2. CONTENTS
• What is ECG
• ECG leads
• Parts of ECG
• Interpetration
• Rate
• Rhythm
• Axis
3. • What is ECG?
- An electrocardiogram is a picture of the electrical conduction of the
heart
- By examining changes from normal on the ECG, clinicians can identify
a multitude of cardiac disease processes
- There are two ways to learn ECG interpretation:
1. pattern recognition
2. understanding the exact electrical vectors recorded by an ECG as
they relate to cardiac electrophysiology
4. ECG
• The standard ECG has 12 leads
• 6 of the leads are considered “limb leads” because they are placed on the arms and/or legs of the individual.
- lead I
- lead II
- lead III
- aVL lead
- aVR lead
- aVF lead
• Leads I, II, III are called bipolar leads as they measure the potential difference (electrical difference) between two
electrodes
• aVL, aVR, aVF lead are unipolar limb leads/ augmented limb leads; these leads are calculated as a combination of
leads I, II and III
• The other six leads are considered “precordial leads” because they are placed on the torso (precordium).
• The six precordial leads are called leads V1, V2, V3, V4, V5 and V6.
9. ECG GRID
• First, the standard 12-lead ECG is a 10-second strip. The bottom one
or two lines will be a full “rhythm strip” of a specific lead, spanning
the whole 10 seconds of the ECG
• Other leads will span only about 2.5 seconds
10. • ECG paper has horizontal and
vertical lines at regular
intervals of 1 mm. every 5th
line (5mm) is thickened
• DURATION
- Denoted by vertical lines
- Interval between 2 thin lines
(1mm)= 0.04 s
- 2 thick lines (5mm) = 0.2 s
• AMPLITUDE
- Denoted by horizontal lines
- 1mm = 0.1mV
- 5mm = 0.5mV
• SPEED OF PAPER
- 2 adjustable speed:
- 25mm/sec
- 50mm/sec
- Usually fixed at 25mm/s;
changed to 50mm/s if heart
rate is very high
11. • The standard approach to reading an ECG includes, in this order:
1. Examining rate
2. Examining the rhythm
3. Examining the axis, intervals and segments
4. Examining everything else
12. • A normal ECG contains waves, intervals, segments and
one complex
• Wave: A positive or negative deflection from baseline
that indicates a specific electrical event. The waves on
an ECG include the P wave, Q wave, R wave, S wave, T
wave and U wave.
• Interval: The time between two specific ECG events.
The intervals commonly measured on an ECG include
the PR interval, QRS interval (also called QRS
duration), QT interval and RR interval.
• Segment: The length between two specific points on
an ECG that are supposed to be at the baseline
amplitude (not negative or positive). The segments on
an ECG include the PR segment, ST segment and TP
segment.
• Complex: The combination of multiple waves grouped
together. The only main complex on an ECG is the QRS
complex.
• Point: There is only one point on an ECG termed the J
point, which is where the QRS complex ends and the
ST segment begins.
13. • The waves recorded by limb lead II are considered as the typical waves
Wave/ segment From - to Cause Duration (seconds) Amplitude (mV)
P wave
- Positive wave
- 1st wave in ECG
- Also called atrial
complex
- When SAN creates
action potential that
causes depolarization
of atrial musculature
0.1 0.1 – 0.12
QRS complex
- Initial ventricular
complex
Onset of Q wave to
end of S wave
Ventricular
depolarization
0.08 – 0.10 S = 0.4
Q wave
- Small negative wave
- Depolarization of
basal portion of
interventricular
septum
0.1 – 0.2
R wave
- R wave is the first
upward deflection
after the P wave
Depolarization of the
apical portion of
interventricular
septum and apical
portion of ventricular
muscle
1
14. Wave/ segment From - to Cause Duration (seconds) Amplitude
(mV)
S wave
- Small negative wave
- Depolarization of basal
portion of ventricular muscle
near the atrioventricular ring
0.4
T wave
- The final ventricular complex
- A positive wave
- Repolarization of ventricular
musculature
0.2 0.3
TP segment
- should always be at baseline
- used as a reference to determine
whether the ST segment is
elevated or depressed, as there
are no specific disease conditions
that elevate or depress the TP
segment
from the end of
the T wave to
the beginning of
the P wave
P-R interval Onset of P wave
to onset of Q
wave
Signifies atrial depolarization
and conduction of impulses
through AV node
Shows the duration of
conduction of impulses from
SAN to ventricles through
atrial muscles and AVN
0.18 (0.12- 0.2)
- More that 0.2 signifies
delay in conduction of
impulse from SAN to
ventricles
- Usually delay occurs in AVN
(AV nodal delay)
-
15. Wave/ segment From - to Cause Duration (seconds) Amplitude
(mV)
Q-T interval Onset of Q wave
to end of T wave
the time from the beginning
of the QRS complex,
representing ventricular
depolarization, to the end of
the T wave, resulting from
ventricular repolarization
(electrical activity in
ventricles)
0.4 – 0.42
- Women have a longer QT
interval than men
- Lower heart rates also result
in a longer QT interval
S-T segment End of S wave to
onset of T wave
Isoelectric
- Any deviation from
isoelectric base indicates
pathological condition
- Elevation: MI
- Depression: myocardial
ischaemia and
hypokalemia
0.08
16. Examining rate
• There are two different rates that can be determined on ECGs.
1. The atrial rate: indicated by the frequency of the P waves
2. ventricular rate: indicated by the frequency of the QRS complexes
• Normally, the atrial rate should be the same as the ventricular rate in
the absence of disease
• R-R interval: distance between two consecutive R waves
• HR: number of R waves per unit time
18. Determining rhythm
• The rhythm is either sinus rhythm or not sinus rhythm.
• Sinus rhythm refers to the origination of the electrical activity coming
from SA node
• This results in an upright P wave in lead II on the ECG.
• If there is a P wave before every QRS complex, and it has a sinus
morphology, then normal sinus rhythm, or NSR, is said to be present
• A sinus morphology is an upright P wave in lead II and biphasic (up
and down) P wave in lead V1.
19. • If the P wave has a morphology different from the typical sinus
morphology, it is termed ectopic, meaning coming from somewhere
other than the sinus node
20. Determining axis
• The axis of the ECG is the major direction of the overall electrical
activity of the heart.
• The mean direction of flow of electrical potential at one instance =
instantaneous mean vector/ instantaneous summated vector
21. Degree of instantaneous mean vector at
different limb leads
• When recording ECG in different limb leads, the degree of vector is
altered
• The direction of current flow is always from –ve point to +ve point
• When the electrical potential flows in a horizontal plane from the
right to left side of the heart, the degree of vector is zero
22. Lead I:
Electrical potential travels from right to left in a horizontal plane.
So the degree of vector is considered zero
Lead II:
Vector is from above downwards and slightly towards the left, ie 60
degrees
Lead III:
From above downward and slightly towards right at 120 degrees
aVR:
From below towards upper part of the heart and slightly towards
right at 210 degrees
aVF:
From above downwards at 90 degrees
aVL:
From below towards upper part of the heart and slightly towards
left at -30 degrees/ +330 degrees
23. Calculated vector/ mean QRS vector
• The instantaneous mean vector cannot be determined by the
recording of ECG
• But, another vector can be calculated by measuring the amplitude of
QRS complex from ECG recorded in standard limb leads
• It is called calculated vector/ means QRS vector
= electrical axis of the heart
= cardiac vector
24. • The normal QRS axis should be between -30 and +90
degrees.
• Left axis deviation is defined as the major QRS vector,
falling between -30 and -90 degrees.
• Right axis deviation occurs with the QRS axis and is
between +90 and +180 degrees.
• Indeterminate axis is between +/- 180 and -90 degrees
25. • The fastest non-specific method to determine the QRS axis is to find
the major direction of the QRS complex — positive or negative — in
leads I and aVF.
• Normal QRS axis:
If the QRS complex is upright (positive) in both lead I and lead aVF, then
the axis is normal
• The electrical vector heads towards
the positive of lead I and the
positive of lead aVF, as indicated by
the arrows.
• The QRS axis is thus between these
two arrows, which falls within the
normal range.
26. • Left axis deviation:
- If the QRS is upright in lead I (positive) and downward in lead aVF
(negative), then the axis is between 0 and -90 degrees.
- However, recalling that left axis deviation is defined as between -30
and -90, this scenario is not always technically left axis deviation.
- In this scenario, the QRS axis could fall between 0 and -30, which is
within normal limits. To further distinguish normal from left axis
deviation in this setting, look at lead II. If lead II is downward
(negative), then the axis is more towards -120, and left axis deviation
is present. If the QRS complex in lead II is upright (positive), then the
axis is more towards +60 degrees, and the QRS axis is normal.
27.
28. • Right axis deviation:
- If the QRS is predominantly negative in lead I and positive in lead aVF,
then the axis is rightward (right axis deviation)
29. • Indeterminate axis:
- If the QRS is downward (negative) in lead I and downward (negative)
in lead aVF, then the axis is indeterminate and sometimes referred to
as “northwestern axis.”
- This finding is uncommon and usually from ventricular rhythms;
however, it can also be from paced rhythms, lead misplacement and
certain congenital heart diseases.
Electrical activity travelling towards an electrode is displayed as a positive (upward) deflection on the screen, and electrical activity travelling away as a negative (downward) deflection. The leads are described by convention as follows:
Lead I - measures the potential difference between the right arm electrode and the left arm electrode. The third electrode (left leg) acts as neutral.
Lead II - measures the potential difference between the right arm and left leg electrode.
Lead III - measures the potential difference between the left arm and left leg electrode.
Atrial enlargements can widen the P wave or increase the P wave amplitude. Ectopic atrial rhythms can alter the normal morphology of the P wave
he normal duration (interval) of the QRS complex is between 0.08 and 0.10 seconds — that is, 80 and 100 milliseconds. When the duration is between 0.10 and 0.12 seconds, it is intermediate or slightly prolonged. A QRS duration of greater than 0.12 seconds is considered abnormal.
The QRS duration will lengthen when electrical activity takes a long time to travel throughout the ventricular myocardium. The normal conduction system in the ventricles is called the His-Purkinje system and consists of cells that can conduct electricity quite rapidly. Thus, normal conduction of an electrical impulse through the atrioventricular, or AV, node, then to the ventricles via the His-Purkinje system, is fast and results in a normal QRS duration. When electrical activity does not conduct through the His-Purkinje system, but instead travels from myocyte to myocyte, a longer time is necessary, and the QRS duration is widened.
A widened QRS duration occurs in the setting of a right bundle branch block, left bundle branch block, non-specific intraventricular conduction delay and during ventricular arrhythmias such as ventricular tachycardia
Q wave is the first downward deflection after the P wave and the first element in the QRS complex. When the first deflection of the QRS complex is upright, then no Q wave is present. The normal individual will have a small Q wave in many, but not all, ECG leads. Abnormalities of the Q waves are mostly indicative of myocardial infarction. The terms “Q wave myocardial infarction” and “non-Q wave myocardial infarction” are earlier designations of different types of MIs ultimately resulting in, respectively, Q wave development or the absence of Q wave development
R wave should be small in lead V1. Throughout the precordial leads (V1-V6), the R wave becomes larger — to the point that the R wave is larger than the S wave in lead V4. The S wave then becomes quite small in lead V6; this is called “normal R wave progression.” When the R wave remains small in leads V3 to V4 — that is, smaller than the S wave — the term “poor R wave progression” is used. Recall that the R wave is usually quite small in lead V1; if the R wave is large in V1 — that is, greater in amplitude than the S wave — significant pathology may be present.
The causes for a R/S wave ratio greater than 1 in lead V1 include right bundle branch block, Wolff-Parkinson-White syndrome, an acute posterior myocardial infarction, right ventricular hypertrophy and isolated posterior wall hypertrophy, which can occur in Duchenne muscular dystrophy.
If a right bundle branch block is present, there may be two R waves, resulting in the classic “bunny ear” appearance of the QRS complex. In this setting, the second R wave is termed “R’” or “R prime.”
S wave may not be present in all ECG leads in a given patient. In the normal ECG, there is a large S wave in V1 that progressively becomes smaller, to the point that almost no S wave is present in V6. A large slurred S wave is seen in leads I and V6 in the setting of a right bundle branch block.
The presence or absence of the S wave does not bear major clinical significance. Rarely is the morphology of the S wave discussed.
In the setting of a pulmonary embolism, a large S wave may be present in lead I — part of the S1Q3T3 pattern seen in this disease state. At times, the morphology of the S wave is examined to determine if ventricular tachycardia or supraventricular tachycardia with aberrancy is present
T waves should be upright in most leads; the exceptions are aVR and V1. Further, T waves should be asymmetric in nature. The second portion of the T wave should have a steeper decline when compared with the incline of the first portion. If the T wave appears symmetric, cardiac pathology such as ischemia may be present. Many abnormal T wave patterns exist and are reviewed in more detail in the relevant ECG Reviews and Criteria sections. These include hyperkalemia, Wellens’ syndrome, left ventricular hypertrophy with repolarization abnormalities, pericarditis (stage III), arrhythmogenic right ventricular dysplasia or ARVD, and hyperacute T waves during myocardial infarction.
During states of tachycardia, the TP segment is shortened and may be difficult to visualize altogether. It is good to examine the TP segment closely for the presence of U waves or atrial activity that could indicate pathology.
Prolongation of the QT interval can result from multiple medications, electrolyte abnormalities — hypocalcemia, hypomagnesemia and hypokalemia — and certain disease states including intracranial hemorrhage.
The QTc is considered prolonged if greater than 450 ms in males and 470 ms in females
The ST segment normally remains isoelectric, thus ST segment depression or ST segment elevation can indicate cardiac pathology.
The ST segment is scrutinized on the ECG for the detection of myocardial ischemia. This can be done in the setting of either exercise or pharmacologic stress testing.
Abnormal ST segments are reviewed based on the causes outlined in the relevant ECG Reviews and Criteria sections and include anterior, posterior and inferior myocardial infarctions, left ventricular hypertrophy, pericarditis and Brugada syndrome
Myocardial ischemia happens if blood supply to the myocardium does not meet the demand. If this imbalance persists, it triggers a cascade of cellular, inflammatory and biochemical events, leading eventually to the irreversible death of heart muscle cells, resulting in MI.
However certain conditions, such as third degree AV nodal block or ventricular tachycardia can alter this normal relationship causing “AV dissociation”. In this setting, the atrial rate (P waves) and ventricular rate (QRS complexes) are at different heart rates
Ectopic atrial rhythms including atrial tachycardia, multifocal atrial tachycardia and junctional rhythms all have P waves that are not of sinus morphology
If there is sinus rhythm, and the heart rate is less than 60 beats per minute, then sinus bradycardia is present. If there is sinus rhythm, and the heart rate is greater than 100 bpm, then sinus tachycardia is present
If there are no P waves present, or the P wave morphology is not normal, then the exact rhythm must be determined. Various arrhythmias — including atrial fibrillation, atrial flutter, and ventricular rhythms such as ventricular tachycardia or ventricular fibrillation
The causes of LAD are listed below. Note that the first three account for almost 90% of ECG tracings with LAD.
Normal variant
Left anterior fascicular block
Left ventricular hypertrophy (rarely with LVH; usually axis is normal)
Left bundle branch block (rarely with LBBB)
Mechanical shift of heart in the chest (lung disease, prior chest surgery, etc.)
Inferior myocardial infarction
Wolff-Parkinson-White syndrome with “pseudoinfarct” pattern
Ventricular rhythms (accelerated idioventricular or ventricular tachycardia)
Ostium primum atrial septal defect
The causes of RAD are listed below.
Normal variant
Right bundle branch block
Right ventricular hypertrophy
Left posterior fascicular block
Dextrocardia
Ventricular rhythms (accelerated idioventricular or ventricular tachycardia)
Lateral wall myocardial infarction
Wolff-Parkinson-White syndrome
Acute right heart strain/pressure overload — also known as McGinn-White Sign or S1Q3T3 that occurs in pulmonary embolus
