d. Size and Time Intervals of EKG Waves. The size of the deflection waves
and particular time intervals are important when you are reading an electrocardiogram.
For example, the duration of a normal "P" wave is between 0.06 and 0.1 seconds, the
time it takes for depolarization current to pass through the atrial musculature. An
increased width of "P" wave may indicate left atrial abnormality or right atrial
hypertrophy (enlargement). The deflection of a normal "P" wave is small due to the thin
walled structure of the atria. A "P" wave is usually no more than 3 mm high. A taller "P"
wave may indicate that atrial enlargement has occurred due to hypertension, coronary
pulmonade, or congenital heart disease.
(1) P-R interval. Measured from the beginning of the P wave to the
beginning of the R wave, this wave pattern represents the conduction time from the
beginning of atrial excitation to the beginning of ventricular excitation. This is the time it
takes for an electrical impulse to travel through the atria and atrioventricular node to the
remaining conducting tissues. A medical condition that disrupts this electrical impulse
will display itself as a P-R interval that is longer than 0.2 seconds, due to the increased
time it takes to travel the conducting tissues. The normal P-R interval is between 0.12
and 0.20 seconds.
(2) Q wave. The Q wave is defined as the first down (negative) deflection
following the P wave but coming before the R wave.
(3) QRS complex. This complex is made up of three waves: the Q, the R,
and the S waves. The QRS complex represents ventricular depolarization. A normal
QRS measurement is less than 0.12 seconds. The QRS complex is larger than the P
wave on an EKG because ventricular depolarization involves a greater muscle mass
than atrial depolarization.
R wave. The R wave is the first upward (positive) deflection that follows
(5) S-T segment. Beginning at the end of the S wave ending at the
beginning of the T wave, this wave represents the time between the end of the spread
of the heart's electrical impulse through the ventricles and repolarization of the
ventricles. When a patient has acute myocardial infarction, the S-T segment is
elevated. When the heart muscle does not receive enough oxygen, the S-T segment is
(6) T wave. Representing repolarization of the ventricular cells, the T wave
is flat when the heart muscle does not receive enough oxygen; for example, in
atherosclerotic heart disease. When the body's potassium level is increased, the T
wave may be elevated. This wave occurs after the QRS complex.
Refer to figure 2-6 to see these waves.