Volume Conductor Principles and ECG Rules of Interpretation
The electrocardiogram uses electrodes on the surface of the body to measure the electrical
activity of the heart. It is possible to place electrodes on the body
surface and measure cardiac potentials because the body acts as a conductor of
the electrical currents generated by the heart. What do these electrodes actually measure?
If
a piece of living ventricular muscle is placed in a bath containing a salt solution to conduct
electrical currents, and electrodes are placed in the
bath on either side of the muscle, no potential difference would be recorded
between the two electrodes when the muscle is in its
polarized, resting state (top panel of figure
at right). The reason for this is that the outside of the cells is
positive relative to the inside because the
resting membrane
potential is be about -90 mV; therefore, no currents will flow along the surface of the muscle
and through the bath. If the left side of the muscle
is stimulated electrically to induce self-propagating
action
potentials, a
wave of depolarization would sweep across the muscle from left-to-right (lower
panel). Midway through this depolarization process, cells on the left (depolarized cells) would be negative on the outside
relative to the inside, while non-depolarized cells on the right of the muscle
would still be polarized (positive on the outside). There would now
exist a potential difference between the positive and negative electrodes.
By convention, a wave of depolarization heading toward the positive
electrode is recorded as a positive voltage (upward deflection in the
recording). After the wave of
depolarization sweeps across the entire muscle mass, all the cells on the
outside are negative, and once again, no potential difference would exist
between the two electrodes.
The
entire process of depolarization and repolarization is depicted in the animated
model to
the right, which is representative of the electrical events that occur in the
atria. In the resting, polarized state, no potential difference is
measured between the positive and negative electrodes (i.e., isoelectric - flat
red line). When the left side of the tissue becomes depolarized
(representing firing of the
SA
node), a wave of depolarization begins to spread across the
atria. During this time, some of the muscle mass temporarily remains
positive on the outside (polarized) and while some is negative (depolarized);
thus, there is a separation of charges which causes a potential difference between the
two electrodes. Because the wave of
depolarization is moving toward the positive electrode, by convention, a
positive voltage (upward deflection) is recorded. The voltage reaches its
maximal positive value when half the tissue is depolarized. Once the entire
atrial mass is depolarized (all cells negative on outside), there is no longer
be a potential difference and the voltage is zero just as it was in the polarized state. When repolarization
occurs, starting first with the left side ( SA nodal region) then moving across the atria,
there will once again be both positive and negative charges on the surface of
the atria, but this time, the negative charges will be closest to the positive
electrode. The wave of repolarization sweeping across the atria away from the
negative electrode and toward the positive electrode causes, by convention, a negative voltage (downward deflection) to occur. Finally, when all of
the cells are repolarized, the measured voltage difference will once again be
zero until
another wave of depolarization occurs.
A similar process occurs within the ventricles with
one major difference: repolarization normally occurs in a direction opposite to
depolarization. In other words,
the last cells in the ventricle to
depolarize are the first to repolarize. This results in a
positive
recording as the ventricles
repolarize as shown in the animated model to the right.
Several important observations and rules emerge from
these volume conductor considerations:
-
A wave of
depolarization traveling toward a positive electrode results in a positive
deflection in the ECG trace.
-
A wave of
depolarization traveling away from a positive electrode results in a
negative deflection.
-
A wave of
repolarization traveling toward a positive electrode results in a negative
deflection.
-
A wave of
repolarization traveling away from a positive electrode results in a
positive deflection.
-
A wave of
depolarization or repolarization traveling perpendicular to an electrode
axis results in a biphasic deflection of equal positive and negative
voltages (i.e., no net deflection).
-
The instantaneous
amplitude of the measured potentials depends upon the orientation of the
positive electrode relative to the mean electrical vector.
-
The voltage amplitude
is directly related to the mass of tissue undergoing depolarization or
repolarization.
The first four rules are derived from the volume
conductor model described above. The fifth rule is also based on volume
conductor principles and could be modeled by placing the positive and negative
electrodes midway on the top and bottom surfaces of the tissue instead of on the
ends. In this case, the positive electrode would first measure a positive
voltage as the wave of depolarization transverse the tissue from the left edge
to the midpoint (toward the electrode), and then the electrode would measure a
negative voltage as the wave moved away from the electrode to the right edge. The sixth rule takes into consideration
that at any given point in time during depolarization in the atria or ventricles
there are many separate waves of depolarization traveling in different
directions relative to the positive electrode. The recording by the
electrode reflects the average, instantaneous direction and magnitude (i.e.,
mean electrical vector) for all of the individual depolarization
waves. The seventh rule simply states that the amplitude of the wave
recorded by the ECG is directly related to the mass of the muscle
undergoing depolarization or repolarization. For example, when the mass of
the left ventricle is increased by hypertrophy, the voltage amplitude of the QRS
complex, which represents ventricular depolarization, is increased in certain
leads.
RK Revised 04/06/07
