Origin of the Electrocardiogram
The waveform of the electrocardiogram (ECG)
recorded from the body surface depends on the
properties of the generator (i.e., cardiac action
potential), the spread of excitation, and the characteristics
of the volume conductor. The electrocardiographic
theory attempts to integrate these
three elements.1
Figure 1–1 shows the temporal relation
between the atrial and ventricular action potentials
and the surface ECG. The P wave is derived
from atrial depolarization, the QRS complex from
ventricular depolarization, the ST segment from
phase 2 (plateau), and the T wave from phase 3
of the ventricular action potential. The activity
of the conducting system is not represented on
the surface ECG. Intracardiac electrodes can be
used to record His bundle potential and signals
from other parts of the conducting system.
theory is based on an elementary model of a
single fiber surrounded by a homogeneous
medium. In such fibers, during resting the positive
and negative boundaries are equal and
opposite in sign for the extent of the conductor.
This means that the polarized surface is closed,
and no potential is recorded (Figure 1–2, top
tracing). When the excitation wave invades
the fiber and reverses the charges across the
membrane, currents flow outside the fiber,
inside the fiber, and across the membrane, as
indicated by the arrows in Figure 1–1 (second
tracing from top). This creates potential differences
of depolarization, which disappear when
the double layer again becomes homogeneous
in the fully activated state, as shown in the
middle tracing of Figure 1–2. Subsequently a
transition from an activated to a resting state
occurs, a process of repolarization shown in
Figure 1–2 (fourth and fifth tracings from top).