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Crit Care Nurse 2003 Oct; 23(5): 58-60
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Ask the Experts
Do you have a clinical, practical, or legal question you’d like to have answered? Send it to us and we’ll pass it on to our "Ask the Experts" panel. Call (800) 394-5995, ext. 8839, to leave your message. Questions may also be faxed to (949) 362-2049, mailed to Ask the Experts, CRITICAL CARE NURSE, 101 Columbia, Aliso Viejo, CA 92656, or sent by e-mail to ccn{at}aacn.org. Questions of the greatest general interest will be answered in this department each and every issue.

Patricia Hahn is a flight nurse with Northwest MedStar, a critical care air transport service in Spokane, Wash. She is also a family nurse practitioner, employed by the Student Health Center at Gonzaga University in Spokane, Wash.

To purchase reprints, contact The InnoVision Group, 101 Columbia, Aliso Viejo, CA 92656. Phone, (800) 809-2273 or (949) 362-2050 (ext 532); fax, (949) 362-2049; e-mail, reprints{at}aacn.org.

Q How do you interpret a 12- lead electrocardiogram (ECG)?

A Patricia Hahn, RN,MN, ARNP, replies:

I asked myself that same question several years ago, and that is when I came up with a systematic approach to ECG interpretation, the ABCDE method. The name of this method is a mnemonic for th e interpretation steps: "A" stands for Analyze rhythm; "B" for Bundle branch block; "C" for Chamber enlargement; "D" for Determine axis; and "E" for Evaluate each area for injury, ischemia, and infarction.

A—Analyze Rhythm

Assess the long lead II tracing at the bottom of the ECG. Interpret the rhythm and measure the PR, QRS, and QT intervals. A QRS duration of 0.12 seconds or greater indicates a delay in ventricular conduction. This delay could be due to a ventricular pacemaker, ventricular rhythm, or a bundle branch block. This finding is important because what depolarizes abnormally will repolarize abnormally; that is, if the QRS complex is distorted because of abnormal ventricular conduction, the ST segments and T waves will also be distorted. In step E, ST segments and T waves are assessed to determine changes related to injury (ST elevation) and ischemia (ST depression and/or T wave inversion). This finding, a wide QRS complex rhythm, will alter the interpretation of these changes on the ECG.

B—Bundle Branch Block

Because you have just measured the QRS complex, if the duration is within normal limits (0.04–0.10 seconds) then no bundle branch block is present and you can move on to the next step. However, if the QRS is equal to or greater than 0.12 seconds, there is a significant delay in conduction, possibly due to a bundle branch block. Look at V1; if the QRS has an rSR’ pattern, a right bundle branch block is present (Figure 1Go). If the QRS complex in V1 looks wide but otherwise fairly normal, with a small R wave and a deep S wave or deep Q wave complex, a left bundle branch block is present (Figure 2Go). With a bundle branch block, all of the ST and T waves will be abnormal. These changes are considered secondary ST-T wave changes and are related to the bundle branch block, rather than primary changes indicating ischemia or injury. If there is a bundle branch block on the ECG, it is difficult, if not impossible, to interpret ischemia, injury, and infarct changes when we get to step E.



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  		Figure 1 Right bundle branch block.

 


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  		Figure 2 Left bundle branch block.

 
C—Chamber Enlargement

On a 12-lead ECG, it is impossible to distinguish between dilation and hypertrophy, you know only that the chamber is enlarged. Enlarged atria can be detected on an ECG with either an increased amplitude of the P wave (P-pulmonale, indicating right atrial enlargement) or an increased duration of the P wave (P-mitrale, indictating left atrial enlargement). As a reminder, normal P-wave amplitude is less than 2.5 mm in height, and normal P-wave duration is <0.11 seconds. Assess lead II, because it tends to best reflect atrial activity.

P-pulmonale, right atrial enlargement, is noted by a tall, peaked P wave with an amplitude exceeding 2.5 mm. P-mitrale, left atrial enlargement, will develop a notched or M-shaped P wave, with a duration that exceeds 0.11 seconds.

Enlargement of the ventricles is assessed by looking at the precordial leads V1 through V6. V1 and V2 tend to be the more right sided of the precordial leads, and V5 and V6 tend to be more left sided. When a chamber is enlarged, it will produce more electrical current. This change is detected on an ECG as increased amplitude, or positive forces recorded as R waves on the ECG. If the right ventricle is enlarged, tall R waves will be detected in V1 and V2, and deep S waves (negative forces) in V5 and V6. Right ventricular enlargement is rarely detected on an ECG, because the left ventricle is normally 2 to 3 times the muscle mass compared with the right ventricle. There are 3 other ECG findings that may also result in tall R waves in V1 and V2, but they will not cause deep S waves in V5 and V6; these findings are right bundle branch block, Wolff-Parkinson-White syndrome, and a posterior wall infarction.

Left ventricular enlargement is a fairly common finding on an ECG. It is detected by tall R waves in V5 and V6, with deep S waves in V1 and V2. Ventricular enlargement will often cause an axis deviation, because the axis tends to shift towards areas of hypertrophy.

D—Determine Axis

The mean QRS axis simply refers to the average direction or flow of current as it depolarizes the myocardium. Generally, current travels down and to the left; therefore, a normal axis would be in the left lower quadrant, 0 to +90° (Figure 3Go). If the axis deviates too far to the right, it would be in the +90° to ±180° quadrant, indicating a right axis deviation; if it deviates too far to the left, (0 to –90°), a left axis deviation is present; and if it falls in the –90° to ±180° quadrant, it is considered an indeterminate axis. Normal axis is extended up to the –30° range, to account for the natural shift of the heart to the left. So, technically –30° to +90° is considered a normal axis.



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  		Figure 3 Axis quadrants.

 
To determine the quadrant, 2 leads should be looked at, lead I and aVF. Remember this simple rule: if current is traveling toward the positive or recording electrode, it will produce a positive QRS complex; if current is traveling away from the positive electrode, it will produce a negative QRS complex (Figure 4Go). First, look at lead I. This lead has its positive electrode located on the left arm and its negative electrode on the right arm. It detects right-to-left movement of current. If current is traveling to the left, lead I will have a positive QRS complex, placing the axis in one of the left quadrants. If lead I has a primarily negative QRS, the current is moving to the right, placing it in one of the right quadrants.



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  		Figure 4 QRS complex polarity.

 
Next, look at aVF. This lead has its positive electrode located on the left foot. It detects upward or downward current. If aVF has a positive QRS complex, current is flowing downward, toward the positive electrode, placing the axis in the lower quadrants. If aVF is negative, the current is flowing upward, away from the positive electrode, placing the axis in the upper quadrants. Determine where the quadrants overlap; the QRS axis lies in that quadrant. To be more specific, look at a hexaxial figure (Figure 5Go). Locate the quadrant, and identify the leads that run through that quadrant. For example, in the left axis quadrant, leads III and aVL bisect that quadrant. Compare the QRS complex in those leads. Which lead has the larger QRS amplitude or size? It does not matter if the QRS complex is positive or negative, we want to know which is larger, as amplitude reflects electrical forces. The lead with the larger QRS complex or amplitude has more electrical current running along its axis. Assign the degree of that lead as the QRS axis. For example, if lead III has the larger QRS complex, a left axis deviation of –60° is present.



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  		Figure 5 Hexaxial figure.

 
If the axis is abnormal, the next step is to determine the cause. An axis deviation may be due to a mechanical shift of the heart or a shift in the electrical current. Generally, your physical assessment or the ECG will reveal the cause. The TableGo provides a list of causes of left and right axis deviations.


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  		Causes of axis deviation

 
E-Evaluate Each Area for Injury, Ischemia, and Infarction

Injury, an acute, yet reversible process, indicates impending myocardial damage. It is detected by ST-segment elevation of 1 mm or more in the limb leads, and 2 mm or more in the precordial leads. Ischemia, also a reversible process, is seen as ST-segment depression and/or T-wave inversion. Infarction indicates permanent myocardial damage. In the limb leads, Q waves will form as a sign of an infarct. A significant Q wave, indicating infarction, must meet 3 criteria:

In the precordial leads (V1 through V6), infarction will be detected by a loss of R waves. Normally R waves progress, becoming larger as you move from V1 to V4, then becoming slightly smaller in leads V5 and V6. Loss of R waves indicates a loss of positive forces because of an infarction.

Myocardial infarctions are described according to the wall of the left ventricle affected. The left ventricle essentially has 5 areas or walls, the anterior, septal, lateral, inferior, and posterior walls. The ECG leads are placed in different positions on the chest and limbs, and look at the various walls of the left ventricle. Leads II, III, and aVF reflect changes in the inferior wall. V1 through V4 assess the anterior and septal walls. Lead I, aVL, V5, and V6 reflect the lateral wall. The posterior wall may be assessed directly with extended precordial leads V7, V8, and V9 or through reciprocal changes in V1 and V2. Right ventricular infarcts often occur with inferior wall infarcts, and occlusion of the right coronary artery. To detect a right ventricular infarct, right-sided precordial leads may be obtained. V4R is the most accurate lead to detect right ventricular infarcts.

This simplified ECG interpretation method, divided into 5 easy steps, covers the basics of 12-lead ECG interpretation, and is not meant to be a comprehensive interpretation. As with any skill, a little practice goes a long way.





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