amplifier is more than 6 dB and the ECG input is not less than 300 microvolts. The R-wave detector output triggers two monostables of durations 10 ms and 5 micro-s respectively. The 10 ms monostable controls the multiplexer so that it outputs Z signal for 10 ms after each R-wave and 1PG signal for rest of the cardiac cycle. The 5 micro-s monostable output, hereafter referred to as R-pulse, and the multiplexer Output F(Z) are connected to the signal processor unit. The signal processor unit is comprised of an 8-bit microprocessor (intel 8085), 2K bytes of EPROM, 6K bytes of RAM, six input-output ports, an analog to digital converter (ADC), two digital to analog converters (DAC), 3 digit numeric display and a small key board. R-pulse and F(Z) from interface unit are connected to RST 7.5 input of microprocessor and analog input of ADC respectively. The start conversion (SC) signal for ADC is provided by the microprocessor through SOD line. The output of one of the DACs (DAC-DZ) is connected to a strip chart recorder for recording the processed IPG waveform. The output of the other DAC (DAC-Z) which outputs basal impedance (Zo) is connected to one of the inputs of increment-Z amplifier of the basic 1PG unit. Keyboard is comprised of six keys, five of them are connected to an input port through a decoder. Four of these keys are labelled as F1, F2, F3 and F4 for selection of the program to be executed by the microprocessor. F1 selects on-line ensemble averaging for any number of cardiac cycle upto 240; F2 selection causes the microprocessor to out on-line average of preceding 8 cardiac cycles; F3 causes microprocessor to output raw IPG waveform as it is and to plot amplitude of C-wave, amplitude of O-wave and R-R interval for each IPG cycle at the end of data acquisition and F4 selection instructs the microprocessor to record an occlusive impedance phlebogram. The fifth key called SDA is used for stopping the data acquisition by the microprocessor. The sixth key is the system 'Reset' key and is connected to the reset input of the microprocessor directly to initiate fresh data acquisition. The on-line ensemble averaging employed in this system yields 40 dB signal to noise ratio in addition to minimisation of respiratory artefacts[3],[6],[7]. Therefore the patient need not hold his breath during investigation by this IPG system. This makes the procedure more comfortable for the patient and the data more reproducible.
[Figure - 2] illustrates various lead configurations for obtaining impedance plethysmographic data in patients. For recording IPG waveforms from neck and thoracic region, with the subject in supine, the current electrodes I1 and I2 are applied around the forehead and feet of the subject respectively and the ECG electrodes E1 and E2 are applied around the base of the neck and around the thorax at xiphistemum (non-standard lead) respectively. Voltage electrode V1 is applied around top of neck and V2 is shorted with E1 for recording IPG from neck location. For recording IPG from thorax, voltage electrodes V1 and V2 are shorted with ECG electrodes E1 and E2 respectively as shown in the figure. For recording IPG data from lower extremities, the current electrode I2 is applied around right foot retaining the position of I1, E1 and E2. The voltage electrode V1 is applied around lower abdomen (below umbilicus) and V2 is applied around mid thigh in right leg for recording IPG waveform right thigh. The electrodes V1 and V2 are then shifted to knee, calf and ankle locations as shown in the figure for recording IPG from right knee, right calf and right ankle respectively. IPG data are also acquired from right knee, right calf and right ankle locations after elevating the right leg by an angle of 45? without disturbing the posture of rest of the body. For assessment of left leg the current electrode 12 is moved to left foot and V1 and V2 are applied as described above for the right leg. Upper extremities are examined in a manner similar to that of lower extremities. The current electrode 12 is applied around right palm retaining the positions of I1, E1 and E2. Voltage electrodes V1 and V2 are applied around upper-arm, elbow, forearm and wrist for acquiring IPG data from these four locations. I2 is then shifted to left palm and data from left upper-arm, left elbow, left fore-arm and left wrist are recorded by placing V1 and V2 accordingly. IPG data from fore-arm location are also recorded from both the upper extremities in hanging down position, 900 abduction and hyperabduction postures of the upper extremity with the subject in sitting position. [Figure:3a] shows typical IPG waveforms recorded from neck, thorax and various locations in the lower extremities and 3b on page 68B gives similar data from upper extremities in a normal subject. The waveforms shown in these figures represent the ensemble average of 50 IPG cycles and have been recorded during normal breathing of the subject. The waveforms are recorded at a paper speed of 100 mm per second. One ohm per second of d7Jdt is represented by 10 mm deflection on Y-axis for thorax and the same is represented by 20 mm deflection on Y-axis for all other locations.
As can be seen from [Figure:3a] and [Figure:3b] each, IPG cycle is preceded with R-pulse and therefore, it is not necessary to record ECG simultaneously with IPG for making measurement in time domain. [Figure:4] on page 69 shows (a) typical IPG waveform recorded from thoracic region and (b) typical IPG waveform recorded from knee location in the lower extremity. As can be seen from [Figure:4(a)], IPG recorded from thorax has one major deflection starting from point B, reaching the maximum at point C and traversing downward till point X. This deflection is caused during ventricular systole and is therefore, named as systolic wave or the 'C-wave'. This wave is followed with another deflection known as 'diastolic wave' or the 'O-wave'. Diastolic wave has several phase changes in its complex represented by points Y, O, Z and A. Following A point is a small positive deflection and is called pre-ejection wave. Time elapsed between R-pulse and phase reversal points described above is measured on X-axis as shown in the figure. The amplitude of the systolic wave or (dZ/dt)m is measured on Yaxis from B to C in ohms per second. However, in the absence of a discernible B point, the height of C from base line is taken as (dZ/dt)m. The interval (RX-RB) gives the left ventricular ejection time (LVET). Time intervals RC and RX are referred to as pulse arrival time (PAT) and pulse termination time (PTT) respectively for impedance plethysmogram recorded from upper or lower extremities (see [Figure:4(b)] on page 69). PAT values can be used for obtaining differential pulse arrival time (DPAT) by subtracting the PAT of a proximal location from that of a distal location. Stroke volume and blood flow index can be calculated from this data using Kubicek's formula and Parulkar's formula described by Babu et al[1] as follows: L dZ Stroke Volume = ? ----- ------ m (RX-RB) Zo dt) Blood Flow Index = 500 (dZ/dt)m x (RX-RB)/Zo.
The authors are thankful to Shri MK Gupta, Asso. Director, E & I Group, BARC, Shri BR Bairi, Head Electronics Division, BARC and Shri KR Gopalakrishnan, Head, Nuclear Instrumentation Section, BARC for guiding and encouraging this work.
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