ELECTROCARDIOGRAM

Electrocardiogram

Because the body fluids are good conductors (ie, because the body

is a volume conductor), fluctuations in potential, representing

the algebraic sum of the action potentials of myocardial fibers,

can be recorded extracellularly. The record of these fluctuations

in potential during the cardiac cycle is the ECG.

The ECG may be recorded by using an active or

exploring electrode connected to an indifferent electrode at

zero potential (unipolar recording) or by using two active

electrodes (bipolar recording). In a volume conductor,

the sum of the potentials at the points of an equilateral

triangle with a current source in the center is zero at all times.

A triangle with the heart at its center (Einthoven triangle,

see below) can be approximated by placing electrodes on both

arms and on the left leg. These are the three standard limb

leads used in electrocardiography. If these electrodes are connected

to a common terminal, an indifferent electrode that

stays near zero potential is obtained. Depolarization moving

toward an active electrode in a volume conductor produces

a positive deflection, whereas depolarization moving in the

opposite direction produces a negative deflection.

The names of the various waves and segments of the ECG

in humans are shown in By convention, an upward

deflection is written when the active electrode becomes positive

relative to the indifferent electrode, and a downward deflection

is written when the active electrode becomes negative.

As can be seen in the P wave is primarily produced

by atrial depolarization, the QRS complex is dominated by ventricular

depolarization, and the T wave by ventricular repolarization.

The U wave is an inconstant finding that may be due to

ventricular myocytes with long action potentials. However, the

contributions to this segment are still undetermined. The intervals

between the various waves of the ECG and the events in the

heart that occur during these intervals are shown in.

BIPOLAR LEADS

Bipolar leads were used before unipolar leads were developed. The standard limb leads each record the differences in potential between two limbs. Because current flows only in the body fluids, the records obtained are those that would be obtained if the electrodes were at the points of attachment of the limbs, no matter where on the limbs the electrodes are placed. In lead I, the electrodes are connected so that an upward deflection is inscribed when the left arm becomes positive relative to the right (left arm positive). In lead II, the electrodes are on the right arm and left leg, with the leg positive; and in lead III, the electrodes are on the left arm and left leg, with the leg positive.

UNIPOLAR LEADS

An additional nine unipolar leads, that is, leads that record the potential difference between an exploring electrode and an indifferent electrode, are commonly used in clinical electrocardiography. There are six unipolar chest leads (precordial leads) designated V1–V6 (Figure 29–6) and three unipolar limb leads: VR (right arm), VL (left arm), and VF (left foot). The indifferent electrode is constructed by connecting electrodes placed on the two arms and the left leg to a central terminal. This “V” lead effectively records a “zero” potential because they are situated such that the electrical activity should be cancelled out. Augmented limb leads, designated by the letter a (aVR, aVL, aVF), are generally used. The augmented limb leads do not use the “V” electrode as the zero, rather, they are recordings between the one, augmented limb and the other two limbs. This increases the size of the potentials by 50% without any change in configuration from the nonaugmented record.

Unipolar leads can also be placed at the tips of catheters and inserted into the esophagus or heart. Although sensitivity can be increased, this is obviously more invasive and thus, not a first step in obtaining electrical readings.