OBTAINING A 12-LEAD ECG
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Signal-averaged ECG
Although most patients will be tested with a 12-lead ECG, some may benefit from being tested with a signal-averaged ECG. This simple, noninvasive test helps identify patients at risk for sudden death from sustained ventricular tachycardia.
The test uses a computer to identify late electrical potentials— tiny impulses that follow normal depolarization. Late electrical potentials can’t be detected by a 12-lead ECG.
Who gets the signal?
Patients prone to ventricular tachycardia—those who have had a recent MI or unexplained syncope, for example—are good candidates for a signal-averaged ECG. Keep in mind that a 12-lead ECG should be done when the patient is free from arrhythmias.
Noise free
Memory jogger
To help you remember
the electrodes for the signal-averaged ECG, think of the phrase “XYZ times 2 and G.” The electrodes are X–, X+,
Y–, Y+, Z–, Z+, and G (ground).
A signal-averaged ECG is a noise-free, surface ECG recording taken from three specialized leads for several hundred heartbeats. (See Electrode placement for a signal-averaged ECG.) The test
Electrode placement for a signal-averaged ECG
Electrodes are placed much differently for a signal-averaged ECG than they are for a 12-lead ECG. Here’s one method:
1.Place the positive X electrode at the left fourth intercostal space, midaxillary line.
2.Place the negative X electrode at the right fourth intercostal space, midaxillary line.
3.Place the positive Y electrode at the left iliac crest.
4.Place the negative Y electrode at the superior aspect of the manubrium of the sternum.
5.Place the positive Z electrode at the fourth intercostal space left of the sternum.
6.Place the ground (G) on the lower right at the eighth rib.
7.Reposition the patient on his side, or have him sit forward. Then place the negative Z electrode on his back, directly posterior to the positive Z electrode.
8.Attach all the leads to the electrodes, being careful not to dislodge the posterior lead.
Now, you can obtain the tracing.
takes approximately 10 minutes. The machine’s computer detects late electrical potentials and then enlarges them so they’re recognizable. The electrodes for a signal-averaged ECG are labeled X–, X+, Y–, Y+, Z–, Z+, and G.
The machine averages signals from these leads to produce one representative QRS complex without artifacts. This process cancels noise, electrical impulses that don’t occur as a repetitious pattern or with the same consistent timing as the QRS complex. That additional noise is filtered out so late electrical potentials can be detected. Muscle noise can’t be filtered, however, so the patient must lie still for the test.
That’s a wrap!
Obtaining a 12-lead ECG
12-lead ECG basics
• Provides 12 different views of the heart’s electrical activity
The limb leads
•Three bipolar limb leads: I, II, and III
•Three unipolar limb leads: aVR, aVL, and aVF
•Record electrical activity in the heart’s frontal plane, providing a view through the middle of the heart from top to bottom
The precordial leads
•Six unipolar precordial (chest) leads: V1 through V6
•Record electrical activity in the heart’s horizontal plane, providing a transverse view through the middle of the heart, dividing it into upper and lower portions
Electrical axis
•Measurement of the electrical impulses flowing through the heart
•Normal axis downward and to the left
•Direction of electrical activity swings away from areas of damage or necrosis and toward areas of hypertrophy
Placing the leads
•Bipolar and unipolar limb leads: electrodes on both arms and the left leg, ground on right leg
•V1: Over fourth intercostal space at the right sternal border
•V2: Over fourth intercostal space at the left sternal border
•V3: Midway between leads V2 and V4
•V4: Over fifth intercostal space at left midclavicular line
•V5: Over fifth intercostal space at left anterior axillary line
•V6: Over fifth intercostal space at left midaxillary line
Views of the heart walls
•Lead I: Lateral wall
•Lead II: Inferior wall
•Lead III: Inferior wall
•Lead aVR: No specific view
•Lead aVL: Lateral wall
•Lead aVF: Inferior wall
•Lead V1: Septal wall
•Lead V2: Septal wall
•Lead V3: Anterior wall
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OBTAINING A 12-LEAD ECG |
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Obtaining a 12-lead ECG (continued) |
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• Lead V4: Anterior wall |
Types of ECGs |
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• Lead V5: Lateral wall |
• Multichannel ECG: all electrodes |
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• Lead V6: Lateral wall |
attached at one time to provide simulta- |
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Other lead placements |
neous views of all leads |
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• Signal-averaged ECG: use of computer |
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• Posterior leads: V7, V8, and V9 are |
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to identify late electrical potentials from |
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placed opposite V4, V5, and V6 on the left |
three specialized leads over hundreds |
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side of the back to view posterior surface |
of beats; identifies patients at risk for |
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of the heart |
sudden cardiac death from ventricular |
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• Right chest leads: placed on right chest |
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tachycardia |
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in mirror image of standard precordial |
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leads to view right ventricle |
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Quick quiz
1.The precordial leads are placed on the:
A.anterior chest starting with the fourth intercostal space at the right sternal border.
B.lateral chest starting with the fourth intercostal space at the left midaxillary line.
C.posterior chest wall starting with the fourth intercostal space at the midscapular line.
D.left leg and both arms.
Answer: A. Lead V1 is placed anteriorly between the fourth and fifth ribs at the right sternal border. Leads V2 to V6 are then placed accordingly.
2.A 12-lead ECG is used to assess function of the:
A.right ventricle.
B.left ventricle.
C.right and left ventricle simultaneously.
D.right and left atria simultaneously.
Answer: B. A 12-lead ECG gives a more complete view of the heart’s electrical activity than a rhythm strip and is used to assess left ventricular function.
3.A posterior-lead ECG is used to assess:
A.posterior myocardial damage.
B.inferior myocardial damage.
C.damage to the interventricular septum.
D.damage to the base of the heart.
Answer: A. The posterior-lead ECG assesses damage to the posterior surface of the heart, an area a standard 12-lead ECG can’t detect.
4. When recording a 12-lead ECG, the paper speed should be set at:
A.10 mm/second.
B.20 mm/second.
C.25 mm/second.
D.50 mm/second.
Answer: C. The correct paper speed for a 12-lead ECG is 25 mm/ second.
5.A signal-averaged ECG measures:
A.electrical impulses from the sinoatrial node.
B.electrical impulses arriving at the atrioventricular node.
C.action potentials of individual cardiac cells.
D.late electrical potentials throughout the heart.
Answer: D. Signal-averaged ECGs measure late electrical potentials, tiny electrical impulses that occur after depolarization and can cause ventricular tachycardia.
6. To record the bipolar limb leads I, II, and III and the unipolar limb leads aVR, aVL, and aVF, place electrodes on:
A.both of the patient’s arms and left leg, with a ground on his right leg.
B.the patient’s right arm and left leg, with a ground on his left arm.
C.both of the patient’s legs and his left arm, with a ground on his right arm.
D.both of the patient’s arms and his right leg, with a ground on his left leg.
Answer: A. To record these bipolar and unipolar limb leads, place electrodes on both of the patient’s arms and left leg, with a ground on his right leg.
OBTAINING A 12-LEAD ECG
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7.On the 12-lead ECG printout, you should write:
A.date, practitioner’s name, patient’s name, and your signature.
B.date, time, practitioner’s name, and special circumstances.
C.practitioner’s name, patient’s name, and your signature.
D.patient’s name, room number, and date.
Answer: B. On the printout, be sure to write the date, time, practitioner’s name, and special circumstances.
Scoring
If you answered all seven questions correctly, great job! You’re the new leader of the 12-lead ECG pack.
If you answered six questions correctly, we’re impressed! Your leadership qualities are obvious, and you should be next in line for a top job.
If you answered fewer than six questions correctly, that’s okay! You’ll be sure to take the lead in the next chapter.
