Electrocardiogram
An electrocardiogram is a graphic representation of voltage variations plotted against time. The variations result from the depolarization and repolarization of the cardiac muscle, which produces electric fields that reach the surface of the body where electrodes are located. An electrocardiographic machine is a galvanometer that records voltage variations, usually on paper tape. The first such machine was developed by Wilhelm Einthoven in 1906. It consisted of a silver-plated quartz string situated in a fixed magnetic field. Voltage variations from the body passed through the string, and the interaction of the electric fields between the magnet and the string resulted in the string’s movement, which was photographed. The modern ECG machine is similar to these early models, but microelectronics and computer interfaces have been incorporated, making them more useful and powerful. Although more convenient to use, these newer machines are no more accu- rate than the original ECG built by Einthoven.
Since the development
of a practical method of recording the ECG, much has been learned about the
electrophysiology of the heart. In the major contribution, Nobel Prize winner Einthoven
described the vector concept and stated that the action current of the
heart, often called the “accession” or “regression” wave, can be represented by
a vector that has magnitude, direction, and sense. The magnitude of the voltage
of the accession wave is the length of the arrow shaft, the direction is
determined with respect to a line of reference, and the sense is indicated
by the presence of an arrowhead on the shaft. In its simplest concept, the
vector represents the magnitude of a single dipole (i.e., a paired
electric charge, minus and plus). Likewise, the electrical effect of a group of
dipoles can be represented by a vector.
The electrocardiogram
is a record of voltage variations plotted against time. The paper on which the ECG
is recorded is ruled in 1-mm-spaced lines, horizontally and vertically. When the
tracing is properly standardized (1-mV change produces 10-mm stylus deflection),
each vertical space represents a voltage change of 0.1 mV, and each horizontal space
an interval of 40 milliseconds (“m-sec” in Plate 2-15).
Each fifth line, horizontal and vertical, is heavy. The time between the heavy lines
is 0.2 second. The voltage change between two heavy lines is 0.5 millivolt (mV).
A P wave is the
result of atrial depolarization and should not exceed 2.5 mm (0.25 mV) in height
in lead II or longer than 0.12 second. The P-R interval, which includes the P
wave plus the P-R segment, is a measure of the interval from the beginning of atrial
depolarization to the beginning of ventricular depolarization. This interval should
not be greater than 0.2 second for rates greater than 60 beats per minute. The Q
wave is the first downward deflection of the QRS complex and represents septal
depolarization. The R wave is the first positive, or upward, deflection
of the QRS complex, nor- mally caused by apical LV depolarization. The S
wave is the first negative deflection after the R wave, caused by
depolarization of the posterior basal region of the left ventricle. The voltage
of the R wave in the precordial leads should not exceed 27 mm. The Q-T interval
is measured from the beginning of the QRS complex to the end of the T wave, including
the QRS complex, S-T segment, and T wave intervals, the latter two constituting the ST interval. The Q-T interval varies with the cardiac rate and should not
be greater than 0.43 second for rates greater than 60 beats/min. The total QRS
interval should not exceed 0.1 second.
The cardiac rate
may be determined by counting the number of R-R intervals within 16 heavy vertical
time lines (15 time spaces) and multiplying by 20. The first interval counted is
coincident with the zero time line (see Plate 2-15).
ELECTROCARDIOGRAPHIC LEADS AND
REFERENCE LINES
The conventional
electrical connections used for recording the ECG are the limb leads, augmented
limb leads, and precordial leads.
The bipolar limb
leads detect electrical variations at two points and display the difference. Lead
I is the connection between the electrodes on the left arm and right arm; the galvanometer
is between these points of contact (see Plate 2-16). When
the left arm is in a positive field of force with respect to the right arm, an
upward (positive) deflection is written in lead I. Lead II is the connection between
the left leg and right arm electrodes. When the left leg is in a positive field
of force with respect to the right arm, an upward deflection is written in lead
II. Lead III is the connection between the left leg and left arm. When the left
leg is in a positive field of force with respect to the left arm, a positive deflection
is written in lead III.
The unipolar augmented
limbs leads register the electrical variations in potential at one point (right
arm, left arm, or left leg) with respect to a point that does not vary significantly
in electrical activity during cardiac contraction (see Plate
2-16). The lead is augmented by virtue of the type of electrical connection,
which results in a trace of increased amplitude, versus the older Wilson unipolar
lead connections. Lead aVR inscribes the electrical potentials of the
right arm with respect to a null point, which is made by uniting the wires from
the left arm and left leg. Lead aVL records the potentials at the left
arm in relation to a connection made by the union of wires from the right arm and
left foot. Lead aVF reveals the potentials at the left foot in reference
to a junction made by the union of wires from the left and right arms.
The unipolar precordial
leads are recorded in chest positions 1 through 6 (see Plate
2-16). The V designa- tion indicates that the movable electrode registers the
electric potential under the electrode with respect to a V, or central terminal,
connection, which is made by connecting wires from the right arm, left arm, and
left leg. The electric potential of the central terminal con- nection does not vary
significantly throughout the cardiac cycle; therefore the recordings made with the
V connection show the electrical variations occurring under the movable precordial
electrode. Position V1 is at the fourth intercostal space to the right
of the sternum; V2 is at the fourth intercostal space to the left of
the sternum; V4 is at the left midclavicular line in the fifth intercostal
space; V3 is halfway between V2 and V4; V5
is at the fifth intercostal space in the anterior axillary line; and V6
is at the fifth intercostal space in the left midaxillary line.
At times, other
precordial lead placements are helpful, including those elevated 2 inches (5 cm)
above the usual positions (EV1, EV2,
etc.), which may help to detect MIs. Precordial leads are also placed 2 inches
below the usual positions (LV1, LV2, etc.) when the heart
is unusually low in the thorax, as in patients with pulmonary emphysema. Leads to
the right of V1 (V3R, V4R, etc.) are used to differentiate
right bundle branch block and right ventricular hypertrophy from the normal condition.
Leads farther to the left (V7, V8, etc.) are used to explore
the left ventricle when it is directed posteriorly.
Reference
Lines
For the various leads, the reference lines of Einthoven are shown in Plate 2-16 as red arrows. For example, the line of reference for lead I connects the left and right arm electrodes. An accession wave (vector) directed toward the arrowhead of any of the red arrows results in an upward (positive) deflection in the ECG. If the electrical activity, or accession wave, is directed toward the tail of the reference arrow, a downward (negative) deflection is written, but if this wave is perpendicular to the line (90 degrees), no deflection (or a small biphasic one) will be written. The height of the ECG wave is proportional to the magnitude of the projection of the accession wave vector on a reference line.
