Vernier Tech Info Library TIL #2036

Question

Why is my EKG trace noisy? Why doesn�t my EKG trace look like the trace in the book?

Answer

The trace that is observed in the book actually uses some filtering to clean up the signal. If you want the data to look just like it does in the book, you need to use the proper experiment file. Analyzing EKG in Biology or Human Physiology with Vernier is set up properly to do smoothing. If problems persist check the following.

There are several things that can increase the noise in an EKG trace. Most of these issues can be addressed using proper electrode placement and placement of the sensor. Software filtering can also be used to clean up a noise signal as well. See the instructions below.

Is an EKG trace visible? If the QRS complex (big spike) in the EKG trace is not visible then there is either a problem with the EKG sensor or a problem with the subject. To test the EKG sensor, collect data while shaking the leads when they are not attached to the subject. You should see a change in the trace. If you do not see a change, something could be wrong with the leads or the sensor.

In a few cases (less than 12%) the subject�s skin may not be conductive enough to pick up a good EKG trace. Abrasive / conductive electrode gel and clinical grade EKG electrodes may need to be used in this case. The easiest way to test this is to record an EKG from another individual. If you get a good EKG from another subject skin conductivity is the problem.

If the EKG trace is visible but is noisy there are several things that you can do to decrease the noise.

1)If you are using a LabQuest, use the battery power (remove the AC adapter). This should decrease the noise.

2)Let the electrodes sit on the skin for 1-2 minutes before you begin recording. This will increase the strength of the signal.

3)Place the grounding electrode on the ulnar process of the wrist. If you are recording from the chest, place the grounding lead on the clavicle. This will increase the strength of the signal and should decrease any noise.

4)Do not place the EKG sensor near computers, cell phones, power outlets or the LabQuest. The EKG sensor is not shielded and can pick up electronic interference and noise. The best location for the EKG sensor is on the subject's lap or knee during a recording.

5)If the EKG trace is still very noisy, drop the sampling rate to 50 samples / sec. This will dramatically decrease the amplitude of the signal, but the EKG trace should be visible. The source of the noise in the room should be isolated.

If the EKG trace is visible but is slightly noisy there are several other things that can be done to clear up the signal.

A)Use SG smoothing to get rid of the noise. To do this, create a calculated column and use the SmoothSG function on the Potential data. The default settings of using 7 data points for smoothing will work for data collected at 100 samples / second.

B)The quality of the EKG trace will increase if the sampling speed is greater. For college clients, 200, 500, 1 or 2K samples per second are preferable. The SG smoothing function should be used to clean up the signal using a calculated column. Change the FILE settings to 7, 15, 21 and 39 samples per smoothing calculation for each of these sampling rates. Smoothing can be done after recording.

Created by: jmelville on July 13 2009
Last updated by: jmelville on August 14 2009