An electrocardiogram measures the electrical signals that travel through the heart while it beats. Doctors are looking at how long it takes the electrical wave to pass through the heart, as well as how much signal there is.
The printout from an ECG is more than just a bunch of squiggly lines. A closer look will reveal that not only are there squiggly lines, but there are also a few letters and numbers scattered about the page. What does it all mean?
All Those Wires
If you have an ECG done, you’ll rest flat on your back and they’ll hook up a bunch of electrodes. It does not hurt. These electrodes allow the doctor to look at the heart’s electrical activity from different angles.
To illustrate how this works, consider these pictures:
These pictures are quite different but they’re all photos of the same thing, just from different angles. Likewise, the different leads used in an ECG give doctors heart-activity pictures from a bunch of different angles. It’s one heart, but the electrical waves look different depending on your perspective.
Chest Electrodes and Their Leads
Six electrodes are attached to the chest at specific places to provide a picture of how the heart is functioning. These six chest electrodes show the electrical signal looking from the horizontal plane.
- V1 and V2 look at the heart from the center (septal)
- V3 and V4 look at the heart from the front (anterior)
- V5 and V6 look at the heart from the side (lateral)
Limb Electrodes and Their Leads
There will also be an electrode for each limb: right arm, right leg, left arm, left leg. One peculiar thing is that the leg electrodes can be positioned anywhere between your torso and ankle. As long as they’re the same location on both sides of the body it doesn’t seem to matter (I’ve had them near my ankle and also had them on my lower abdomen, and was confused about the variation depending on who’s doing the test – but now it makes more sense). These limb electrodes generate views from a vertical plane.
When measuring the flow of electricity toward the right arm, this is called augmented vector right (aVR)
- When measuring the flow of electricity toward the left arm, this is called augmented vector left (aVL)
- When measuring the flow of electricity toward the left foot, this is called augmented vector foot (aVF)
- When measuring the flow of electricity toward the right foot – nope, that doesn’t happen. They only consider the left electrode; the one on the right is neutral.
Maybe you’re puzzled, as I was, about how it can be a 12-lead ECG when there are only ten electrodes (6 chest + 4 limbs). It turns out that leads are not synonymous with electrodes. The six electrodes for the chest are the same as the leads, but to understand the others, we have to recognize that those leads are made up of pairs of electrodes. Think back to high school science class and what we learned about electricity. Or maybe it might help to picture the terminals on your car battery, where we know that electricity flows from the negative to the positive.
An ECG also measures the flow between the limb electrodes (from negative to positive):
- Lead I measures the flow from the right arm’s electrode to the left arm’s electrode
- Lead II measures the flow from the right arm to the left leg
- Lead III measures the flow from the left arm to the left leg
If you ever want to have even a fuzzy idea of what your ECG shows, it’s important to know about the leads, because the printout of your heart’s rhythm will reference those leads. The letters and numbers on the ECG’s printout are referring to the lead used for that portion of the tracing. The ten electrodes give twelve leads: six chest and six limb.
The Strip (Printout)
The printout from an ECG shows the heart’s electrical signal as waves.
- If there is no electrical impulse, the line on the ECG is basically flat. This is called the baseline.
- When an electrical impulse is travelling toward a lead, the line on the graph will move upward.
- When an electrical impulse is travelling away from a lead, the line on the graph will move downward.
Just as the front and back of my nativity scene look very different, waves look different depending on which lead is being used. For example, this shows the same heartbeats as seen from two different leads:
But what does it mean? Notice that the squiggles seem to have a pattern. Every distinct portion of the ECG means something specific. First the heart is at rest, and the horizontal line showing no electrical activity is the baseline.
The P-wave indicates the heart’s electrical signal traveling through the atria (top chambers of the heart).
Following the P-wave, the QRS complex shows the electrical signal traveling through the ventricles (bottom part of the heart). When this happens, the ventricles contract and the atria re-set.
The T-wave indicates that the ventricles are resetting and getting ready for the next heartbeat.
When we combine what we know about leads with what we know about these waves, we can look at the printout from an ECG and see how it all fits together. This illustration is color-coded to show which leads view the heart from different angles, and where that information shows on the ECG’s printout.
Certainly this isn’t enough information to allow patients to read an ECG, but it does give us some background to have a hope of understanding what doctors are saying if they ever attempt to explain what they’re seeing on the printout of our electrocardiogram.
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About Pericarditis ECG’s:
- Pericarditis ECG Review
- ECG Basics: Pericarditis