A look at 12-lead ECG interpretation

That’s progress for ya!

All about waves

As you examine each lead, note where changes occur so you can identify the area of the heart that’s affected. Remember that

P waves should be upright; however, they may be inverted in lead aVR or biphasic or inverted in leads III, aVL, and V1.

PR intervals should always be constant, just like QRS-complex durations. QRS-complex deflections will vary in different leads. Observe for pathologic Q waves. (See Normal findings in pediatric ECGs.)

Elevated one minute, depressed the next

ST segments should be isoelectric or have minimal deviation. ST-segment elevation greater than 1 mm above the baseline and ST-segment depression greater than 0.5 mm below the baseline are considered abnormal. Leads facing an injured area will have ST-segment elevations, and leads facing away will display ST-segment depressions.

That old, changeable T wave

Ages

and stages

Normal findings in pediatric ECGs

In a neonate, dominant R waves in the chest leads and upright T waves are normal findings. By the end of the first week of life, the T wave in lead V1 becomes inverted and remains inverted through age 7.

An axis that falls between 0 and +90 degrees is considered normal. An axis between +90 and +180 degrees indicates right axis deviation, and one between 0 and –90 degrees indicates left axis

Hexaxial reference system

The hexaxial reference system consists of six bisecting lines, each representing one of the six limb leads, and a circle, representing the heart. The intersection of all lines divides the circle into equal, 30-degree segments.

Shifting degrees

Note that +0 degrees appears at the 3 o’clock position (positive pole lead I). Moving counterclockwise, the degrees become increasingly negative, until reaching ±180 degrees, at the 9 o’clock position (negative pole lead I).

The bottom half of the circle contains the corresponding positive degrees. However, a positive-degree designation doesn’t necessarily mean that the pole is positive.

This diagram is called the hexaxial reference system. Use it to determine your patient’s electrical axis.

deviation. An axis between –180 and –90 degrees indicates extreme axis deviation and is called an indeterminate axis.

To determine your patient’s electrical axis, use one of the two methods described here, the quadrant method or the degree method. (See Axis deviation across the life span.)

Quadrant method

The quadrant method, a fast, easy way to plot the heart’s axis, involves observing the main deflection of the QRS complex in leads I and aVF. (See Quadrant method.) Lead I indicates whether impulses are moving to the right or left, and lead aVF indicates whether they’re moving up or down.

Quadrant method

This chart will help you quickly determine the direction of a patient’s electrical axis. Just observe the deflections of the QRS complexes in leads I and aVF. Then check the chart to determine whether the patient’s axis is normal or has a left, right, or extreme axis deviation.

Ages

and stages

Axis deviation across the life span

Right axis deviation, between +60 degrees and +160 degrees, is normal in a neonate due to dominance of the right ventricle. By age 1, the axis shifts to fall between +10 degrees and +100 degrees as the left ventricle becomes dominant.

Left axis deviation commonly occurs in elderly people. This axis shift may result from fibrosis of the anterior fascicle of the left bundle branch or thickness of the left ventricular wall, which increases by 25% between ages 30 and 80.

INTERPRETING A 12-LEAD ECG

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If the QRS-complex deflection is positive or upright in both leads, the electrical axis is normal. If lead I is upright and lead aVF points down, left axis deviation exists.

Right on and right in

When lead I points downward and lead aVF is upright, right axis deviation exists. Both waves pointing down signal extreme axis deviation.

Degree method

A more precise axis calculation, the degree method gives an exact degree measurement of the electrical axis. (See Degree method.) It also allows you to determine the axis even if the QRS complex isn’t clearly positive or negative in leads I and aVF. To use this method, follow these steps.

Review all six leads, and identify the one that contains either the smallest QRS complex or the complex with an equal deflection above and below the baseline.

Use the hexaxial diagram to identify the lead perpendicular to this lead. For example, if lead I has the smallest QRS complex, then the lead perpendicular to the line representing lead I would be lead aVF.

After you’ve identified the perpendicular lead, examine its QRS complex. If the electrical activity is moving toward the positive pole of a lead, the QRS complex deflects upward. If it’s moving away from the positive pole of a lead, the QRS complex deflects downward.

Plot this information on the hexaxial diagram to determine the direction of the electrical axis.

Axis deviation

Finding a patient’s electrical axis can help confirm a diagnosis or narrow the range of possible diagnoses. (See Causes of axis deviation, page 262.) Factors that influence the location of the axis include the heart’s position in the chest, the heart’s size, the patient’s body size or type, the conduction pathways, and the force of the electrical impulses being generated.

Remember that electrical activity in the heart swings away from areas of damage or necrosis, so the damaged part of the heart will be the last area to be depolarized. For example, in right

Memory jogger

Think of

the QRScomplex deflections in leads I and aVF as thumbs pointing up or down. Two thumbs up is normal; anything else is abnormal.

Heart size can influence the location of the electrical axis. Tell me, how does my size look to you...?

Step 2

Locate the axis for lead III on the hexaxial diagram. Then find the axis perpendicular to it, which is the axis for lead aVR.

Step 3

Now, examine the QRS complex in lead aVR, noting whether the deflection is positive or negative. As you can see, the QRS complex for this lead is negative. This tells you that the electric current is moving toward the negative pole of aVR, which, on the hexaxial diagram, is in the right lower quadrant at +30 degrees. So the electrical axis here is normal at +30 degrees.