An electrocardiogram is simply a measure of voltage changes in the body. Any large electrical event can be detected.
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, its necessary to have more than just one set of electrodes. (Electrical activity causes a voltage change only if its 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]
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).
Three additional limb leads can be obtained by mixing combinations of electrodes. These are leads R, L, and F.
This new ECG lead will detect electricity best if its 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.
Lead L sees electrical activity best if its 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.
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]
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).
Its 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]
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 hearts 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 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 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.
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:
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?
Look at the QT interval. Is it over half the R-R distance, or over 10 boxes in length?
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