If you set up a safely small, usually high frequency A.C. constant current through a segment of your human subject, and then measure the resulting A.C. voltage across that segment, you can resolve the resistance of that segment based on the simple formula R = E/I. And this resistance, along with major and minor fluctuations that can be seen in it, can open up new avenues of physiometry data about your subject. The term impedance is usually used instead of resistance, because the response of the body to that constant current has been found to be a complex value, including both real (resistance) and imaginary (reactance) components. The term bioimpedance is used frequently to refer to this technique. And as you might guess, these impedance methods can be used to explore a wide range of subjects and preparations.

Some bioimpedance systems are bipolar because these systems only require two electrodes to be placed on the subject. These two electrodes are placed at either end of the segment of interest. And with just two electrodes, the same electrodes function to both supply the constant current, which is often referred to as the excitation current, as well as to actually sense, to detect the impedance that the segment imposes on the excitation signal. Placing just two electrodes on your subject seems simple, but this bipolar approach can be sensative to the quality of the electrode to skin contact path. And the bipolar approach works VERY poorly if you instrument multiple segments on the same subject. The different excitation frequencies will clash wildly, especially if the operating frequencies are close to each other.

Some bioimpedance systems are tetrapolar. This approach separates the two electrodes that supply the constant current excitation signal, from the two electrodes required to sense the resulting impedance across the segment. Using four electrodes instead of two seems more complicated, but the tetrapolar configuration minimizes the role of the quality of the electrode to skin contact path for the actual sensing electronics. And a tetrapolar approach also opens up another important benefit -- multi-channel sensing. As long as the excitation current flows linearly through the multiple segments you wish to measure, that same excitation current easily supports multiple sensing channels. The tetrapolar approach supplies impedance sensing across a number of segments on your subject at the same time.

A recent enhancement to bioimpedance is the use of multiple frequencies to allow you to evaluate how the impedance of your segment changes over a range of frequencies. While computationally intensive, this 'swept frequency' impedance technique can supply fairly fine-grained information about the segment.

Applying electrodes for impedance work is NOT for the faint of heart! You can read our Skin Surface Electrodes 101 primer as a starting point. And note that solid column electrodes should NOT be used for impedance work! But by far the most common misunderstanding has to do with the way the excitation current flows through the body. It will take the most direct, straight path from one excitation electrode to the other. So, for example, if you place one excitation electrode on the right hand, and the other excitation electrode on the right foot, there will be NO excitation current flowing in the left leg! The excitation current will flow down one arm, down the torso, down the right leg, and thats it. There are a number of other issues that can surface as well.

UFI fas been supplying impedance based equipment for many years. For example, our Checktrode line of contact impedance testers use a bipolar connection and a lower frequency to supply a good indication of the quality of the connection between two electrodes and the subject.

And our 2991 Impedance Converter is really in a league by itself. This device supplies a bipolar impedance front-end which you manually balance to match the impedance of your subject or specimen. Then, any changes in the impedance of your subject are sensed and amplified substantially. How else would you measure gill function in aquatic subjects? The 2991 really is a universal signal conditioner!

With some important changes, including a self-balancing bipolar impedance front-end, our Resp1 impedance pneumograph uses the change in impedance between two electrodes across the chest to supply a respiration signal. Inhaling adds air, and so drops the impedance, while exhaling removes air, and the impedance increases. And adding an ECG specific filter and amplifier allows the Resp1-EKG to supply both Respiration and amplified ECG from just three electrodes!

The UFI THRIM has been available for many years, and through a number of design iterations. THRIM stands for Tetrapolar High Resolution Impedance Meter, and the THRIM does indeed use a tetrapolar impedance measurement scheme, and supports up to four channels; the THRIM supplies simultaneous impedance on up to four body segments at the same time. The excitation frequency is fixed at 51.2 KHz., and each sense channel supplies Resistance (Ro) Reactance (Xc), and the two pulsatile channels Delta R (the amplified change in Ro, equivalent to the PPG signal) and dR/dt, the electronic first derivative of the Delta R signal. The THRIM also includes a robust three zone isolation scheme and allows for battery operation.

We also have a swept frequency version of the THRIM in development, and have a prototype up and running. The frequency range is approximately 3 to 300 KHz., and a single tetrapolar channel is currently operational. You will need to contact us if you are interested in more information about this device.

If you still have questions, or if you don't see what you need, drop us a line, and we will see how we can help you!

Product Families:
Bioamps/signal conditioners
Ambulatory data loggers
Hot Flash Recording
BAERCOM™ hearing tester
PC-based instruments
Multi-subject systems
Test instruments