LEAD PLACEMENT

  An average EKG machine contains: 1) electrode wires with gel patches; 2) a computer which converts the signal into a waveform and adjusts for electrical artifact; 3) a screen to view the cardiac rhythm in real time (many incorporate a 6 second delay), a printer to produce a hard copy of the waveform, and many have a method of storing digitially the waveform data. The data is collected from the patient using a total of 10 electrodes. Electrodes are patches made of electro-conductive material, usually an electrolyte gel. They attach to the patients chest and limbs using a tape like adhesive. Excessive hair can decrease conductivity and compromise the data collection. The electrodes connect to the EKG machine through wires. The procedure is non-invasive and painless, and the actual data sampling for a 12 lead EKG usually takes only 5-10 seconds.. Below is a series of diagrams and explanations of the leads represented by combining specific pairs of electrodes. Remember: an ELECTRODE is a physical patch which connects to the patient; a LEAD is a specific vector in which voltage is measured. In many leads, a central terminal or common ground is used. This is a computed virtual negative ground calculated by combining values from several electrodes. The central terminal allows the EKG to provide information about vectors not represented by specific pairs of electrodes. This will be explained further below.


 







Lead I is the first of three standard limb leads (I, II, III). Limb leads measure cardiac depolarization in the frontal (coronal) plane.

The negative electrode is connected to the RIGHT ARM.
The positive electrode is connected to the LEFT ARM.

The axis is 0 degrees.

When an action potential starts on the right and proceeds toward the left side of the heart, a positive inflection will be seen in lead one. This holds true for all leads. Whenever a current proceeds toward a positive electrode, an upright inflection is seen on the EKG tracing.

 







Lead II is used alone quite frequently. Normal rhythms present with a prominant P wave and a tall QRS.

The negative electrode is connected to the RIGHT ARM.
The positive electrode is connected to the LEFT LEG.

The axis is +60 degrees.

In all the limb leads, the electrodes may be positioned close to the torso. For convenience, they are often placed at the shoulders and hips.

 







Lead III is the last of the three standard limb leads.

The negative electrode is connected to the LEFT ARM.
The positive electrode is connected to the LEFT LEG.

The axis is 120 degrees.

 






In addition to the 3 basic limb leads, there are 3 augmented limb leads. These leads provide additional vector views of cardiac depolarization in the frontal plane.

Unlike leads I, II, III, the augmented leads utilize a central negative terminal. This virtual "electrode" is calculated by the EKG computer to measure vectors originating roughly at the center of the heart.

Augmented Lead (RIGHT) avR

The negative electrode is the central termininal.
The positive electrode is connected to the RIGHT ARM.

The axis is -150 degrees.

 






Augmented Lead (LEFT) avL

The negative electrode is the central termininal.
The positive electrode is connected to the LEFT ARM.

The axis is -30 degrees.

 






Augmented Lead (FEET) avF

The negative electrode is the central termininal.
The positive electrode is connected to the LEFT LEG.

The axis is +90 degrees.

 






As discussed above, each limb lead consists of the difference in electrical potential between two electrodes. One electrode is designated negative and one positive.

Leads I, II, III together form the vectors of an equilateral triangle (0, 60, 120 degrees). This is called Einthoven's triangle. Einthoven found that the sum of current of Lead I + Lead III = Lead II.

He also found that since the heart was approximately centered in the triangle, the vectors could all be shifted inward. Taking the edges of the trangle and drawing them together until their centers intersect results in a triaxial reference figure. This image can be seen in the vectors webpage.










Super-imposing the augmented leads over Einthoven's triangle demonstrates the vector relationship. Using a common terminal the augmented leads provide vector views not offered by the limb leads. They AUGMENT the coronal view of the heart with additional measurements. Their vectors are: -150, -30, +90 degrees. See the vector page for comprehensive diagrams of the limb leads.
 







Precordial leads make up the other 6 leads and electrodes connected during an EKG. These measure depolarization in the axial plane using a central terminal. These leads provide essental information used in localizing ischemic or infarcted tissue during a cardiac event.

The electrodes are placed as shown. The vectors measured are:

V1 +120 degrees
V2 +90 degrees
V3 +75 degrees
V4 +60 degrees
V5 +30 degrees
V6 0 degrees

 

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