A Look at the 12-Lead ECG
The 12-lead ECG is a diagnostic test that helps identify pathologic conditions, especially angina and acute myocardial infarction (AMI). It gives a more complete view of the heart’s electrical activity than a rhythm strip and can be used to assess left ventricular function. Patients with other conditions that affect the heart’s electrical system may also benefit from a 12-lead ECG. (See Why a 12-lead ECG?,)
Interdependent Evidence
Just like other diagnostic tests, a 12-lead ECG must be viewed alongside other clinical evidence. Always correlate the patient’s ECG results with their history, physical assessment findings, laboratory results, and medication regimen.
Remember, too, that an ECG can be done in various ways, including over a telephone line. (See Transtelephonic cardiac monitoring) Transtelephonic monitoring, in fact, has become increasingly important as a tool for assessing patients at home and in other nonclinical settings.
How Leads Work
The 12-lead ECG records the heart’s electrical activity using a series of electrodes placed on the patient’s extremities and chest wall. The 12 leads include three bipolar limb leads (I, II, and III), three unipolar augmented limb leads (aVR, aVL, and aVF), and six unipolar precordial, or chest, leads (V1, V2, V3, V4, V5, and V6). These leads provide 12 different views of the heart’s electrical activity. (See A look at the leads.)
Up, Down, and Across
Scanning up, down, and across the heart, each lead transmits information about a different area. The waveforms obtained from each lead vary depending on the location of the lead in relation to the wave of depolarization, or electrical stimulus, passing through the myocardium.
Limb Leads
The six limb leads record electrical activity in the heart’s frontal plane. This plane is a view through the middle of the heart from top to bottom. Electrical activity is recorded from the anterior to the posterior axis.
Precordial Leads
The six precordial leads provide information on electrical activity in the heart’s horizontal plane, a transverse view through the middle of the heart, dividing it into upper and lower portions. Electrical activity is recorded from either a superior or an inferior approach.
The Electrical Axis
Besides assessing 12 different leads, a 12-lead ECG records the heart’s electrical axis. The axis is a measurement of electrical impulses flowing through the heart.
As impulses travel through the heart, they generate small electrical forces called instantaneous vectors. The mean of these vectors represents the force and direction of the wave of depolarization through the heart. That mean is called the electrical axis. It’s also called the mean instantaneous vector and the mean QRS vector.
Havin’ a Heart Wave
In a healthy heart, impulses originate in the sinoatrial node, travel through the atria to the atrioventricular node, and then to the ventricles. Most of the movement of the impulses is downward and to the left, the direction of a normal axis.
Swingin’ on an Axis
In an unhealthy heart, axis direction varies. That’s because the direction of electrical activity swings away from areas of damage or necrosis and toward areas of hypertrophy. Knowing the normal deflection of each lead will help you evaluate whether the electrical axis is normal or abnormal.
Obtaining a 12-Lead ECG
To obtain an ECG, you’ll need to:
- gather the appropriate supplies
- explain the procedure to the patient
- attach the electrodes properly
- know how to use the ECG machine.
Let’s take a closer look at obtaining an ECG.
Explain the Procedure
Tell the patient that the practitioner has ordered an ECG, and explain the procedure. Emphasize that the test takes only a few minutes and that it’s a safe, painless, and risk-free way to evaluate cardiac function.
Answer the patient’s questions, and offer reassurance. Preparing him or her well helps alleviate anxiety and promote cooperation.
Prepare the Patient
Ask the patient to lie in a supine or semi-Fowlers position with his or her arms at his or her sides. Ensure privacy, and expose the patient’s arms, legs, and chest, draping for comfort. Make sure the patient’s legs are uncrossed. Remove electrical devices, including cell phones, from the area because of the interference they may cause. (See Obtaining a pediatric ECG.)
Skin Preparation
Select the areas where you’ll attach the electrodes. Choose spots that are flat and fleshy, and avoid areas that are muscular or bony. Dry the skin if it is wet or diaphoretic. Clip the area if it’s excessively hairy. Clean excess oil or other substances from the skin to enhance electrode contact. Follow your hospital policy for any additional skin preparation such as the use of a nonalcohol wipe or abrasive skin prep pad. Remember—the better the electrode contact, the better the recording.
Make the Recording
The 12-lead ECG offers 12 different views of the heart, just as 12 photographers snapping the same picture would produce 12 different snapshots. To help ensure an accurate recording—or set of “pictures”—the electrodes must be applied correctly. Inaccurate placement of an electrode by greater than 5/8″ (1.5 cm) from its stand ardized position may lead to inaccurate waveforms and an incorrect ECG interpretation.
The 12-lead ECG requires four electrodes on the limbs and six across the front of the chest wall.
Going Out on a Limb Lead
To record the bipolar limb leads I, II, and III and the unipolar limb leads aVR, aVL, and aVF, place electrodes on both of the patient’s arms and on their left leg. The right leg also receives an electrode, but that electrode acts as a ground and doesn’t contribute to the waveform.
Where the Wires Go
Finding where to place the electrodes on the patient is easy because each leadwire is labeled or color-coded. (See Monitoring the limb leads.) For example, a wire—usually white—might be labeled “RA” for right arm. Another wire—usually red—might be labeled “LL” for left leg. Precordial leads are also labeled or color-coded according to which wire corresponds to which lead.
No Low Leads Allowed
To record the six precordial leads (V1 through V6), position the electrodes on specific areas of the anterior chest wall. It is vital that the practitioner knows the exact placement of each electrode on the patient as incorrect placement can lead to a false diagnosis of infarction or negative changes on the ECG. (See Positioning precordial electrodes)
- Place lead V1 over the fourth intercostal space at the right sternal border. To find the space, locate the sternal notch at the second rib and feel your way down along the sternal border until you reach the fourth intercostal space (V1 and V2 electrodes that are placed in a more superior location can mimic an anterior wall MI and cause T-wave inversion).
- Place lead V2 just opposite V1, over the fourth intercostal space at the left sternal border.
- Place lead V3 midway between V2 and V4. Tip: Placing lead V4 before lead V3 makes it easier to see where to place lead V3.
- Place lead V4 over the fifth intercostal space at the left midclavicular line.
- Place lead V5 over the fifth intercostal space at the left anterior axillary line.
- Place lead V6 over the fifth intercostal space at the left midaxillary line. If you’ve placed leads V4 through V6 correctly, they should line up horizontally.
Give Me More Electrodes!
In addition to the 12-lead ECG, two other ECGs may be used for diagnostic purposes: the posterior-lead ECG and the right chest–lead ECG. These ECGs use chest leads to assess areas stand ard 12-lead ECGs can’t.
Seeing Behind Your Back
Because of lung and muscle barriers, the usual chest leads can’t “see” the heart’s posterior surface to record myocardial damage there. Some practitioners add three posterior leads to the 12-lead ECG: leads V7, V8, and V9. These leads are placed opposite anterior leads V4, V5, and V6, on the left side of the patient’s back, following the same horizontal line.
On rare occasions, a practitioner may request right-sided posterior leads. These leads are labeled V7R, V8R, and V9R and are placed on the right side of the patient’s back. Their placement is a mirror image of the electrodes on the left side of the back. This type of ECG provides information on the right posterior area of the heart.
Checking Out the Right Chest
The usual 12-lead ECG evaluates only the left ventricle. If the right ventricle needs to be assessed for damage or dysfunction, the practitioner may order a right chest–lead ECG. For example, a patient with an inferior wall MI might have a right chest–lead ECG to rule out right ventricular involvement.
With this type of ECG, the six leads are placed on the right side of the chest in a mirror image of the stand ard precordial lead placement. Electrodes start at the left sternal border and swing down under the right breast area.
Know Your Machine
After you understand how to position the electrodes, familiarize yourself with the ECG machine. Machines come in two types: multichannel recorders and single-channel recorders. Because single-channel recorders are rarely used, we’ll only discuss a multichannel recorder.
With a multichannel recorder, you’ll attach all electrodes to the patient at once and the machine prints a simultaneous view of all leads. To begin recording the patient’s ECG, follow the following steps:
- Plug the cord of the ECG machine into a grounded outlet. If the machine operates on a charged battery, it may not need to be plugged in.
- Enter the patient’s identification data as prompted by the ECG machine.
- Prepare the patient’s skin, and then place the electrodes on the patient’s chest, arms, and legs.
- Make sure all leads are securely attached.
- Instruct the patient to relax, lie still, and breathe normally. Ask them not to talk during the recording to prevent distortion of the ECG tracing.
- Verify that the ECG paper speed selector is set to 25 mm/sec. If necessary, calibrate or stand ardize the machine according to the manufacturer’s instructions.
- Press the appropriate button to record the ECG. If you’re performing a right chest–lead ECG, select the appropriate button for recording or note it on the ECG hardcopy.
- Observe the quality of the tracing. When the machine finishes the recording, turn it off.
- Remove the electrodes and , if necessary, clean the patient’s skin. If the ECG machine you’re using also transmits a copy to a central monitoring area, make sure the copy has been transmitted.
Interpreting the Recording
ECG tracings from multichannel machines will all look the same. (See Multichannel ECG recording.) The printout will show the patient’s name and identification number. At the top of the printout, you’ll see the patient’s heart rate and wave durations, measured in seconds.
Some machines can also record ST-segment elevation and depression. The name of the lead appears next to each 6-second strip.
Remember, ECGs are legal documents. They belong on the patient’s chart and must be saved for future reference and comparison with baseline strips.
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 (SAECG). 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, coronary heart disease, cardiomyopathy, or unexplained syncope, for example—are good cand idates for an SAECG. Keep in mind that a 12-lead ECG should be done when the patient is free from arrhythmias.
Noise-Free
An SAECG 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 takes approximately 7 to 10 minutes. The machine’s computer detects late electrical potentials and then enlarges them so they’re recognizable. The electrodes for an SAECG 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. Multiple electric signals from the heart are averaged to remove interference and reveal small variations in the QRS complex, usually called “late potentials.” These late potentials may represent a predisposition toward potentially dangerous ventricular arrhythmias. Muscle noise can’t be filtered, however, so the patient must lie still for the test.
