ECG
Dr Jassim M. Ibrahim
Internal Medicine Specialist
Board certified in Internal Medicine
Lecturer at college of medicine/ HMU
MBCHB, Msc, FKBMS
Definitions
• Electrocardiography
Electrocardiography is the technique by which
electrical activities of the heart are studied.
• Electrocardiograph
Electrocardiograph is the instrument (machine) by
which electrical activities of the heart are recorded.
• Electrocardiogram
(ECG or EKG from electrokardiogram) 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.
Physiological basis of ECG
• Normally, the cardiac stimulus (electrical
signal) is generated in an automatic way by
pacemaker cells in the sinoatrial (SA) node,
located in the high right atrium (RA).
• The stimulus then spreads through the RA and
left atrium (LA).
• Next, it traverses the atrioventricular (AV)
node and the bundle of His, which comprise
the AV junction.
• The stimulus then sweeps into the left and
right ventricles (LV and RV) by way of the left
and right bundle branches, which are
continuations of the bundle of His.
• The cardiac stimulus spreads rapidly and
simultaneously to the left and right ventricular
muscle cells through the Purkinje fibers.
• Electrical activation of the atria and ventricles,
respectively, leads to sequential contraction of
these chambers (electromechanical coupling).
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
6. Coronary artery disease
7. Hypertrophy of heart chambers.
Image result for normal ECG record of a cardiac cycle
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ECG Leads
ECG is recorded in 12 leads that record the same
electric cardiac cycle but from different views.
1) Bipolar leads
2) Unipolar leads
1) Bipolar limb leads (standard limb leads):
Limb lead I (right arm and left arm)
Limb lead II (right arm and left leg)
Limb lead III (left arm and left leg)
Bipolar limb leads (standard limb leads)
Bipolar limb leads (standard limb leads)
(Einthoven's triangle)
2) Unipolar leads
a) Unipolar Limb Leads(augmented limb
leads)
aVR lead
aVL lead
aVF lead.
Both Limb Leads
b) Unipolar Chest Leads
V1 : Over 4th intercostal space near right sternal margin
V2 : Over 4th intercostal space near left sternal margin
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.
Unipolar Chest Leads
ECG paper
• small squares 1×1 mm
• large squares 5×5 mm
• Time duration of different ECG waves is plotted horizontally
on X- axis.
1 small square=1 mm = 0.04 second
1 large square=5 mm = 0.20 second
5 large squares =1 sec.
• Amplitude of ECG waves is plotted vertically on Y-axis.
1 small square=1 mm = 0.1 mV
1 large square=5 mm = 0.5 mV
2 large squares =1 mv
Normal duration and amplitude measurements in
•
ECG
Wave/Segment From – To Cause Duration (second) Amplitude (mV)
P wave – Atrial depolarization
duration 0.1 seconds
amplitude 0.1 to 0.12mV
QRS complex - Onset of Q wave to the end of S wave
(Ventricular depolarization and atrial repolarization)
0.08 to 0.10 seconds
Q = 0.1 to 0.2 mV
R = l mV
S = 0.4 mV
T wave – Ventricular repolarization
0.2 seconds
0.3 mV
P-R interval Onset of P wave to onset of Q wave
(Atrial depolarization and conduction through AV node)
0.18 (0.12 to 0.2) seconds
Q-T interval Onset of Q wave and end of T wave
Ventricular depolarization and ventricular repolarization
0.4 to 0.42 seconds
S-T segment
End of S wave and onset of T wave Isoelectric
0.08 seconds
How to analyze ECG:
• Calculate heart rate (normal or bradycardia or tachycardia)
• Determine the rhythm (sinus or not- regular or not)
• Determine the cardiac axis (its normal or left or right axis
deviation)
• Assess the waveform morphology
• Measurements of different waves, intervals and segments in
ECG:
1) P wave (present or not, its duration and amplitude)
2) QRS wave (its shape, duration and amplitude)
3) T wave
4) PR interval
5) ST segment
• Look for sign of chamber enlargement
Heart Rate
• Calculation of heart rate:
• HR= 1500/ No. of small squares between 2 RR waves
• HR= 300/ No. of large squares between 2 RR waves
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.
• So the speed is in mm/second
• 25 mm/second = 60×25 mm/minutes = 1500mm/min. (1500 small
squares/min.) = 300 large squares / min
• If heart rate is very high, speed of the paper is changed to
50 mm/second.
Rhythm
• Regular rhythm
The heart rhythm is sinus regular rhythm if:
• the distance between two successive RR waves is the same,
• all other waves are present
• and P wave precedes every QRS complex
Irregular rhythm
Cardiac axis
Normal cardiac axis is about 59°
It is close to lead II
Axis deviation
Left axis deviation
Right axis deviation
• Physiological causes of left axis deviation:
• During expiration (diaphragm moves up)
• Lying down
• Short fatty persons
• Intraabdominal mass
• Pregnancy
• Physiological causes of right axis deviation:
• During inspiration (diaphragm moves down)
• Standing
• Tall thin persons
Pathological causes of axis deviation
• Left axis deviation occurs in left ventricular
hypertrophy, left bundle-branch block and posterior
wall infarction.
• Right axis deviation occurs due to right ventricular
hypertrophy, right bundle-branch block and anterior
wall infarction.
Any Questions?
