Definitions
Electrocardiography
Electrocardiography is the technique by which electrical activities of the heart are studied. The spread of excitation through myocardium produces local electrical potential. This low-intensity current flows through the body, which acts as a volume conductor.
This current can be picked up from surface of the body by using suitable electrodes and recorded in the form of electrocardiogram. This technique was discovered by Dutch physiologist, Einthoven Willem, who is considered the father of electrocardiogram (ECG).
Electrocardiograph
Electrocardiograph is the instrument (machine) by which electrical activities of the heart are recorded.
Electrocardiogram
Electrocardiogram (ECG or EKG from electrokardiogram in Dutch) is the record or graphical registration of electrical activities of the heart, which occur prior to the onset of mechanical activities. It is the summed electrical activity of all cardiac muscle fibers recorded
from surface of the body.
USES OF ECG
Electrocardiogram is useful in determining and
diagnosing the following:
1. Heart rate
2. Heart rhythm
3. Abnormal electrical conduction
4. Poor blood flow to heart muscle (ischemia)
5. Heart attack
552 Section 8 t Cardiovascular System
6. Coronary artery disease
7. Hypertrophy of heart chambers.
ELECTROCARDIOGRAPHIC GRID
The paper that is used for recording ECG is called ECG paper. ECG machine amplifies the electrical signals produced from the heart and records these signals on a moving ECG paper. Electrocardiographic grid refers to the markings (lines) on ECG paper. ECG paper has horizontal and vertical lines at regular intervals of 1 mm. Every 5th line (5 mm) is thickened.
 |
| ECG grid |
DURATION
Time duration of different ECG waves is plotted
horizontally on X-axis.
On X-axis
1 mm = 0.04 second
5 mm = 0.20 second
AMPLITUDE
Amplitude of ECG waves is plotted vertically on Y-axis.
On Y-axis
1 mm = 0.1 mV
5 mm = 0.5 mV
SPEED OF THE PAPER
Movement of paper through the machine can be
adjusted by two speeds, 25 mm/second and 50 mm/
second. Usually, speed of the paper during recording
is fixed at 25 mm/second. If heart rate is very high,
speed of the paper is changed to 50 mm/second.
ECG LEADS
ECG is recorded by placing series of electrodes on the
surface of the body. These electrodes are called ECG
leads and are connected to the ECG machine.
Electrodes are fixed on the limbs. Usually, right arm,
left arm and left leg are chosen. Heart is said to be in
the center of an imaginary equilateral triangle drawn by
connecting the roots of these three limbs. This triangle
is called Einthoven triangle.
Einthoven Triangle and Einthoven Law
Einthoven triangle is defined as an equilateral triangle
that is used as a model of standard limb leads used to
record electrocardiogram. Heart is presumed to lie in
the center of Einthoven triangle.
Electrical potential generated from the heart appears
simultaneously on the roots of the three limbs, namely
the left arm, right arm and the left leg.
Refer next Chapter for Einthoven law.
ECG is recorded in 12 leads, which are generally
classified into two categories.
I. Bipolar leads
II. Unipolar leads.
BIPOLAR LIMB LEADS
Bipolar limb leads are otherwise known as standard
limb leads. Two limbs are connected to obtain these
leads and both the electrodes are active recording
electrodes, i.e. one electrode is positive and the other
one is negative (Fig. 94.1).
Standard limb leads are of three types:
a. Limb lead I
b. Limb lead II
c. Limb lead III.
Limb Lead I
Lead I is obtained by connecting right arm and left arm.
Right arm is connected to the negative terminal of the
instrument and the left arm is connected to the positive
terminal.
Limb Lead II
Lead II is obtained by connecting right arm and left leg.
Right arm is connected to the negative terminal of the
instrument and the left leg is connected to the positive
terminal.
FIG
 |
| Position of electrodes for limb leads |
Limb Lead III
Lead III is obtained by connecting left arm and left leg.
Left arm is connected to the negative terminal of the
instrument and the left leg is connected to the positive
terminal.
UNIPOLAR LEADS
Here, one electrode is active electrode and the other
one is an indifferent electrode. Active electrode is
positive and the indifferent electrode is serving as a
composite negative electrode.
Unipolar leads are of two types:
1. Unipolar limb leads
2. Unipolar chest leads.
1. Unipolar Limb Leads
Unipolar limb leads are also called augmented limb leads
or augmented voltage leads. Active electrode is connected
to one of the limbs. Indifferent electrode is obtained by
connecting the other two limbs through a resistance.
Unipolar limb leads are of three types:
i. aVR lead
ii. aVL lead
iii. aVF lead.
i. aVR lead
Active electrode is from right arm. Indifferent electrode
is obtained by connecting left arm and left leg.
ii. aVL lead
Active electrode is from left arm. Indifferent electrode is
obtained by connecting right arm and left leg.
iii. aVF lead
Active electrode is from left leg (foot). Indifferent electrode
is obtained by connecting the two upper limbs.
2. Unipolar Chest Leads
Chest leads are also called ‘V’ leads or precardial chest
leads. Indifferent electrode is obtained by connecting the
three limbs, viz. left arm, left leg and right arm, through
a resistance of 5000 ohms. Active electrode is placed
on six points over the chest (Fig. 94.2). This electrode
is known as the chest electrode and the six points over
the chest are called V1, V2, V3, V4, V5 and V6. V indicates
vector, which shows the direction of current flow.
Position of chest leads:
V1 : Over 4th intercostal space near right sternal
margin
V2 : Over 4th intercostal space near left sternal
margin
FIGURE 94.2: Position of electrodes for chest leads
(V1 to V6)
V3 : In between V2 and V4
V4 : Over left 5th intercostal space on the mid
clavicular line
V5 : Over left 5th intercostal space on the anterior
axillary line
V6 : Over left 5th intercostal space on the mid
axillary line.
 |
| points of contact for chest leads in ecg |
WAVES OF NORMAL ECG
Normal ECG consists of waves, complexes, intervals and segments. Waves of ECG recorded by limb lead II are considered as the typical waves. Normal electrocardiogram has the following waves, namely P,Q, R, S and T. Einthoven had named the waves of ECG starting from the middle of the English alphabets (P) instead of starting from the beginning (A).
Major Complexes in ECG are..
- ‘P’ wave, the atrial complex
- ‘QRS’ complex, the initial ventricular complex
- ‘T’ wave, the final ventricular complex
- ‘QRST’, the ventricular complex.
‘P’ WAVE
‘P’ wave is a positive wave and the first wave in ECG. It
is also called atrial complex.‘P’ wave is produced due to the depolarization of atrial
musculature. Depolarization spreads from SA node to
all parts of atrial musculature. Atrial repolarization is not
recorded as a separate wave in ECG because it merges
with ventricular repolarization (QRS complex).
Duration of p wave
Normal duration of ‘P’ wave is 0.1 second.
Amplitude
Normal amplitude of ‘P’ wave is 0.1 to 0.12 mV.
Morphology of p wave
‘P’ wave is normally positive (upright) in leads I, II,
aVF, V4, V5 and V6. It is normally negative (inverted) in
aVR. It is variable in the remaining leads, i.e. it may be
positive, negative, biphasic or flat.
Clinical Significance of p wave
Variation in the duration, amplitude and morphology
of ‘P’ wave helps in the diagnosis of several cardiac
problems such as:
1. Right atrial hypertrophy: ‘P’ wave is tall (more than
2.5 mm) in lead II. It is usually pointed
2. Left atrial dilatation or hypertrophy: It is tall and
broad based or M shaped
3. Atrial extrasystole: Small and shapeless ‘P’ wave,
followed by a small compensatory pause
4. Hyperkalemia: ‘P’ wave is absent or small
5. Atrial fibrillation: ‘P’ wave is absent
6. Middle AV nodal rhythm: ‘P’ wave is absent
7. Sinoatrial block: ‘P’ wave is inverted or absent
8. Atrial paroxysmal tachycardia: ‘P’ wave is inverted
9. Lower AV nodal rhythm: ‘P’ wave appears after QRS
complex.
INTERVALS AND SEGMENTS OF ECG
‘P-R’ INTERVAL
‘P-R’ interval is the interval between the onset of ‘P’ wave and onset of ‘Q’ wave.
‘P-R’ interval signifies the atrial depolarization and conduction of impulses through AV node. It shows the
duration of conduction of the impulses from the SA node to ventricles through atrial muscle and AV node.
‘P’ wave represents the atrial depolarization. Short isoelectric (zero voltage) period after the end of ‘P’
wave represents the time taken for the passage of depolarization within AV node.
Duration
Normal duration of ‘P-R interval’ is 0.18 second and varies between 0.12 and 0.2 second. If it is more than 0.2 second, it signifies the delay in the conduction of impulse from SA node to the ventricles. Usually, the delay occurs in the AV node. So it is called the AV nodaldelay.
Clinical Significance
Variation in the duration of ‘P-R’ intervals helps in the diagnosis of several cardiac problems such as:
1. It is prolonged in bradycardia and first degree heart block
2. It is shortened in tachycardia, Wolf-Parkinson-White syndrome, Lown-Ganong-Levine syndrome,
Duchenne muscular dystrophy and type- II glycogen storage disease.
‘Q-T’ INTERVAL
‘Q-T’ interval is the interval between the onset of ‘Q’ wave and the end of ‘T’ wave.
‘Q-T’ interval indicates the ventricular depolarization and ventricular repolarization, i.e. it signifies the
electrical activity in ventricles.
Duration
Normal duration of Q-T interval is between 0.4 and 0.42
second.
Clinical Significance
1. ‘Q-T’ interval is prolonged in long ‘Q-T’ syndrome, myocardial infarction, myocarditis, hypocalcemia
and hypothyroidism
2. ‘Q-T’ interval is shortened in short ‘Q-T’ syndrome and hypercalcemia.
‘S-T’ SEGMENT
‘S-T’ segment is the time interval between the end of ‘S’ wave and the onset of ‘T’ wave. It is an isoelectric
period.
J Point
The point where ‘S-T’ segment starts is called ‘J’ point. It is the junction between the QRS complex and ‘S-T’ segment.
Duration of ‘S-T’ Segment
Normal duration of ‘S-T’ segment is 0.08 second.
Clinical Significance
Variation in the duration of ‘S-T’ segment and its deviation from isoelectric base indicates the pathological
conditions such as:
1. Elevation of ‘S-T’ segment occurs in anterior or inferior myocardial infarction, left bundle branch
block and acute pericarditis. In athletes, ‘S-T’ segment is usually elevated
2. Depression of ‘S-T’ segment occurs in acute myocardial ischemia, posterior myocardial infarction,
ventricular hypertrophy and hypokalemia
3. ‘S-T’ segment is prolonged in hypocalcemia
4. ‘S-T’ segment is shortened in hypercalcemia.
‘R-R’ INTERVAL
‘R-R’ interval is the time interval between two consecutive ‘R’ waves.
Significance
‘R-R’ interval signifies the duration of one cardiac
cycle.
Duration
Normal duration of ‘R-R’ interval is 0.8 second.
Significance of Measuring ‘R-R’ Interval
Measurement of ‘R-R’ interval helps to calculate:
1. Heart rate
2. Heart rate variability.
1. Heart Rate
Heart rate is calculated by measuring the number of ‘R’
waves per unit time.
Calculation of heart rate
Time is plotted horizontally (X-axis). On X-axis, interval
between two thick lines is 0.2 sec (see above). Time
duration for 30 thick lines is 6 seconds. Number of ‘R’
waves (QRS complexes) in 6 seconds (30 thick lines)
is counted and multiplied by 10 to obtain heart rate. For
the sake of convenience, the ECG paper has special
time marking at every 3 seconds. So it is easy to find the
time duration of 6 seconds.
2. Heart Rate Variability
Heart rate variability (HRV) refers to the beat-tobeat
variations. Under resting conditions, the ECG of
healthy individuals exhibits some periodic variation in
‘R-R’ intervals. This rhythmic phenomenon is known as
respiratory sinus arrhythmia (RSA), since it fluctuates
with the phases of respiration. ‘R-R’ interval decreases
during inspiration and increases during expiration
(Chapter 96).
Significance of Heart Rate Variability
HRV decreases in many clinical conditions like:
1. Cardiovascular dysfunctions such as hypertension
2. Diabetes mellitus
3. Psychiatric problems such as panic and anxiety
