Introduction

An electrocardiogram (ECG) measures the electrical activity of the heart muscle by using electrodes placed on the skin. These electrodes detect the small electrical changes that are a consequence of cardiac muscle depolarization followed by repolarization during each cardiac cycle (heartbeat). ECG is measured in millivolt (mV) and the signal usually ranges between 1 and 5 mV. 

Availability, support and advice

For availability of ECG equipment within your unit, contact your lab coordinator. ECG equipment from BIOPAC or VU-AMS can also be borrowed from SOLO through the Helpdesk or by contacting labsupport@fsw.leidenuniv.nl.

Protocol

Below, a general protocol of measuring ECG with a two lead configuration common at the FSW is described. Note that an adequate training is required before working with physiological data acquisition equipment and collecting such data from participants. If you are uncertain about the procedure, please contact your supervisor or lab coordinator. 

Materials

ECG equipment Gloves 3 disposable electrodes (Ag/AgCl) Scrub gel (NuPrep) Cotton buds/cotton wool pads Alcohol wipes

​​​​​​Preparation

It is important to reduce the impedance (resistance) of the skin before applying the electrodes, by removing oils and dead skin cells. It is recommended to use a scrub gel (NuPrep). The gel is gently rubbed on the skin with a cotton bud or pad. Afterwards, the skin is dried with a clean cotton pad. Apply the electrodes directly after cleaning the skin.

Electrode placement

The number of electrodes used and their placement can differ depending on the type of study. A common method is to use three electrodes (two lead configuration), one of which is placed under the right collarbone (negative, V-), one on the left on the bottom rib (positive, V+), and one on the right just below the ribs (ground). When drawing an imaginary line between the positive and negative electrode, the heart should be located in the middle. The placement of the ground does not matter, as long as it is not located between the positive and negative electrode. Note that when using the wired BIOPAC system with multiple modules that use a ground (e.g. ECG100C and GSR100C), only one ground should be applied to avoid ground loops. The ground of the ECG can be omitted. See BIOPAC grounding guidelines for more information.

Which leads (specifically: which colour) should be attached to these electrodes depends on the system that you are working with. With the Biopac system, you should attach the black lead to the grounding electrode on the right, below the ribs. The red lead should be attached to the electrode on the left, on the ribs, and the white lead to the electrode on the right, below the collarbone. When using the wireless Biopac module, you can fix the strap with the transmitter around the participant’s waist and then attach the leads to the electrode. Ensure the strap is placed between the Biopac transmitter and the skin, so that the transmitter does not come into contact with the skin. With the VU-AMS system, you attach the green lead to the grounding electrode on the right, below the ribs. The black lead should be attached to the electrode on the left, on the ribs, and the white lead to the electrode on the right, below the collar bone.

Biopac	VU-AMS - Figure adapted from source: Nederend, ten Harkel, Blom, Berntson, & de Geus, 2017

Afterwards

Remove the leads and, if present, the wireless module. The participant can then remove the electrodes him/herself. Removing the electrodes can be painful for some people. Depending on the sensitivity of the participant’s skin, there may be red marks visible. These will normally fade within a few hours. Give the participant a tissue to remove any excess gel, or allow the participant to wash the areas in question with water and soap. The electrodes can be discarded in the bin. After each participant the equipment he/she has been in contact with must be cleaned. This could apply to the leads that were attached to the electrodes and, in the case of the wireless Biopac module, the transmitter. Clean these components carefully with an alcohol wipe. Ensure that no liquids enter the equipment. The strap must be cleaned after use with Incidin Plus.

Tips & Tricks

• The participant must move as little as possible to prevent artefacts in the data.
• The participant must be comfortable and sit in a natural posture with both feet on the floor.
• Check the ECG signal before starting actual data collection. The figure below shows a typical ECG signal (recorded with BIOPAC AcqKnowledge).
• When the data looks irregular or shows a flat line, check whether the leads are properly attached and whether the electrodes are still properly attached.
• If the data looks upside down (i.e., when the R peaks are pointing down) it is likely that the V+ and V- leads have been swapped around. Check whether the V- lead is attached to the electrode under the collarbone and the V+ lead to the electrode on the left-hand side below the bottom rib.

Resources

• Nederend, I., ten Harkel, A. D. J., Blom, N. A., Berntson, G. G., & de Geus, E. J. D. (2017). Impedance cardiography in healthy children and children with congenital heart disease: Improving stroke volume assessment. International Journal of Psychophysiology, 120, 136-147