Physiology Lab (Phsl 3063, Phsl 3701)

Fall 2012












Lesson Summaries

Review Material


Atlas of Human Cardiac Anatomy


BME Department

Physiology Department

Institute of Technology








Lesson 6 - ECG II Tutorial

Lesson 6 (ECGII) will show you more information you can derive from an ECG; such as how to find the direction your heart lies in your chest (known as mean electrical axis). You will be using a bipolar limb lead configuration, so particular attention should be given to how the leads are placed on the body (see BIOPAC manual for lead setup). Below is a picture showing Einthoven's triangle with respect to the heart.

.inthoven triangle on body
Image from -

So...who cares about Einthoven...well you do for this module. Einthoven's Law states mathematically that Lead I + Lead III = Lead II. So if you know two of the three leads, you can figure out the third mathematically. BUT this is not straight forward one dimensional math, it is VECTOR math which deals in more than one dimension, in our case two: x- and y- coordinates (described as the "frontal plane" in your BIOPAC manual). Luckily for you, we assume everything is occurring in the same plane, so we won't have to mess with the z plane. So for those of you who don't exactly know what a vector is, you really only have to know two things for this module: 1) Vectors have a magnitude and 2) Vectors have a direction.

riangle to vectors

In the above example, the left side shows the bipolar lead setup from the BIOPAC manual. To the right, the vectors are moved so that the negative end of the lead vectors are located at the heart and directed in a positive direction (because the electrical activity of the heart moves in a positive direction). Notice the angle of the vector is maintained.

Most of your data analysis for this module will involve finding the Mean Electrical Magnitude (MEM) and Mean Electrical Axis (MEA) from an ECG trace when your body is doing different activities. How do you do this?

CG data to vector

Above you can see a simplified exercise to find the MEM and MEA. On the right side there are two ECG traces; LEAD I and LEAD III. Here we assume a couple of things: 1) that the QRS is the most prominent electrical event in the heart and 2) that the R wave is a good approximation for the value of the QRS wave. That is why the magnitude of the representative vectors for each lead are taken from the R wave. In this example, the vector for LEAD I has a magnitude of 1mV and a direction of 0° and the vector for LEAD III has a magnitude of 1mV and a direction of 120°. By plotting these vectors on the graph, Einthoven's Law allows you to figure out the LEAD II vector. By drawing PERPENDICULAR lines from the positive end of each lead vector and finding where these perpendicular lines intersect, you can find the MEM and MEA. In this example, MEM = 2mV and MEA = 60°.

A couple of take home messages from this Lab: 1) What determines the Mean Electrical Magnitude? For most, the bigger the heart, the bigger the magnitude (the catch is that the heart has to function normal, at least electrically speaking). 2) What determines the Mean Electrical Axis? Ahhh, what you spend most of your data analysis figuring out. MEA basically tells you where the heart sits in the chest (which way the apex of the heart is pointing). So two big factors are gravity and how the surrounding tissues come in contact with the heart. If you stand up, which way will your heart apex go? If you inhale, what will the lung tissue do to the position of your heart apex? YOU THINK ABOUT IT







This page was last updated on August 11, 2008