C H A P T E R 4 2
Introduction to the cardiovascular system
647
The P wave immediately precedes the contraction of
the atria. The QRS complex immediately precedes the
contraction of the ventricles and then relaxation of the
ventricles during the T wave. The repolarisation of the
atria (the Ta wave) occurs during the QRS complex and
usually is not seen on an ECG. In certain conditions of
atrial hypertrophy, the Ta wave may appear around the
QRS complex.
In addition to the five waves, several areas represent
critical points on the ECG. These include the following:
PR interval
: Reflects the normal delay of conduction
at the AV node
QT interval
: Reflects the critical timing of repolari
sation of the ventricles
ST segment
: Reflects important information about
the repolarisation of the ventricles
A person with a normal ECG pattern and a heart
rate within the normal range for that person’s age
group is said to be in normal sinus rhythm. However,
abnormalities in the shape or timing of each part of an
ECG tracing help to reveal the presence of particular
cardiac disorders.
Arrhythmias
A disruption in cardiac rate or rhythm is called an
arrhythmia
or
dysrhythmia
. Various factors, such as
drugs, acidosis, decreased oxygen levels, changes in the
electrolytes in the area and build-up of waste products,
can change the cardiac rate and rhythm. Arrhythmias
can arise because of changes in the automaticity or con
ductivity of the heart cells. They are significant because
they interfere with the work of the heart and can disrupt
the cardiac output, which eventually will affect every
cell in the body. Several different types of arrhythmias
may occur.
Sinus arrhythmias
The SA node is influenced by the autonomic nervous
system to change the rate of firing to meet the body’s
P
R
Time
Millivolts
Q
Q
S
S
R
T
T
P
PQ
(PR) QRS
ST
QT
Key:
Each vertical square represents 0.1 mV
of electrical charge.
Each horizontal square equals 0.04 sec of time.
P wave = Electrical changes associated with atrial depolarisation
QRS complex = Electrical changes associated with ventricular depolarisation
T wave = Electrical changes associated with ventricular repolarisation
The electrical changes associated with atrial repolarisation normally coincide with the QRS complex
and are obscured by it.
Approximate values for normal intervals:
PQ (PR) interval—0.16 sec
QT interval—0.3 sec
QRS interval—0.08 sec
P wave—0.08 sec
ST interval—0.1 sec
FIGURE 42.5
The normal
electrocardiogram waveform.
