ECG Rhythm Interpretation

Module I
ECG Basics

Course Objectives
To recognize the normal rhythm of the
heart - Normal Sinus Rhythm.
To recognize the 13 most common
rhythm disturbances.
To recognize an acute myocardial
infarction on a 12-lead ECG.

Learning Modules

ECG Basics
How to Analyze a Rhythm
Normal Sinus Rhythm
Heart Arrhythmias
Diagnosing a Myocardial Infarction
Advanced 12-Lead Interpretation

Normal Impulse Conduction

Sinoatrial node
AV node
Bundle of His
Bundle Branches
Purkinje fibers

Impulse Conduction & the ECG

Sinoatrial node
AV node
Bundle of His
Bundle Branches
Purkinje fibers

The PQRST
P wave - Atrial
depolarization
QRS - Ventricular
depolarization
T wave - Ventricular
repolarization

The PR Interval
Atrial depolarization
+
delay in AV junction
(AV node/Bundle of His)

(delay allows time for

the atria to contract
before the ventricles
contract)

Pacemakers of the Heart

SA Node - Dominant pacemaker
(60 - 100 beats/minute)
AV Node - Back-up pacemaker
(40 - 60 beats/minute)
Ventricular cells - Back-up pacemaker
(20 - 45 beats/minute)

The ECG Paper

Horizontally
One small box - 0.04 s
One large box - 0.20 s

Vertically
One large box - 0.5 mV

The ECG Paper (cont)

3 sec

3 sec

Every 3 seconds (15 large boxes) is

marked by a vertical line.
This helps when calculating the heart
rate.

End of Module I
ECG Basics

ECG Rhythm Interpretation

Module II
How to Analyze a Rhythm

Course Objectives
To recognize the normal rhythm of the
heart - Normal Sinus Rhythm.
To recognize the 13 most common
rhythm disturbances.
To recognize an acute myocardial
infarction on a 12-lead ECG.

Learning Modules

ECG Basics
How to Analyze a Rhythm
Normal Sinus Rhythm
Heart Arrhythmias
Diagnosing a Myocardial Infarction
Advanced 12-Lead Interpretation

Rhythm Analysis

Step 1:
Step 2:
Step 3:
Step 4:
Step 5:

Calculate rate.
Determine regularity.
Assess the P waves.
Determine PR interval.
Determine QRS duration.

Step 1: Calculate Rate

3 sec

3 sec

Option 1
Count the # of R waves in a 6 second
rhythm strip, then multiply by 10.

Interpretation?
9 x 10 = 90 bpm

Step 1: Calculate Rate

R wave

Option 2
Find a R wave that lands on a bold line.

Step 1: Calculate Rate

3 1 1
0 5 0 7 6 5
0 0 0 5 0 0

Option 2 (cont)
Memorize the sequence:
300 - 150 - 100 - 75 - 60 - 50

Interpretation? Approx. 1 box less than

100 = 95 bpm

Step 2: Determine regularity

R

Look at the R-R distances (using a caliper or

markings on a pen or paper).
Regular (are they equidistant apart)?
Occasionally irregular? Regularly irregular?
Irregularly irregular?
Interpretation?

Regular

Step 3: Assess the P waves

Are there P waves?

Do the P waves all look alike?
Do the P waves occur at a regular rate?
Is there one P wave before each QRS?
Interpretation? Normal P waves with 1 P
wave for every QRS

Step 4: Determine PR interval

Normal: 0.12 - 0.20 seconds.

(3 - 5 boxes)

Interpretation? 0.12 seconds

Step 5: QRS duration

Normal: 0.04 - 0.12 seconds.

(1 - 3 boxes)

Interpretation? 0.08 seconds

Rhythm Summary

Rate
Regularity
P waves
PR interval
QRS duration
Interpretation?

90-95 bpm
regular
normal
0.12 s
0.08 s

Normal Sinus Rhythm

End of Module II
How to Analyze a Rhythm

ECG Rhythm Interpretation

Module III
Normal Sinus Rhythm

Learning Modules

ECG Basics
How to Analyze a Rhythm
Normal Sinus Rhythm
Heart Arrhythmias
Diagnosing a Myocardial Infarction
Advanced 12-Lead Interpretation

Normal Sinus Rhythm (NSR)

Etiology: the electrical impulse is formed
in the SA node and conducted normally.
This is the normal rhythm of the heart;
other rhythms that do not conduct via
the typical pathway are called
arrhythmias.

NSR Parameters
Rate
60 - 100 bpm
Regularity
regular
P waves
normal
PR interval
0.12 - 0.20 s
QRS duration
0.04 - 0.12 s
Any deviation from above is sinus
tachycardia, sinus bradycardia or an
arrhythmia

Arrhythmia Formation
Arrhythmias can arise from problems in
the:
Sinus node
Atrial cells
AV junction
Ventricular cells

SA Node Problems
The SA Node can:
fire too slow
fire too fast

Sinus Bradycardia
Sinus Tachycardia

Sinus Tachycardia may be an appropriate

response to stress.

Atrial Cell Problems

Atrial cells can:
fire occasionally
from a focus
fire continuously
due to a looping
re-entrant circuit

Premature Atrial
Contractions (PACs)
Atrial Flutter

Teaching Moment
A re-entrant
pathway occurs
when an impulse
loops and results
in selfperpetuating
impulse
formation.

Atrial Cell Problems

Atrial cells can also:
fire continuously
Atrial Fibrillation
from multiple foci
or
Atrial Fibrillation
fire continuously
due to multiple
micro re-entrant
wavelets

Teaching Moment
Multiple micro reentrant wavelets
refers to wandering
small areas of
activation which
generate fine chaotic
impulses. Colliding
wavelets can, in turn,
generate new foci of
activation.

Atrial tissue

AV Junctional Problems
The AV junction can:
fire continuously
Paroxysmal
Supraventricular
due to a looping
Tachycardia
re-entrant circuit
block impulses
AV Junctional Blocks
coming from the
SA Node

Ventricular Cell Problems

Ventricular cells can:
fire occasionally
Premature Ventricular
Contractions (PVCs)
from 1 or more foci
Ventricular Fibrillation
fire continuously
from multiple foci
fire continuously
Ventricular Tachycardia
due to a looping
re-entrant circuit

End of Module III

Normal Sinus Rhythm

ECG Rhythm Interpretation

Module IV a
Sinus Rhythms and
Premature Beats

Learning Modules

ECG Basics
How to Analyze a Rhythm
Normal Sinus Rhythm
Heart Arrhythmias
Diagnosing a Myocardial Infarction

Arrhythmias

Sinus Rhythms
Premature Beats
Supraventricular Arrhythmias
Ventricular Arrhythmias
AV Junctional Blocks

Sinus Rhythms
Sinus Bradycardia
Sinus Tachycardia

Rhythm #1

Rate?
Regularity?
P waves?
PR interval?
QRS duration?

30 bpm
regular
normal
0.12 s
0.10 s

Interpretation? Sinus Bradycardia

Sinus Bradycardia
Deviation from NSR
- Rate
< 60 bpm

Sinus Bradycardia
Etiology: SA node is depolarizing slower
than normal, impulse is conducted
normally (i.e. normal PR and QRS
interval).

Rhythm #2

Rate?
Regularity?
P waves?
PR interval?
QRS duration?

130 bpm
regular
normal
0.16 s
0.08 s

Interpretation? Sinus Tachycardia

Sinus Tachycardia
Deviation from NSR
- Rate
> 100 bpm

Sinus Tachycardia
Etiology: SA node is depolarizing faster
than normal, impulse is conducted
normally.
Remember: sinus tachycardia is a
response to physical or psychological
stress, not a primary arrhythmia.

Premature Beats
Premature Atrial Contractions
(PACs)
Premature Ventricular Contractions
(PVCs)

Rhythm #3

Rate?
Regularity?
P waves?
PR interval?
QRS duration?

70 bpm
occasionally irreg.
2/7 different contour
0.14 s (except 2/7)
0.08 s

Interpretation? NSR with Premature Atrial

Contractions

Premature Atrial Contractions

Deviation from NSR
These ectopic beats originate in the
atria (but not in the SA node),
therefore the contour of the P wave,
the PR interval, and the timing are
different than a normally generated
pulse from the SA node.

Premature Atrial Contractions

Etiology: Excitation of an atrial cell
forms an impulse that is then conducted
normally through the AV node and
ventricles.

Teaching Moment
When an impulse originates anywhere in
the atria (SA node, atrial cells, AV node,
Bundle of His) and then is conducted
normally through the ventricles, the QRS
will be narrow (0.04 - 0.12 s).

Rhythm #4

Rate?
Regularity?
P waves?
PR interval?
QRS duration?

60 bpm
occasionally irreg.
none for 7th QRS
0.14 s
0.08 s (7th wide)

Interpretation? Sinus Rhythm with 1 PVC

PVCs

Deviation from NSR

Ectopic beats originate in the ventricles
resulting in wide and bizarre QRS
complexes.
When there are more than 1 premature
beats and look alike, they are called
uniform. When they look different, they are
called multiform.

PVCs

Etiology: One or more ventricular cells

are depolarizing and the impulses are
abnormally conducting through the
ventricles.

Teaching Moment
When an impulse originates in a
ventricle, conduction through the
ventricles will be inefficient and the QRS
will be wide and bizarre.

Ventricular Conduction

Normal

Abnormal

Signal moves rapidly

through the ventricles

Signal moves slowly

through the ventricles

End of Module IV a
Sinus Rhythms and
Premature Beats

ECG Rhythm Interpretation

Module IV b
Supraventricular and
Ventricular Arrhythmias

Learning Modules

ECG Basics
How to Analyze a Rhythm
Normal Sinus Rhythm
Heart Arrhythmias
Diagnosing a Myocardial Infarction
Advanced 12-Lead Interpretation

Arrhythmias

Sinus Rhythms
Premature Beats
Supraventricular Arrhythmias
Ventricular Arrhythmias
AV Junctional Blocks

Supraventricular Arrhythmias
Atrial Fibrillation
Atrial Flutter
Paroxysmal Supraventricular
Tachycardia

Rhythm #5

Rate?
Regularity?
P waves?
PR interval?
QRS duration?

100 bpm
irregularly irregular
none
none
0.06 s

Interpretation? Atrial Fibrillation

Atrial Fibrillation
Deviation from NSR
No organized atrial depolarization, so
no normal P waves (impulses are not
originating from the sinus node).
Atrial activity is chaotic (resulting in an
irregularly irregular rate).
Common, affects 2-4%, up to 5-10% if
> 80 years old

Atrial Fibrillation
Etiology: Recent theories suggest that it
is due to multiple re-entrant wavelets
conducted between the R & L atria.
Either way, impulses are formed in a
totally unpredictable fashion. The AV
node allows some of the impulses to
pass through at variable intervals (so
rhythm is irregularly irregular).

Rhythm #6

Rate?
Regularity?
P waves?
PR interval?
QRS duration?

70 bpm
regular
flutter waves
none
0.06 s

Interpretation? Atrial Flutter

Atrial Flutter
Deviation from NSR
No P waves. Instead flutter waves (note
sawtooth pattern) are formed at a rate
of 250 - 350 bpm.
Only some impulses conduct through
the AV node (usually every other
impulse).

Atrial Flutter
Etiology: Reentrant pathway in the right
atrium with every 2nd, 3rd or 4th
impulse generating a QRS (others are
blocked in the AV node as the node
repolarizes).

Rhythm #7

Rate?
Regularity?
P waves?
PR interval?
QRS duration?

74 148 bpm
Regular regular
Normal none
0.16 s none
0.08 s

Interpretation? Paroxysmal Supraventricular

Tachycardia (PSVT)

PSVT
Deviation from NSR
The heart rate suddenly speeds up,
often triggered by a PAC (not seen
here) and the P waves are lost.

PSVT
Etiology: There are several types of
PSVT but all originate above the
ventricles (therefore the QRS is narrow).
Most common: abnormal conduction in
the AV node (reentrant circuit looping in
the AV node).

Ventricular Arrhythmias
Ventricular Tachycardia
Ventricular Fibrillation

Rhythm #8

Rate?
Regularity?
P waves?
PR interval?
QRS duration?

160 bpm
regular
none
none
wide (> 0.12 sec)

Interpretation? Ventricular Tachycardia

Ventricular Tachycardia
Deviation from NSR
Impulse is originating in the ventricles
(no P waves, wide QRS).

Ventricular Tachycardia
Etiology: There is a re-entrant pathway
looping in a ventricle (most common
cause).
Ventricular tachycardia can sometimes
generate enough cardiac output to
produce a pulse; at other times no pulse
can be felt.

Rhythm #9

Rate?
Regularity?
P waves?
PR interval?
QRS duration?

none
irregularly irreg.
none
none
wide, if recognizable

Interpretation? Ventricular Fibrillation

Ventricular Fibrillation
Deviation from NSR
Completely abnormal.

Ventricular Fibrillation
Etiology: The ventricular cells are
excitable and depolarizing randomly.
Rapid drop in cardiac output and death
occurs if not quickly reversed

End of Module IV b
Supraventricular and
Ventricular Arrhythmias

ECG Rhythm Interpretation

Module IV c
AV Junctional Blocks

Course Objectives
To recognize the normal rhythm of the
heart - Normal Sinus Rhythm.
To recognize the 13 most common
rhythm disturbances.
To recognize an acute myocardial
infarction on a 12-lead ECG.

Learning Modules

ECG Basics
How to Analyze a Rhythm
Normal Sinus Rhythm
Heart Arrhythmias
Diagnosing a Myocardial Infarction
Advanced 12-Lead Interpretation

Arrhythmias

Sinus Rhythms
Premature Beats
Supraventricular Arrhythmias
Ventricular Arrhythmias
AV Junctional Blocks

AV Nodal Blocks
1st Degree AV Block
2nd Degree AV Block, Type I
2nd Degree AV Block, Type II
3rd Degree AV Block

Rhythm #10

Rate?
Regularity?
P waves?
PR interval?
QRS duration?

60 bpm
regular
normal
0.36 s
0.08 s

Interpretation? 1st Degree AV Block

1st Degree AV Block

Deviation from NSR
PR Interval

> 0.20 s

1st Degree AV Block

Etiology: Prolonged conduction delay in
the AV node or Bundle of His.

Rhythm #11

Rate?
Regularity?
P waves?
PR interval?
QRS duration?

50 bpm
regularly irregular
nl, but 4th no QRS
lengthens
0.08 s

Interpretation? 2nd Degree AV Block, Type I

2nd Degree AV Block, Type I

Deviation from NSR
PR interval progressively lengthens,
then the impulse is completely blocked
(P wave not followed by QRS).

2nd Degree AV Block, Type I

Etiology: Each successive atrial impulse
encounters a longer and longer delay in
the AV node until one impulse (usually
the 3rd or 4th) fails to make it through
the AV node.

Rhythm #12

Rate?
Regularity?
P waves?
PR interval?
QRS duration?

40 bpm
regular
nl, 2 of 3 no QRS
0.14 s
0.08 s

Interpretation? 2nd Degree AV Block, Type II

2nd Degree AV Block, Type II

Deviation from NSR
Occasional P waves are completely
blocked (P wave not followed by QRS).

2nd Degree AV Block, Type II

Etiology: Conduction is all or nothing
(no prolongation of PR interval);
typically block occurs in the Bundle of
His.

Rhythm #13

Rate?
Regularity?
P waves?
PR interval?
QRS duration?

40 bpm
regular
no relation to QRS
none
wide (> 0.12 s)

Interpretation? 3rd Degree AV Block

3rd Degree AV Block

Deviation from NSR
The P waves are completely blocked in
the AV junction; QRS complexes
originate independently from below the
junction.

3rd Degree AV Block

Etiology: There is complete block of
conduction in the AV junction, so the
atria and ventricles form impulses
independently of each other. Without
impulses from the atria, the ventricles
own intrinsic pacemaker kicks in at
around 30 - 45 beats/minute.

Remember
When an impulse originates in a ventricle,
conduction through the ventricles will be
inefficient and the QRS will be wide and
bizarre.

End of Module IV c
AV Junctional Blocks

ECG Rhythm Interpretation

Module V
Acute Myocardial Infarction

Course Objectives
To recognize the normal rhythm of the
heart - Normal Sinus Rhythm.
To recognize the 13 most common
heart arrhythmias.
To recognize an acute myocardial
infarction on a 12-lead ECG.

Learning Modules

ECG Basics
How to Analyze a Rhythm
Normal Sinus Rhythm
Heart Arrhythmias
Diagnosing a Myocardial Infarction
Advanced 12-Lead Interpretation

Diagnosing a MI
To diagnose a myocardial infarction you
need to go beyond looking at a rhythm
strip and obtain a 12-Lead ECG.
12-Lead
ECG

Rhythm
Strip

The 12-Lead ECG

The 12-Lead ECG sees the heart
from 12 different views.
Therefore, the 12-Lead ECG helps
you see what is happening in
different portions of the heart.
The rhythm strip is only 1 of these 12
views.

The 12-Leads
The 12-leads include:
3 Limb leads
(I, II, III)
3 Augmented leads
(aVR, aVL, aVF)
6 Precordial leads
(V1- V6)

Views of the Heart

Some leads get a
good view of the:
Anterior portion
of the heart

Inferior portion
of the heart

Lateral portion
of the heart

ST Elevation
One way to
diagnose an
acute MI is to
look for
elevation of
the ST
segment.

ST Elevation (cont)
Elevation of the
ST segment
(greater than 1
small box) in 2
leads is
consistent with a
myocardial
infarction.

Anterior View of the Heart

The anterior portion of the heart is best
viewed using leads V1- V4.

Anterior Myocardial Infarction

If you see changes in leads V1 - V4
that are consistent with a myocardial
infarction, you can conclude that it is
an anterior wall myocardial infarction.

Putting it all Together

Do you think this person is having a
myocardial infarction. If so, where?

Interpretation
Yes, this person is having an acute anterior
wall myocardial infarction.

Other MI Locations
Now that you know where to look for an
anterior wall myocardial infarction lets
look at how you would determine if the MI
involves the lateral wall or the inferior wall
of the heart.

Other MI Locations
First, take a look
again at this
picture of the heart.

Anterior portion
of the heart

Inferior portion
of the heart

Lateral portion
of the heart

Other MI Locations
Second, remember that the 12-leads of the ECG look at
different portions of the heart. The limb and augmented
leads see electrical activity moving inferiorly (II, III and
aVF), to the left (I, aVL) and to the right (aVR). Whereas, the
precordial leads see electrical activity in the posterior to
anterior direction.

Limb Leads

Augmented Leads

Precordial Leads

Other MI Locations
Now, using these 3 diagrams lets figure where
to look for a lateral wall and inferior wall MI.
Limb Leads

Augmented Leads

Precordial Leads

Anterior MI
Remember the anterior portion of the heart is
best viewed using leads V1- V4.
Limb Leads

Augmented Leads

Precordial Leads

Lateral MI
So what leads do you think
the lateral portion of the
heart is best viewed?
Limb Leads

Leads I, aVL, and V5- V6

Augmented Leads

Precordial Leads

Inferior MI
Now how about the
inferior portion of the
heart?
Limb Leads

Leads II, III and aVF

Augmented Leads

Precordial Leads

Putting it all Together

Now, where do you think this person is
having a myocardial infarction?

Inferior Wall MI
This is an inferior MI. Note the ST elevation
in leads II, III and aVF.

Putting it all Together

How about now?

Anterolateral MI
This persons MI involves both the anterior wall
(V2-V4) and the lateral wall (V5-V6, I, and aVL)!

End of Module V
Acute Myocardial Infarction

ECG Rhythm Interpretation

Module VI
Advanced 12-Lead Interpretation

Course Objectives
To recognize the normal rhythm of the
heart - Normal Sinus Rhythm.
To recognize the 13 most common
heart arrhythmias.
To recognize an acute myocardial
infarction on a 12-lead ECG.

Learning Modules

ECG Basics
How to Analyze a Rhythm
Normal Sinus Rhythm
Heart Arrhythmias
Diagnosing a Myocardial Infarction
Advanced 12-Lead Interpretation

The 12-Lead ECG

The 12-Lead ECG contains a wealth of
information. In Module V you learned that
ST segment elevation in two leads is
suggestive of an acute myocardial
infarction. In this module we will cover:
ST Elevation and non-ST Elevation MIs
Left Ventricular Hypertrophy
Bundle Branch Blocks

ST Elevation and
non-ST Elevation MIs

ST Elevation and non-ST Elevation MIs

When myocardial blood supply is abruptly
reduced or cut off to a region of the heart, a
sequence of injurious events occur beginning
with ischemia (inadequate tissue perfusion),
followed by necrosis (infarction), and eventual
fibrosis (scarring) if the blood supply isn't
restored in an appropriate period of time.
The ECG changes over time with each of
these events

ECG Changes
Ways the ECG can change include:
ST elevation &
depression

T-waves
peaked
Appearance
of pathologic
Q-waves

flattened

inverted

ECG Changes & the Evolving MI

There are two
distinct patterns
of ECG change
depending if the
infarction is:

Non-ST Elevation

ST Elevation

ST Elevation (Transmural or Q-wave), or

Non-ST Elevation (Subendocardial or non-Q-wave)

ST Elevation Infarction
The ECG changes seen with a ST elevation infarction are:

Before injury Normal ECG

Ischemia

ST depression, peaked T-waves,

then T-wave inversion

Infarction

ST elevation & appearance of

Q-waves

Fibrosis

ST segments and T-waves return to

normal, but Q-waves persist

ST Elevation Infarction
Heres a diagram depicting an evolving infarction:
A. Normal ECG prior to MI
B. Ischemia from coronary artery occlusion
results in ST depression (not shown) and
peaked T-waves
C. Infarction from ongoing ischemia results in
marked ST elevation
D/E. Ongoing infarction with appearance of
pathologic Q-waves and T-wave inversion
F. Fibrosis (months later) with persistent Qwaves, but normal ST segment and Twaves

ST Elevation Infarction
Heres an ECG of an inferior MI:
Look at the
inferior leads
(II, III, aVF).
Question:
What ECG
changes do
you see?
ST elevation
and Q-waves

Extra credit:
What is the
rhythm? Atrial fibrillation (irregularly irregular with narrow QRS)!

Non-ST Elevation Infarction

Heres an ECG of an inferior MI later in time:
Now what do
you see in the
inferior leads?
ST elevation,
Q-waves and
T-wave
inversion

Non-ST Elevation Infarction

The ECG changes seen with a non-ST elevation infarction are:

Before injury Normal ECG

Ischemia

ST depression & T-wave inversion

Infarction

ST depression & T-wave inversion

Fibrosis

ST returns to baseline, but T-wave

inversion persists

Non-ST Elevation Infarction

Heres an ECG of an evolving non-ST elevation MI:
Note the ST
depression
and T-wave
inversion in
leads V2-V6.

Question:
What area of
the heart is
infarcting?

Anterolateral

Left Ventricular Hypertrophy

Left Ventricular Hypertrophy

Compare these two 12-lead ECGs. What stands
out as different with the second one?

Normal

Left Ventricular Hypertrophy

Answer: The QRS complexes are very tall

(increased voltage)

Left Ventricular Hypertrophy

Why is left ventricular hypertrophy characterized by tall
QRS complexes?
As the heart muscle wall thickens there is an increase in
electrical forces moving through the myocardium resulting
in increased QRS voltage.

LVH

Increased QRS voltage

ECHOcardiogram

Left Ventricular Hypertrophy

Criteria exists to diagnose LVH using a 12-lead ECG.
For example:
The R wave in V5 or V6 plus the S wave in V1 or V2
exceeds 35 mm.

However, for now, all

you need to know is
that the QRS voltage
increases with LVH.

Bundle Branch Blocks

Bundle Branch Blocks

Turning our attention to bundle branch blocks
Remember normal
impulse conduction is
SA node
AV node
Bundle of His
Bundle Branches
Purkinje fibers

Normal Impulse Conduction

Sinoatrial node
AV node
Bundle of His
Bundle Branches
Purkinje fibers

Bundle Branch Blocks

So, depolarization of
the Bundle Branches
and Purkinje fibers are
seen as the QRS
complex on the ECG.
Therefore, a conduction
block of the Bundle
Branches would be
reflected as a change in
the QRS complex.

Right
BBB

Bundle Branch Blocks

With Bundle Branch Blocks you will see two changes
on the ECG.
1. QRS complex widens (> 0.12 sec).
2. QRS morphology changes (varies depending on ECG lead,
and if it is a right vs. left bundle branch block).

Bundle Branch Blocks

Why does the QRS complex widen?
When the conduction
pathway is blocked it
will take longer for
the electrical signal
to pass throughout
the ventricles.

Right Bundle Branch Blocks

What QRS morphology is characteristic?
For RBBB the wide QRS complex assumes a
unique, virtually diagnostic shape in those
leads overlying the right ventricle (V1 and V2).
V1

Rabbit Ears

Left Bundle Branch Blocks

What QRS morphology is characteristic?
For LBBB the wide QRS complex assumes a
characteristic change in shape in those leads
opposite the left ventricle (right ventricular
leads - V1 and V2).

Normal

Broad,
deep S
waves

Summary
This Module introduced you to:
ST Elevation and Non-ST Elevation MIs
Left Ventricular Hypertrophy
Bundle Branch Blocks
Dont worry too much right now about trying to
remember all the details. Youll focus more on
advanced ECG interpretation in your clinical
years!

End of Module VI
Advanced 12-Lead
Interpretation

ECG Rhythm Interpretation

Module VII a
Reading 12-Lead ECGs

Course Objective
To systematically analyze a 12-lead
ECG.

Learning Modules

ECG Basics
How to Analyze a Rhythm
Normal Sinus Rhythm
Heart Arrhythmias
Diagnosing a Myocardial Infarction
Advanced 12-Lead Interpretation
Reading 12-Lead ECGs

Reading 12-Lead ECGs

The 12-Lead ECG contains information that will assist
you in making diagnostic and treatment decisions in your
clinical practice. In previous modules you learned how to
read and interpret parts of the ECG. Now, we will bring all
that you have learned together so that you can
systematically read and interpret a 12-lead ECG.

Reading 12-Lead ECGs

The best way to read 12-lead ECGs is to develop a stepby-step approach (just as we did for analyzing a rhythm
strip). In these modules we present a 6-step approach:
1. Calculate RATE
2. Determine RHYTHM
3. Determine QRS AXIS
4. Calculate INTERVALS
5. Assess for HYPERTROPHY
6. Look for evidence of INFARCTION

Rate Rhythm Axis Intervals Hypertrophy Infarct

If you use the rhythm
strip portion of the
12-lead ECG the total
length of it is always
10 seconds long. So
you can count the
number of R waves
in the rhythm strip
and multiply by 6 to
determine the beats
Rate? 12 (R waves) x 6 = 72 bpm
per minute.

Rate

Rhythm Axis Intervals Hypertrophy Infarct

Tip: the rhythm strip portion of the 12-lead ECG is a good

place to look at when trying to determine the rhythm
because the 12 leads only capture a few beats.
Rhythm?
1 of 12 leads

Lead II
Rhythm strip

Atrial
fibrillation

Rate Rhythm

Axis Intervals Hypertrophy Infarct

Axis refers to the mean QRS axis (or vector) during ventricular
depolarization. As you recall when the ventricles depolarize (in a
normal heart) the direction of current flows leftward and downward
because most of the ventricular mass is in the left ventricle. We like
to know the QRS axis because an abnormal axis can suggest
disease such as pulmonary hypertension from a pulmonary
embolism.

Rate Rhythm

Axis Intervals Hypertrophy Infarct

The QRS axis is determined by overlying a circle, in the frontal

plane. By convention, the degrees of the circle are as shown.
The normal QRS axis lies between -30o and +90o.
A QRS axis that falls between -30o
and -90o is abnormal and called left
axis deviation.

-90o
-60o

-120o
-150o

A QRS axis that falls between +90o

and +150o is abnormal and called
right axis deviation.

-30o

180o
o

A QRS axis that falls between +150

and -90o is abnormal and called
superior right axis deviation.

0o
30o

150o
120o

90o

60o

Rate Rhythm

Axis Intervals Hypertrophy Infarct

We can quickly determine whether the QRS axis is

normal by looking at leads I and II.
QRS negative (R < Q+S)

If the QRS complex is

overall positive (R > Q+S)
in leads I and II, the QRS
axis is normal.
In this ECG what leads
have QRS complexes
that are negative?
equivocal?
QRS equivocal (R = Q+S)

Rate Rhythm

Axis Intervals Hypertrophy Infarct

How do we know the axis is normal when the QRS

complexes are positive in leads I and II?

Rate Rhythm

Axis Intervals Hypertrophy Infarct

The answer lies in the fact that each frontal lead

corresponds to a location on the circle.
Limb leads

-90o

I = +0o

-60o

-120o

II = +60o

avR -150o

-30o

III = +120o
Augmented leads
avL = -30o

180o

0o
30o

150o

avF = +90o
avR = -150

III

120o

90o

avF

60o

II II

avL
II

Rate Rhythm

Axis Intervals Hypertrophy Infarct

Since lead I is orientated at 0o a wave of depolarization directed towards

it will result in a positive QRS axis. Therefore any mean QRS vector
between -90o and +90o will be positive.
-90o
-60o

-120o
-150o

-30o

180o

0o I
30o

150o
120

90o

60o

Rate Rhythm

Axis Intervals Hypertrophy Infarct

Since lead I is orientated at 0o a wave of depolarization directed towards

it will result in a positive QRS axis. Therefore any mean QRS vector
between -90o and +90o will be positive.
Similarly, since lead II is orientated at 60o
-120o
a wave of depolarization directed towards
it will result in a positive QRS axis.
-150o
Therefore any mean QRS vector between
-30o and +150o will be positive.

-90o
-60o
-30o

180o

0o I
30o

150o
120

90o

60o

II

Rate Rhythm

Axis Intervals Hypertrophy Infarct

Since lead I is orientated at 0o a wave of depolarization directed towards

it will result in a positive QRS axis. Therefore any mean QRS vector
between -90o and +90o will be positive.
Similarly, since lead II is orientated at 60o
-120o
a wave of depolarization directed towards
it will result in a positive QRS axis.
-150o
Therefore any mean QRS vector between
-30o and +150o will be positive.

-90o
-60o
-30o

180o

Therefore, if the QRS complex is positive

in both leads I and II the QRS axis must 150o
be between -30o and 90o (where leads I
and II overlap) and, as a result, the axis
120o
must be normal.

0o I
30o

90o

60o

II

Rate Rhythm

Axis Intervals Hypertrophy Infarct

Now using what you just learned fill in the following table. For example, if
the QRS is positive in lead I and negative in lead II what is the QRS axis?
(normal, left, right or right superior axis deviation)
-90o

QRS Complexes

I
+
+

II
+
-

Axis
normal
left axis deviation

-120o

-60o

-150o

-30o

180o

0o I
30o

150o
120

90o

60o

II

Rate Rhythm

Axis Intervals Hypertrophy Infarct

if the QRS is negative in lead I and positive in lead II what is the QRS
axis? (normal, left, right or right superior axis deviation)
-90o

QRS Complexes

I
+
+

II
+
-

Axis
normal
left axis deviation
right axis deviation

-120o

-60o

-150o

-30o

180o

0o I
30o

150o
120

90o

60o

II

Rate Rhythm

Axis Intervals Hypertrophy Infarct

if the QRS is negative in lead I and negative in lead II what is the QRS
axis? (normal, left, right or right superior axis deviation)
-90o

QRS Complexes

I
+
+

II
+
-

Axis
normal
left axis deviation
right axis deviation
right superior
axis deviation

-120o

-60o

-150o

-30o

180o

0o I
30o

150o
120

90o

60o

II

Rate Rhythm

Axis Intervals Hypertrophy Infarct

Is the QRS axis normal in this ECG?

No, there is left axis

deviation.

The QRS is
positive in I
and negative
in II.

Rate Rhythm

Axis Intervals Hypertrophy Infarct

To summarize:
The normal QRS axis falls between -30o and +90o because ventricular
depolarization is leftward and downward.
Left axis deviation occurs when the axis falls between -30o and -90o.
Right axis deviation occurs when the axis falls between +90o and +150o.
Right superior axis deviation occurs when the axis falls between between
+150o and -90o.
A quick way to determine
the QRS axis is to look at the
QRS complexes in leads I
and II.

QRS Complexes

II

Axis

normal

left axis deviation

right axis deviation

right superior
axis deviation

SUMMARY Rate Rhythm Axis Intervals Hypertrophy Infarct

To summarize VII a:
1. Calculate RATE
2. Determine RHYTHM
3. Determine QRS AXIS

Normal
Left axis deviation
Right axis deviation
Right superior axis deviation

SUMMARY Rate Rhythm Axis Intervals Hypertrophy Infarct

In VII b we will cover the next 3 steps:
1.
2.
3.
4.
5.
6.

Calculate RATE
Determine RHYTHM
Determine QRS AXIS
Calculate INTERVALS
Assess for HYPERTROPHY
Look for evidence of INFARCTION

Rate Rhythm Axis

Intervals Hypertrophy Infarct

Intervals refers to the length of the PR and QT intervals

and the width of the QRS complexes. You should have
already determined the PR and QRS during the rhythm
step, but if not, do so in this step.
In the following few slides well review what is a normal
and abnormal PR, QRS and QT interval. Also listed are a
few common causes of abnormal intervals.

Rate Rhythm Axis

Intervals Hypertrophy Infarct

PR interval
< 0.12 s

0.12-0.20 s

> 0.20 s

High catecholamine
states
Wolff-Parkinson-White

Normal

AV nodal blocks

Wolff-Parkinson-White

1st Degree AV Block

Rate Rhythm Axis

Intervals Hypertrophy Infarct

QRS complex
< 0.10 s

Normal

0.10-0.12 s

> 0.12 s

Incomplete bundle
branch block

Bundle branch block

PVC
Ventricular rhythm

Incomplete bundle branch block

3rd degree AV block with

ventricular escape rhythm

Remember: If you have a BBB determine if it is a right or left

BBB. If you need a refresher see Module VI.

Rate Rhythm Axis

Intervals Hypertrophy Infarct

QT interval
The duration of the QT interval is
proportionate to the heart rate. The faster
the heart beats, the faster the ventricles
repolarize so the shorter the QT interval.
Therefore what is a normal QT varies
with the heart rate. For each heart rate you
need to calculate an adjusted QT interval,
called the corrected QT (QTc):
QTc = QT / square root of RR interval

Rate Rhythm Axis

Intervals Hypertrophy Infarct

QTc interval
< 0.44 s

> 0.44 s

Normal

Long QT

Long QT

Torsades de Pointes

A prolonged QT can be very dangerous. It may predispose an individual to a type of

ventricular tachycardia called Torsades de Pointes. Causes include drugs, electrolyte
abnormalities, CNS disease, post-MI, and congenital heart disease.

Rate Rhythm Axis

Intervals Hypertrophy Infarct

QT = 0.40 s
RR = 0.68 s
Square root of
RR = 0.82
QTc = 0.40/0.82
= 0.49 s

PR interval?
0.16 seconds

QRS width?
0.08 seconds

QTc interval?
0.49 seconds

Interpretation of intervals? Normal PR and QRS, long QT

Rate Rhythm Axis

Intervals Hypertrophy Infarct

RR
23 boxes

17 boxes

10 boxes

13 boxes

QT

Normal QT

Long QT

Tip: Instead of calculating the QTc, a quick way to estimate if the

QT interval long is to use the following rule:
A QT > half of the RR interval is probably long.

Rate Rhythm Axis Intervals

Hypertrophy Infarct

In this step of the 12-lead ECG analysis, we use the ECG

to determine if any of the 4 chambers of the heart are
enlarged or hypertrophied. We want to determine if there
are any of the following:

Right atrial enlargement (RAE)

Left atrial enlargement (LAE)
Right ventricular hypertrophy (RVH)
Left ventricular hypertrophy (LVH)

Rate Rhythm Axis Intervals

Hypertrophy Infarct

In Module VI we introduced the concept of left

ventricular hypertrophy. As you remember the QRS
voltage increases with LVH and is characterized by tall
QRS complexes in certain leads. Similarly for right
ventricular hypertrophy we look at the QRS complexes for
changes in voltage patterns.
With right and left atrial enlargement we analyze the P
wave (since the P wave represents atrial depolarization).
Here we also look for changes in voltage patterns.

Rate Rhythm Axis Intervals

Hypertrophy Infarct

Right atrial enlargement

Take a look at this ECG. What do you notice about the P waves?

The P waves are tall, especially in leads II, III and avF.
Ouch! They would hurt to sit on!!

Rate Rhythm Axis Intervals

Hypertrophy Infarct

Right atrial enlargement

To diagnose RAE you can use the following criteria:
II
V1 or V2

> 2 boxes (in height)

P > 2.5 mm, or

P > 1.5 mm

> 1 boxes (in height)

Remember 1 small
box in height = 1 mm

A cause of RAE is RVH from pulmonary hypertension.

Rate Rhythm Axis Intervals

Hypertrophy Infarct

Left atrial enlargement

Take a look at this ECG. What do you notice about the P waves?

Notched

Negative deflection

The P waves in lead II are notched and in lead V1 they

have a deep and wide negative component.

Rate Rhythm Axis Intervals

Hypertrophy Infarct

Left atrial enlargement

To diagnose LAE you can use the following criteria:
II
V1

> 0.04 s (1 box) between notched peaks, or

Neg. deflection > 1 box wide x 1 box deep

Normal

LAE

A common cause of LAE is LVH from hypertension.

Rate Rhythm Axis Intervals

Hypertrophy Infarct

Right ventricular hypertrophy

Take a look at this ECG. What do you notice about the axis and QRS
complexes over the right ventricle (V1, V2)?

There is right axis deviation (negative in I, positive in II) and

there are tall R waves in V1, V2.

Rate Rhythm Axis Intervals

Hypertrophy Infarct

Right ventricular hypertrophy

Compare the R waves in V1, V2 from a normal ECG and one from
a person with RVH.
Notice the R wave is normally small in V1, V2 because the right
ventricle does not have a lot of muscle mass.
But in the hypertrophied right ventricle the R wave is tall in V1, V2.

Normal

RVH

Rate Rhythm Axis Intervals

Hypertrophy Infarct

Right ventricular hypertrophy

To diagnose RVH you can use the following criteria:

V1

Right axis deviation, and

R wave > 7mm tall

A common
cause of RVH
is left heart
failure.

Rate Rhythm Axis Intervals

Hypertrophy Infarct

Left ventricular hypertrophy

Take a look at this ECG. What do you notice about the axis and QRS
complexes over the left ventricle (V5, V6) and right ventricle (V1, V2)?
The deep S waves
seen in the leads over
the right ventricle are
created because the
heart is depolarizing
left, superior and
posterior (away from
leads V1, V2).

There is left axis deviation (positive in I, negative in II) and there

are tall R waves in V5, V6 and deep S waves in V1, V2.

Rate Rhythm Axis Intervals

Hypertrophy Infarct

Left ventricular hypertrophy

To diagnose LVH you can use the following criteria*:

avL

R in V5 (or V6) + S in V1 (or V2) > 35 mm, or

R > 13 mm
S = 13 mm

* There are several

other criteria for the
diagnosis of LVH.

R = 25 mm

A common cause of LVH

is hypertension.

Rate Rhythm Axis Intervals

Hypertrophy Infarct

A 63 yo man has longstanding, uncontrolled hypertension. Is there evidence

of heart disease from his hypertension? (Hint: There a 3 abnormalities.)

Yes, there is left axis deviation (positive in I, negative in II), left atrial enlargement
(> 1 x 1 boxes in V1) and LVH (R in V5 = 27 + S in V2 = 10 > 35 mm).

Rate Rhythm Axis Intervals Hypertrophy

Infarct

When analyzing a 12-lead ECG for evidence of an

infarction you want to look for the following:
Abnormal Q waves
ST elevation or depression
Peaked, flat or inverted T waves

These topics were covered in Modules V and VI where

you learned:
ST elevation (or depression) of 1 mm in 2 or more
contiguous leads is consistent with an AMI
There are ST elevation (Q-wave) and non-ST elevation
(non-Q wave) MIs

Rate Rhythm Axis Intervals Hypertrophy

Infarct

Tip: One way to determine if Q waves (and R waves) are abnormal is by

looking at the width and using the following mantra (read red downwards):
Any
Any
Any

Any Q wave in V1
Any Q wave in V2
Any Q wave in V3

20
30
30

A Q wave > 20 msec in V4

A Q wave > 30 msec in V5
A Q wave > 30 msec in V6

30
30
30
30

A Q wave > 30 msec in I

A Q wave > 30 msec in avL
A Q wave > 30 msec in II
A Q wave > 30 msec in avF

R40
R50

A R wave > 40 msec in V1

A R wave > 50 msec in V2

(i.e. 0.02 sec or width of a box)

Rate Rhythm Axis Intervals Hypertrophy

Infarct

This mantra corresponds to the ECG in the following way:

30

30

Any R40

20

30

Any R50

30

30

Any

30

SUMMARY Rate Rhythm Axis Intervals Hypertrophy Infarct

To summarize:
1. Calculate RATE
2. Determine RHYTHM
3. Determine QRS AXIS

Normal
Left axis deviation
Right axis deviation
Right superior axis deviation

SUMMARY Rate Rhythm Axis Intervals Hypertrophy Infarct

To summarize:
1.
2.
3.
4.

Calculate RATE
Determine RHYTHM
Determine QRS AXIS
Calculate INTERVALS

PR
QRS
QT

SUMMARY Rate Rhythm Axis Intervals Hypertrophy Infarct

To summarize:
1. Calculate RATE
2. Determine RHYTHM
3. Determine QRS AXIS
4. Calculate INTERVALS
5. Assess for HYPERTROPHY

Right and left atrial enlargement

Right and left ventricular hypertrophy

SUMMARY Rate Rhythm Axis Intervals Hypertrophy Infarct

To summarize:
1.
2.
3.
4.
5.

Calculate RATE
Determine RHYTHM
Determine QRS AXIS
Calculate INTERVALS
Assess for HYPERTROPHY

6. Look for evidence of INFARCTION

Abnormal Q waves
ST elevation or depression
Peaked, flat or inverted T waves

SUMMARY Rate Rhythm Axis Intervals Hypertrophy Infarct

To summarize:
1.
2.
3.
4.
5.
6.

Calculate RATE
Determine RHYTHM
Determine QRS AXIS
Calculate INTERVALS
Assess for HYPERTROPHY
Look for evidence of INFARCTION

Now to finish this module lets analyze a 12-lead ECG!

SUMMARY Rate Rhythm Axis Intervals Hypertrophy Infarct

A 16 yo young man ran into a guardrail while riding a motorcycle.
In the ED he is comatose and dyspneic. This is his ECG.

SUMMARY Rate Rhythm Axis Intervals Hypertrophy Infarct

What is the rate?

Approx. 132 bpm (22 R waves x 6)

SUMMARY Rate Rhythm Axis Intervals Hypertrophy Infarct

What is the rhythm?

Sinus tachycardia

SUMMARY Rate Rhythm Axis Intervals Hypertrophy Infarct

What is the QRS axis? Right axis deviation (- in I, + in II)

SUMMARY Rate Rhythm Axis Intervals Hypertrophy Infarct

What are the PR, QRS PR = 0.12 s, QRS = 0.08 s, QTc = 0.482 s
and QT intervals?

SUMMARY Rate Rhythm Axis Intervals Hypertrophy Infarct

Is there evidence of
atrial enlargement?

No (no peaked, notched or negatively

deflected P waves)

SUMMARY Rate Rhythm Axis Intervals Hypertrophy Infarct

Is there evidence of
No (no tall R waves in V1/V2 or V5/V6)
ventricular hypertrophy?

SUMMARY Rate Rhythm Axis Intervals Hypertrophy Infarct

Infarct: Are there abnormal
Q waves?
30

30

30

30

Yes! In leads V1-V6 and I, avL

Any

R40

20

Any

R50

30

Any

30

SUMMARY Rate Rhythm Axis Intervals Hypertrophy Infarct

Infarct: Is the ST elevation
or depression?

Yes! Elevation in V2-V6, I and avL.

Depression in II, III and avF.

SUMMARY Rate Rhythm Axis Intervals Hypertrophy Infarct

Infarct: Are there T wave
changes?

No

SUMMARY Rate Rhythm Axis Intervals Hypertrophy Infarct

ECG analysis: Sinus tachycardia at 132 bpm, right axis deviation,
long QT, and evidence of ST elevation infarction in the
anterolateral leads (V1-V6, I, avL) with reciprocal changes (the
ST depression) in the inferior leads (II, III, avF).
This young man suffered an
acute myocardial infarction after
blunt trauma. An
echocardiogram showed
anteroseptal akinesia in the left
ventricle with severely
depressed LV function
(EF=28%). An angiogram
showed total occlusion in the
proximal LAD with collaterals
from the RCA and LCX.

End of Module VII

Reading 12-Lead ECGs

THANK YOU

Please note: Important condition specific ECGS to follow in next class.

LET THE HEART DO WHAT ITS

BEST AT
LOVE