MicroEKG Manual

What is an ECG?


MicroEKG Details

The Machinery

An electrocardiogram is simply a measure of voltage changes in the body. Any large “electrical event” can be detected.
The electrically-active tissues in the body are the muscles and nerves. Small brief changes in voltage can be detected as these tissues “fire” electrically. The heart is a muscle with well-coordinated electrical activity, so the electrical activity within the heart can be easily be detected from the outside of the body.

The electrical changes that occur as the heart beats can be detected by attaching two electrodes and measuring the voltage between them over time. To do this, the ECG machine uses a roll of paper that moves at a specific speed to represent passage of time, and has a stylus that moves up or down with voltage to draw out the electrical events.

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The heated stylus moves upward with positive voltage, downward with negative voltage, tracing out voltage versus time on the moving heat-sensitive paper. To detect electrical activity moving in many different directions, it’s necessary to have more than just one set of electrodes. (Electrical activity causes a voltage change only if it’s moving towards or away from an electrode. If the electrical wave is moving at 90 degrees between the electrodes, it will not be detected.)  [Chapter Menu]

The Standard Leads

By attaching electrodes to the left arm, right arm, and either leg, we obtain the three “standard limb leads,” named I, II, and III. Each of these leads measures voltage between two points on the body: left arm vs. right arm (Lead I), left arm vs. foot (Lead II), and right arm vs. foot (Lead III).

lead1.gif (1624 bytes) Lead I is obtained by measuring the voltage between the left arm and right arm. The left arm is the positive pole. An electrical wave moving towards the left arm will cause an upward deflection of the ECG machine stylus on the paper. Lead I is most useful for seeing electrical activity moving in a horizontal direction.
Lead II connects the right arm to the leg, and therefore best sees electricity moving down and leftward.

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lead3.gif (1690 bytes) Lead III compares voltage in left arm and the leg, and will measure electricity moving down and rightward. Lead II and Lead III are positive at the foot.   [Chapter Menu]

The Augmented Leads

Three additional limb leads can be obtained by mixing combinations of electrodes. These are leads R, L, and F.
To create these limb leads, two electrodes are connected together to create an “average” electrode, then connected through the ECG machine to the remaining electrode.

Lead F is created by connecting the two arms together to create an “average” electrode. To the ECG machine, this combination looks like a single electrode midway between the two arms — directly in the center of the body above the heart. This “average” electrode is connected through the ECG machine to the foot electrode.

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This new ECG lead will detect electricity best if it’s moving straight up or down. The foot is the positive electrode, so a downward motion of electricity will make the ECG stylus move upward on the paper. Lead F responds most strongly to electrical activity that is moving straight up or down. Its “viewpoint” lies midway between lead II and lead III.

leadl.gif (1803 bytes) Lead L is created by connecting the right arm and the leg together, then comparing this “average” electrode to the left arm electrode. The left arm electrode is positive, meaning that electricity moving to the left will cause an upward motion of the ECG stylus.

Lead L sees electrical activity best if it’s traveling in a leftward and slightly upward direction. Lead L detects electrical motion from a viewpoint midway between lead I and lead III. To see the direction in which lead L best detects electricity, imagine a line between the right arm and foot. Find the halfway point, then draw an arrow from this halfway point to the left hand. This arrow shows the “direction” of lead L; about 30 degrees above horizontal. Knowing the direction of the various limb leads helps you localize pathology.

To create lead R, the left arm and the foot electrodes are connected together, then the voltage of this “average” electrode is compared to the right arm lead. This gives us a look at the electrical activity from a rightward and slightly upward direction.

Lead R is positive at the right arm electrode. This means that if electricity is moving in a normal leftward direction, lead R will record a downward wave.

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Lead R has a direction of 30 degrees above horizontal, aiming to the right. Because lead R “aims” in the opposite direction from the other limb leads, its ECG waveforms will usually be “upside down” by comparison. In fact, lead R lies exactly in the middle of the normal direction of electrical activity, but with the positive pole aiming away. This means that every electrical wave in lead R should have a net negative direction, unless pathology is present.  [Chapter Menu]

Limb Lead Vectors

If the directions of all ECG limb leads are drawn out and superimposed on each other, the “EKG limb lead vector diagram” is created. This diagram shows the orientation in which each limb lead best detects electrical activity. In the vector diagram, the arrow indicates the direction in which that lead is positive (that is, motion in that direction will cause an upward motion of the ECG stylus).

We assign the left side of lead I the value of zero degrees (no particular reason — that’s just the way ECG folks have always done it). Clockwise from lead I is positive, counterclockwise is negative.

Notice that each lead is 30 degrees apart. Lead L is pointing at negative 30 degrees. Lead II is oriented at plus 60 degrees, while Lead F’s direction is plus 90 degrees.

vector.gif (2152 bytes)

It’s important to know the orientation of the limb leads. For example, if the QRS wave is strongly downward (negative) in lead I, you can conclude that electricity is traveling in an abnormally rightward direction, indicating a conduction blockage in the heart. As another example, if you see signs of a heart attack in the leads that “look down” (lead III, lead II, and lead F), you can conclude that the heart attack affects the bottom portion of the heart.  [Chapter Menu]

The Chest Leads

In addition to the six limb leads, a 12-lead ECG includes six “chest” leads. The chest leads “sample” the electrical activity over small areas of the heart. The chest leads look at the heart’s electrical activity in a slightly off-horizontal plane around the front of the chest. This detects problems that might not be obvious from the standard limb leads, which measure electricity in a vertical plane.
The chest leads are often called V-leads. The electrode over the chest is the positive electode, while the limb electrodes are all averaged together to form a general ground electrode.

e_leads.gif (1754 bytes) The precordial (chest) leads start with V1, placed beneath the 4th rib to the right of the sternum. Lead V2 is opposite V1 at the left side of the sternum. V3 is halfway to lead V4, which is placed below rib 5 directly down from the middle of the clavicle. Lead V5 is straight around the chest from V4, in line with the front of the armpit. V6 is directly around from V5, straight down from the middle of the armpit.

The chest leads detect electricity moving in a front-to-back as well as side-to-side direction. Each lead responds to electricity moving directly towards or away from it. These leads are useful for diagnosing problems on the front surface of the heart, and for diagnosing ventricular hypertrophy.  [Chapter Menu]

The ECG Grid

The paper on which the ECG is drawn is divided up into 1 millimeter lines horizontally and vertically. The vertical lines represent passage of time. Because the paper moves at a rate of 25 mm per second, each 1 mm line represents 0.04 seconds of time. Every fifth line is darkened to help with counting. The time between large boxes (darkened lines) is 0.2 seconds, and five large boxes equals one second.

The vertical direction represents the strength of electrical voltage. Positive voltage moves the stylus up, negative voltage moves it downward. Each millimeter vertically represents 0.1 millivolt. Ten vertical boxes is one millivolt.

The horizontal direction represents passage of time.  [Chapter Menu]

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Approach to the ECG

To avoid missing something important, it helps to have a “system.” A standard approach to every ECG may prevent an unfortunate “miss” of a critical finding. Your standard plan should allow you to check every wave form and every interval:

What is the rhythm? What is the rate?

Look for P waves. Are they all the same shape? Are there any inverted Ps other than in lead R? Does the PR interval vary? Are there any non-conducted P waves? Is the P wave abnormally wide or high?

Check the PR interval. Is there first degree AV block? Is the PR abnormally short?

Look at the QRS complex in each lead. Is the QRS axis normal? Is the QRS width normal? Do the wave forms suggest conduction block? Are there significant Q waves? Is the precordial R wave pattern normal? Are the QRS complexes too small or too large?

Look at the ST segments. Is there an abnormality? Is the abnormality diagnostic of ischemia, infarction, or ventricular strain?
Check the T waves. Is the shape normal? Are there inverted Ts in I, II, or V3-V6?

Look at the QT interval. Is it over half the R-R distance, or over 10 boxes in length?

Once you spot an abnormality, check for other findings that firm up the diagnosis. For example, when you spot a large R in V1, you check for axis deviation, ST depression, and the orientation of the P wave in V1.  [Chapter Menu]

Go to Chapter 2, Parts of the ECG

All material referenced through this menu is excerpted from copyrighted works by Bruce Argyle, MD. You are welcome to use selected portions, as long as appropriate credit is given. The credit for the text referenced through this menu is:

Argyle, B., MicroEKG Computer Program Manual.
Mad Scientist Software, Alpine, Utah

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