Basic 12-lead electrocardiography



Basic 12-lead electrocardiography





The 12-lead electrocardiogram (ECG) is a diagnostic test that helps identify pathologic conditions, especially ischemia and acute myocardial infarction (MI). It provides a more complete view of the heart’s electrical activity than a rhythm strip and can be used to assess left ventricular function more effectively. Patients with conditions that affect the heart’s electrical system may also benefit from a 12-lead ECG, including those with:



  • cardiac arrhythmias


  • heart chamber enlargement or hypertrophy


  • digoxin or other drug toxicity


  • electrolyte imbalances


  • pulmonary embolism


  • pericarditis


  • pacemakers


  • hypothermia.

Like other diagnostic tests, a 12-lead ECG must be viewed in conjunction with other clinical data. Therefore, always correlate the patient’s ECG results with the history, physical assessment findings, and results of laboratory and other diagnostic studies as well as the drug regimen.

Remember, too, that an ECG can be done in various ways, including over a telephone line. (See Transtelephonic cardiac monitoring, page 246.) In fact, transtelephonic monitoring has become increasingly important as a tool for assessing patients at home and in other nonclinical settings.


Fundamentals

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, 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 ECG leads.)


Scanning up, down, and across, each lead transmits information about a different area of the heart. The waveforms obtained
from each lead vary depending on the lead’s location in relation to the wave of depolarization passing through the myocardium.



Limb leads

The six limb leads record electrical activity in the heart’s frontal plane, a view through the middle of the heart from top to bottom. Electrical activity is recorded
from the anterior to the posterior axes.


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.


Electrical axis

As well as assessing 12 different leads, a 12-lead ECG records the heart’s electrical axis. The term axis refers to the direction of depolarization as it spreads 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—the electrical axis. The electrical axis is also called the mean instantaneous vector and the mean QRS vector.

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.

In an unhealthy heart, axis direction varies. That’s because the direction of electrical activity travels 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.

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Aug 18, 2016 | Posted by in CRITICAL CARE | Comments Off on Basic 12-lead electrocardiography

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