678
UNIT 4
Maintenance of the Body
18
representing atrial repolarization is normally obscured by the
large QRS complex being recorded at the same time.
Te
P-R interval
is the time (about 0.16 s) from the beginning
of atrial excitation to the beginning of ventricular excitation. If
the Q wave is visible (which is oFen not the case), it marks the be-
ginning of ventricular excitation, and for this reason this interval
is sometimes called the
P-Q interval
. Te P-R interval includes
atrial depolarization (and contraction) as well as the passage of the
depolarization wave through the rest of the conduction system.
During the
S-T segment
of the ECG, when the action po-
tentials of the ventricular myocytes are in their plateau phases,
the entire ventricular myocardium is depolarized. Te
Q-T in-
terval
, lasting about 0.38 s, is the period from the beginning of
ventricular depolarization through ventricular repolarization.
Figure 18.18
relates the parts of an ECG to the sequence of
depolarization and repolarization in the heart.
Homeostatic Imbalance
18.6
In a healthy heart, the size, duration, and timing of the deflection
waves tend to be consistent. Changes in the pattern or timing of
the ECG may reveal a diseased or damaged heart or problems
with the heart’s conduction system
(Figure 18.19)
. ±or example,
an enlarged R wave hints of enlarged ventricles, an S-² segment
that is elevated or depressed indicates cardiac ischemia, and a
prolonged Q-² interval reveals a repolarization abnormality that
increases the risk of ventricular arrhythmias.
Check Your Understanding
10.
Which part of the intrinsic conduction system directly excites
ventricular myocardial cells? In which direction does the
depolarization wave travel across the ventricles?
11.
Describe the electrical event in the heart that occurs during
each of the following: (a) the QRS wave of the ECG; (b) the T
wave of the ECG; (c) the P-R interval of the ECG.
For answers, see Appendix H.
Heart Sounds
Describe normal heart sounds, and explain how heart
murmurs differ.
Auscultating (listening to) the thorax with a stethoscope will
reveal two sounds during each heartbeat. Tese
heart sounds
,
oFen described as lub-dup, are associated with the heart valves
closing. (Te top of ±igure 18.21 shows the timing of heart
sounds in the cardiac cycle.)
Te basic rhythm of the heart sounds is lub-dup, pause, lub-
dup, pause, and so on, with the pause indicating the period
when the heart is relaxing. Te first sound occurs as the AV
valves close. It signifies the point when ventricular pressure rises
above atrial pressure (the beginning of ventricular systole, dis-
cussed in the next section). Te first sound tends to be louder,
longer, and more resonant than the second. Te second sound
occurs as the SL valves snap shut at the beginning of ventricular
relaxation (diastole) resulting in a short, sharp sound.
1
2
3
4
5
6
Atrial depolarization, initiated by
the SA node, causes the P wave.
With atrial depolarization complete,
the impulse is delayed at the AV node.
Ventricular depolarization begins at
apex, causing the QRS complex. Atrial
repolarization occurs.
Ventricular depolarization is complete.
Ventricular repolarization begins at
apex, causing the T wave.
Ventricular repolarization is
complete.
P
R
T
Q
S
P
R
T
Q
S
P
R
T
Q
S
P
R
T
Q
S
P
R
T
Q
S
P
R
T
Q
S
SA node
AV node
Depolarization
Repolarization
Figure 18.18
The sequence of depolarization and
repolarization of the heart related to the deflection waves
of an ECG tracing.
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