General Description
As the heart undergoes
depolarization and
repolarization , the electrical
currents that are generated
spread not only within the heart,
but also throughout the body.
This electrical activity generated
by the heart can be measured by
an array of electrodes placed on
the body surface. The recorded
tracing is called an
electrocardiogram (ECG, or EKG).
A "typical" ECG tracing is shown
to the right. The different waves
that comprise the ECG represent
the sequence of depolarization
and repolarization of the atria
and ventricles. The ECG is
recorded at a speed of 25 mm/
sec, and the voltages are
calibrated so that 1 mV = 10 mm
in the vertical direction.
Therefore, each small 1-mm
square represents 0.04 sec (40
msec) in time and 0.1 mV in
voltage. Because the recording
speed is standardized, one can
calculate the heart rate from the
intervals between different
waves.
P wave
The P wave represents the wave
of depolarization that spreads
from the SA node throughout the
atria, and is usually 0.08 to 0.1
seconds (80-100 ms) in
duration. The brief isoelectric
(zero voltage) period after the P
wave represents the time in
which the impulse is traveling
within the AV node (where the
conduction velocity is greatly
retarded) and the bundle of
His. Atrial rate can be calculated
by determining the time interval
between P waves. Click here to
see how atrial rate is calculated.
The period of time from the
onset of the P wave to the
beginning of the QRS complex is
termed the P-R interval, which
normally ranges from 0.12 to
0.20 seconds in duration. This
interval represents the time
between the onset of atrial
depolarization and the onset of
ventricular depolarization. If the
P-R interval is >0.2 sec, there is
an AV conduction block, which is
also termed a first-degree heart
block if the impulse is still able to
be conducted into the ventricles.
QRS complex
The QRS complex represents
ventricular
depolarization. Ventricular rate
can be calculated by determining
the time interval between QRS
complexes. Click here to see how
ventricular rate is calculated.
The duration of the QRS complex
is normally 0.06 to 0.1 seconds.
This relatively short duration
indicates that ventricular
depolarization normally occurs
very rapidly. If the QRS complex
is prolonged (> 0.1 sec),
conduction is impaired within
the ventricles. This can occur
with bundle branch blocks or
whenever a ventricular foci
(abnormal pacemaker site)
becomes the pacemaker driving
the ventricle. Such an ectopic foci
nearly always results in impulses
being conducted over slower
pathways within the heart,
thereby increasing the time for
depolarization and the duration
of the QRS complex.
The shape of the QRS complex in
the above figure is idealized. In
fact, the shape changes
depending on which recording
electrodes are being used. The
shape will also change when
there is abnormal conduction of
electrical impulses within the
ventricles. The figure to the right
summarizes the nomenclature
used to define the different
components of the QRS complex.
ST segment
The isoelectric period (ST
segment) following the QRS is
the time at which the entire
ventricle is depolarized and
roughly corresponds to the
plateau phase of the ventricular
action potential. The ST segment
is important in the diagnosis of
ventricular ischemia or hypoxia
because under those conditions,
the ST segment can become
either depressed or elevated.
T wave
The T wave represents
ventricular repolarization and is
longer in duration than
depolarization (i.e., conduction of
the repolarization wave is slower
than the wave of
depolarization). Sometimes a
small positive U wave may be
seen following the T wave (not
shown in figure at top of page).
This wave represents the last
remnants of ventricular
repolarization. Inverted or
prominent U waves indicates
underlying pathology or
conditions affecting
repolarization.
Q-T interval
The Q-T interval represents the
time for both ventricular
depolarization and repolarization
to occur, and therefore roughly
estimates the duration of an
average ventricular action
potential. This interval can range
from 0.2 to 0.4 seconds
depending upon heart rate. At
high heart rates, ventricular
action potentials shorten in
duration, which decreases the Q-
T interval. Because prolonged Q-
T intervals can be diagnostic for
susceptibility to certain types of
tachyarrhythmias, it is important
to determine if a given Q-T
interval is excessively long. In
practice, the Q-T interval is
expressed as a "corrected Q-T
( QTc)" by taking the Q-T interval
and dividing it by the square root
of the R-R interval (interval
between ventricular
depolarizations). This allows an
assessment of the Q-T interval
that is independent of heart
rate. Normal corrected Q-Tc
intervals are less than 0.44
seconds.
There is no distinctly visible wave
representing atrial repolarization
in the ECG because it occurs
during ventricular
depolarization. Because the
wave of atrial repolarization is
relatively small in amplitude (i.e.,
has low voltage), it is masked by
the much larger ventricular-
generated QRS complex.
ECG tracings recorded
simultaneous from different
electrodes placed on the body
produce different characteristic
waveforms.