Abstract

Impedance plethysmography (IPG) is a safe, noninvasive method for measuring peripheral hemodynamics. The purpose of is to describe the IPG technique and its potential use by physical therapists in making hemodynamic evaluations. Impedance plethysmography requires the attachment of four circumferential Mylar band electrodes around a limb. We use a cardiograph to introduce a 4-mA current (I) at a frequency of 100 kHz in the two outer electrodes. The voltage (V) is sensed in the two center electrodes, and the resulting impedance (Z) is calculated using Ohm's law (Z = V/l) as shown in (Figure 1) . Arterial blood flow can be calculated using an impedance-related volume conduction equation. Impedance plethysmography has been shown to be economical, and any limb or limb segment can be evaluated. Applications are presented for the assessment of arterial blood flow, peripheral arterial disease, deep vein thrombosis, and venous insufficiency. Impedance plethysmography offers the physical therapist a safe and relatively simple technique to assess the peripheral vascular status of the patient.

Background

Plethymography is a non-invasive diagnostic treatment used for screening and patient follow-ups with various arterial and venous pathologies.

This treatment is concerned with the measurement of volume and volume displacement of blood. The screening provides a circulatory assessment via a waveform representation of pulsatile peripheral blood flow. Instrumentation providing blood volume parameters exists but nothing to measure volume directly.

An example of this instrumentation is the use of an ultrasound. While ultrasound provides hemodynamic (hemodynamic refers to the forces generated by the heart and the motion of blood through the cardiovascular system) data on vein segments, plethysmography provides information that is indirectly related to venous volume changes. The data obtained is not specific to venous function because limb volume changes may be caused by several factors.

Figure 1. Impedance plethysmography.

Plethysmograph Components

The main part of a very simple plethysmograph (Figure 2) is a container of suitable size and shape in which the part, for example, hand, leg, or finger, is placed. In animal experiments a kidney, heart, or spleen may be studied. The container is filled with water and hermetically sealed. (In Figure 2 a rubber cuff is used.)

Changes in the water level in the container reflect fluctuations in blood volume in the organ or part and are recorded in the form of a curve called a plethysmogram. The plethysmogram shows small fluctuations in blood pressure corresponding to pulse and somewhat larger ones corresponding to respiration; large variations reflect vascular reactions to various stimuli. More advanced methods of plethysmography include (1) photoplethysmography, in which light is directed through an organ, such as an ear or finger, onto a photoelectric cell, or light is reflected from the organ, (2) rheoplethysmography, and (3) dielectrography, or rheocardiography. The last two methods are based on the direct recording of fluctuations in the electrical properties of an organ, which reflect the dynamics of the organ’s blood supply.

Figure 2. Plethysmograph Components.

Plethysmography procedures

Rapid changes are typically associated with changes in blood volume or movement artifact. If movement is controlled, information specific to blood volume can be obtained. Further separation of arterial and venous flow effects can be observed through electronic filtration. Venous flow changes typically involve long transient time constants with duration of seconds or minutes.

Venous displacement measurements are typically associated with shifts in body position and limb compressions which allow measurements of magnitude and duration. Four main types of plethysmography exist. They include Air-Displacement, Photoelectric, Strain gauge, and Impedance. Air-Displacement plethysmography (Figure 3) deals with measuring the volume of an object by indirectly measuring the volume of air it displaces inside a closed chamber. The human body volume is measured when a patient sits in an enclosed chamber and displaces a volume of air equal to his or her own body volume.

Figure 3. Air Displacement Plethysmography.

By subtracting the remaining volume of air inside the chamber when the patient is inside from the volume of air in the chamber empty, you get the body volume. Photoelectric plethysmography is concerned with assessment based on cutaneous blood volume. An electrode consisting of an infrared LED and a photosensor is attached to the skin. Light transmitted into the skin is scattered and absorbed by tissue in the illuminated field. Blood attenuates the reflected light and intensity of reflected light changes with blood tissue density. The voltage signal generated by the photosensor is amplified by a DC circuit. Low frequencies are passed which produces relatively stable tracing. This corresponds to blood density in the underlying tissue. Strain gauge plethysmography uses a transducer filled with mercury or indiumgallium metal alloy conductor.

Stretching the strain gauge causes a decrease is diameter causing an increase in voltage. When wrapped around a limb segment, the gauge provides a circumferential measurement that can be used to compute area. The “slice volume” of the limb segment changes as the limb volume expands and contracts. The final type of plethysmography is impedance plethysmography. A weak current is passed through a limb and the electrical resistance to current flow is measured. Four conductive bands are taped around the limb as outer and inner pairs of electrodes. The inner pair is then used to measure electrical resistance.

Photo Sensor Plethysmography Design & Work Procedures   as a type of Plethysmography kinds

The finger is placed into a box with a red LED on one side and a Cadmium Sulfide (CdS) cell on the other side (Figure 4). The resistance of the CdS cell varies with the intensity of the light hitting it, and this intensity depends on the amount of blood in the finger.

The change in the resistance is transduced into a change in voltage, and in its raw form gives a voltage range of 0 to 2.5 volts. We want to measure the variation in this signal caused by the pulsatility of blood flow in the finger. By carefully examining the raw signal on an oscilloscope, we found that this variation is at most 10mV. This signal must be amplified into a 0-5 V range to take advantage of the full range of the A/D converter.  

The raw signal also includes a great deal of noise which must be filtered out. After being converted to digital, the signal is analyzed using the PIC microcontroller, and the output is displayed on a 10-LED ladder. In one mode, the LEDs are set up to all light up when the CdS detects little light from having most of it blocked by the blood flow, and all turn off when encountering much light, corresponding to no fluid. In the second mode, the heart rate in beats per minute (bpm) is calculated from the period of the signal and different LEDs are lighted to indicate the bpm range in which the heart rate falls.

Figure 4.

Transduction, Amplification, and Filtration

Single supply design (Vcc = 5V) was required to power the circuit components.

TRANDUCTION

The CdS cell changes resistance in response to the amount of light it receives.  To work with this signal requires that it be transduced into a voltage. The CdS cell was placed in series with a 47 kOhm resistor, and the signal was taken between them. The signal then ranged from 0 to Vcc/2 or 2.5 volts as the CdS resistance varied.

AMPLIFICATION

We chose the inverting configuration of an amplifier (Figure 5) as it is common to use this kind in single supply design. These amplifiers can output voltages quite near the power supply voltage. And since we wanted to measure the changes of the 2.5 voltage signal due to the blood flow, we needed to amplify the changes rather than the entire signal. We also needed the signal in the 0-5V range after amplification, rather than being centered around 0V. A virtual ground was employed to achieve this by amplifying the changes with respect to the Vcc/2 volts. amplifier was used and attached to the positive terminal of the op-amp to function as a virtual ground. Below is a diagram of the design constructed so far.

Figure 5. Amplification By Invering Amplifier.

Notice that the inverting configuration was chosen because it can easily be modified to also act as a filter as we shall soon see. A secondary gain stage was implemented using the same configuration to invert the inverted signal from the primary gain stage.

The gain of an inverting amplifier is equal to -Rf/Ri where Rf is the feedback resistance and Ri is the input resistance. Because the gain stages will function with the filtering, we will consider the gain in the next section.

FILTRATION

Once again, the signal we want to end up with consisted of tiny variations (changes in light intensity due to blood flow into and out of the finger), superimposed on a large constant signal (average light flowing through finger). Recall that we only want the time varying part of the signal amplified, and if we were to amplify the raw signal, the DC (constant) part of the signal would saturate the amplifier before obtaining desired amplification of the AC (time varying) part.

To get rid of the DC signal we used a high-pass filter (Figure 6) because DC signals are extremely low frequency. We had to be careful, however, not to attenuate the pulse signal, which is usually about 1 Hz (equivalent to one heart beat per second). High frequency noise was a concern as well with most of it coming from the 120 Hz signal of the light fixture (due to positive and negative portions of the 60 Hz power noise). Considering these requirements, a band-pass filter with a frequency band from .5 Hertz to 10 Hertz was chosen. We decided that a second-order simple band-pass filter, which could be implemented by modifying the amplifier design, would be sufficient. A capacitor in series with the input resistance constitutes a high-pass filter, and one in parallel with the feedback resistance constitutes a low-pass. RC values were selectively chosen to set not only the pass-band frequencies, but also the gains.

Figure 6. High Pass Filter.

DISPLAYING THE PULSE AND MEASURING THE HEART RATE

To measure the amoung of blood flow in the finger as well as the time between each beat on a 10-LED ladder. The algorithms to perform this task upon a push of a button are explained below.

To display the blood flow measured by the photoplethysmograph we used a ladder of ten LEDs, with the number of lit LEDs increasing as the amount of the blood in the finger increased.  This was done by setting a threshold for each LED--the higher the LED on the bar, the higher the threshold and vice versa.  When the digital signal went above the threshold for a given LED, the PIC microcontroller would output a logic high, thus lighting the LED.  In order to have enough outputs from the PIC to control all the LEDs in the bar, we could only have one input into the PIC. 

In addition to showing the signal using the LEDs, we also wished to display information about heart rate.  We did this by lighting 5 LEDs (every other bar on the ladder) to indicate a heart rate above an appropriate threshold (> 40bpm, > 50bpm, etc.).  The circuit's mode of operation (heart rate or pulse) was controlled by a push button on the printed circuit board containing the microcontroller.  We used a state machine (see diagram below) to control the process of calculating the number of heart beats per minute (bpm).  First, we found the minimum and maximum value of the signal over a 3 second interval, then we chose a low threshold at the minimum value plus a quarter of the range and a high threshold at the maximum value minus a quarter of the range.  We started a counter when the signal passed the high threshold from below and stopped the counter when the signal passed the high threshold again after having passed the low threshold.  The heart rate, in beats per minute, was then calculated from this period.  The displayed heart rate was a weighted average of the current calculation and the previous two (a [1 1 2] filter) (Figure 7).

Figure 7.

oscilloscope printouts of the raw signal after the signal processing (Figure 8)

Figure 8.