CHAPTER 2 • ELECTRICAL ACTIVITY OF THE HEART
27
(Fig. 2.13). The repeating waves of the ECG
represent the sequence of depolarization and
repolarization of the atria and ventricles. The
ECG does not measure absolute voltages, but
voltage changes from a baseline (isoelectric)
voltage. ECGs are generally recorded on paper
at a speed of 25 mm/s and with a vertical cali-
bration of 1 mV/cm.
By convention, the first wave of the ECG is
the P wave (Fig. 2.13). It represents the wave
of depolarization that spreads from the SA node
throughout the atria; it is usually 0.08 to 0.1 sec-
onds in duration (Table 2-4). No distinctly vis-
ible wave represents atrial repolarization in the
ECG because it is masked by ventricular depo-
larization and is of relatively small amplitude.
The brief isoelectric (zero voltage) period after
the P wave represents the time in which the
atrial cells are depolarized and the impulse is
traveling within the AV node, where conduction
velocity is greatly reduced. The period of time
from the onset of the P wave to the beginning
of the QRS complex, the PR interval, normally
ranges from 0.12 to 0.20 seconds. This interval
represents the time between the onset of atrial
depolarization and the onset of ventricular
depolarization. If the PR interval is >0.2 sec-
onds, a conduction defect (usually within the
AV node) is present (e.g., first-degree AV block).
The QRS complex represents ventricular
depolarization. The duration of the QRS com-
plex is normally 0.06 to 0.1 seconds, indi-
cating that ventricular depolarization occurs
rapidly. If the QRS complex is prolonged
(>0.1
seconds),
conduction
is
impaired
within the ventricles. Impairment can occur
with defects (e.g., bundle branch blocks) or
aberrant conduction, or it can occur when an
ectopic ventricular pacemaker drives ventric-
ular depolarization. Such ectopic foci nearly
always cause impulses to be conducted over
slower pathways within the heart, thereby
increasing the time for depolarization and the
duration of the QRS complex.
The isoelectric period (ST segment) follow-
ing the QRS is the period at which the entire
ventricle is depolarized and roughly corre-
sponds to the plateau phase of the ventricular
action potential. The ST segment is impor-
tant in the diagnosis of ventricular ischemia,
in which the ST segment can become either
depressed or elevated, indicating nonuniform
membrane potentials in ventricular cells. The
T wave represents ventricular repolarization
(phase 3 of the action potential) and lasts
longer than depolarization.
During the QT interval, both ventricular
depolarization and repolarization occur. This
interval roughly estimates
the
duration
of
ventricular action potentials. The QT interval
can range from 0.2 to 0.4 seconds depending
on heart rate. At high heart rates, ventricular
action potentials are shorter, decreasing the QT
interval. Because prolonged QT intervals can be
diagnostic for susceptibility to certain types of
arrhythmias, it is important to determine if a
given QT interval is excessively long. In prac-
tice, the QT interval is expressed as a corrected
QT (QTc) interval by taking the QT interval and
dividing it by the square root of the RR interval
(the interval between ventricular depolariza-
tions). This calculation allows the QT interval
to be assessed independent of heart rate. Nor-
mal corrected QTc intervals are <0.44 seconds.
TABLE 2-4
SUMMARY OF ECG WAVES, INTERVALS, AND SEGMENTS
ECG C O M P O N E N T
( r e p r e s e n t s
| n O R M A L D U R A T IO N (S )
P wave
Atrial depolarization
0.08-0.10
QRS complex
Ventricular depolarization
0.06-0.10
T wave
Ventricular repolarization
1
PR interval
Atrial depolarization plus AV nodal delay
0.12-0.20
ST segment
Isoelectric period of depolarized ventricles
1
QT interval
Length of depolarization plus repolarization—
corresponds to action potential duration
0.20-0.402
'Duration not norm ally measured.
2High heart rates reduce the action potential duration and therefore the QT interval.
previous page 40 Cardiovascular Physiology Concepts  2nd Edition read online next page 42 Cardiovascular Physiology Concepts  2nd Edition read online Home Toggle text on/off