The Different Recording Modes

          There are two primary recording modes utilized in cardiac electrophysiology, unipolar and bipolar. While many people profess to be familiar with the more commonly used bipolar, in reality there are very few Allied Professionals in the lab that truly understand the differences and similarities between these two recording modes. Both unipolar and bipolar recordings are collected using the same technique and both of these recording modes are based upon unipolar electrograms.

          An electrogram is recorded by comparing the difference in electrical activity between a positive and a negative electrode. Electrical activation at both points is collected and then one signal is subtracted from the other. In order to do this, there are three things that must be present. We need one electrode with a positive polarity which we will call electrode #1. A second electrdoe with a negative polarity is also needed. The second electrode will be referred to as electrode #2. Our final electrode (electrode #3) acts as a reference electrode. At the beginning, both the electrodes # 1 and #2 act as positive electrodes from which two unipolar electrograms are recorded. Electrode #3 acts as the negative pole for these first two electrograms. After the initial pair of unipolar electrograms are recorded, one is subtracted from the other. The final result is an electrogram that displays information on the electrical activity that occurs between electrodes #1 and #2. Everything that is done as far as electrogram recording is tied to this process.

          A unipolar lead is used to record the electrical voltages at the location of the positive electrode relative to the negative electrode which has close to zero potential.3  A bipolar lead records the electrical voltage at the location of one electrode in reference to a second electrode that does not have a zero potential. To do this, two unipolar signals are recorded. The signal from one electrode is inverted and then added to the signal recorded from the second electrode.1 After one electrogram is subtracted from the other, what remains is the electrical activity that has transpired between the two electrodes. This signal is then run through a band pass filter with a high pass at 30Hz and a low pass at 300Hz. To learn more about filters, click here. Unipolar electrograms are also filtered though usually at a lower high pass setting such as 2Hz. The low pass filter for unipolar signals is set to 300Hz as well.

          The basic concepts of unipolar and bipolar signals may be applied throughout all recording aspects in cardiac electrophysiology. It should be noted that understanding the differences between these two recording modes is just the first step towards comprehending what information is provided by each of them. There are other concepts such as filtering and electrode placement that must be taken into consideration when evaluating electrograms.

Unipolar or Bipolar? How you record a signal will determine what you see…

          Now that you understand the difference in how unipolar and bipolar signals are acquired, you can begin to explore the differences in what these two types of electrograms show you. In general terms, bipolar focuses only on the local activation between the two recording electrodes. Unipolar provides a much broader view as seen from the location of the positive recording electrode. To help understand the differences between these two recording modes, the following analogy has been provided.

          You are at home, sitting in your office doing some work on the computer. It is a pleasant Saturday afternoon and the clock shows 2:45pm. Outside the house, you hear the purring growl of what sounds like the engine of a fine tuned, yet powerful sports car. You glance across the room to the window on the far wall just in time to see a blue sports car go zipping by. Being the sports car enthusiast, you rush to the window to take a better look, but alas, the car is already gone. It was impossible to identify the car as you only got a quick glimpse of it. What you can say for certain is that at 2:45pm a blue sports car passed your window.

         Two weeks later, you are at home enjoying another leisurely Saturday afternoon. Because all your work is done, you are not at the computer in the office. For this afternoon, you are sitting in the living room in front of the fire, reading a good suspense thriller. Across the room from you is a large picture window. The clock shows that is 2:51. Once again, you hear that awesome sounding engine that you heard two weeks ago. You look across the room through the window and you see a blue sports car that appears to be a Porsche model similar to the 928S4. You wonder if it may be a new model, but it passes the picture window before you can get a positive identification. By the time you get out of your seat and move to the window, the car has performed another vanishing act.

          Another week has passed, and this Saturday, you are up on the roof clearing out leaves from the rain gutters. It is a cold, blustery autumn day and you would much rather be sitting in your comfy chair by the fire with your book, but you didn’t get the leaf guards installed on your rain gutters and the weather service is predicting heavy rains. You kick yourself for not having the leaf guards installed and set about to get the job done unaware, that life is to reward you. Down the street, you hear that distinctive performance sports car sound. You look down the street and see the blue sports car coming around the corner. From your vantage point on the roof, you have more time to examine this sweet machine. It is indeed a Porsche. It looks very similar to the 928S4 yet it has a sleeker body style and a few new aerodynamic modifications. You can also see that it has black leather interior and the driver is the cute young blonde who lives a few doors down. You watch as she pulls into the driveway and into the garage. Perhaps, you ponder, it is time for a neighborly visit.

           Right then, you hear the screech of tires and the wail of sirens as numerous police cars come zipping around the corner where the Porsche just came from. The chopping sounds of helicopter blades fill the air as a black helicopter appears over the trees and starts to circle the house of the beautiful young Porsche driver. Police cars skid to a stop and the house is raided. In a few moments, the young lady is being carted off in a police car. Curiosity gets the better of you and you climb down off the roof to inquire what was going on. It turns out that your lovely young neighbor was involved with an international crime ring. The authorities had been trying to find her when one of the investigators got a tip on a stolen blue Porsche. They had track the car by satellite and moved in for the arrest.

          The view from inside your office is similar to a bipolar view with a smaller electrode spacing such as 2mm. You can get an indication of when something happened at that location, but it is difficult to determine much more than that. In terms of intracardiac electrograms, bipolar recordings with 2mm electrode spacing provides information on the precise moment an electrical signal passed by the region between the positive and negative electrodes.

          The view from the picture window can be equated with a bipolar view with electrode spacing of 10mm. You get a bit more information on what is happening, but you still can’t see the big picture. Note that as you increase the distance between the electrodes, the bipolar signal takes on more characteristics of a unipolar recording. More information on this can be found in the section on electrode spacing.

          The view from the roof is similar to that obtained using intracardiac unipolar. You can now see much more information including where the signal was coming from and where it was going.

          The satellite view used to track the blue sports car is the global perspective that would be equivalent to our surface ECG. The electrodes for the surface 12 lead are located completely outside the region of interest and provide information on all the electrical activity in the heart.

          By considering the different viewpoints provided by the different recording modes, you can gain a very solid understanding of the information provided by both unipolar and bipolar electrograms.