11/13/24, 7\:32 PM Guide | ECG interpretation

ECG interpretation

Table of contents
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Introduction

This guide demonstrates how to read an ECG using a systematic approach. If you want to put your ECG interpretation
knowledge to the test, check out our ECG quiz.

Con

Before beginning ECG interpretation, you should check the following details\:
Con
Check the date and time that the ECG was performed.
Check the calibration of the ECG (usually 25mm/s and 10mm/1mV).

Heart rate

What is a normal adult heart rate?

Normal\: 60-100 bpm
Tachycardia\: > 100 bpm
Bradycardia\: \< 60 bpm

Regular heart rhythm

If a patient has a regular heart rhythm, their heart rate can be calculated using the following method\:
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Count the number of large squares present within one R-R interval.
Divide 300 by this number to calculate heart rate.
Heart rate calculation example
4 large squares in an R-R interval
300/4 = 75 beats per minute
How to calculate a heart rate on a normal ECG

Irregular heart rhythm

If a patient's heart rhythm is irregular, the
signi
Count the number of complexes on the rhythm strip (each rhythm strip is typically 10 seconds long).
Multiply the number of complexes by 6 (giving you the average number of complexes in 1 minute).
Heart rate calculation example
10 complexes on a rhythm strip
10 x 6 = 60 beats per minute

Heart rhythm

A patient's heart rhythm can be regular or irregular.
Irregular rhythms can be either\:
Regularly irregular (i.e. a recurrent pattern of irregularity)
Irregularly irregular (i.e. completely disorganised)
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Mark out several consecutive R-R intervals on a piece of paper, then move them along the rhythm strip to check if the
subsequent intervals are similar.
Hint
If you are suspicious of atrioventricular block (AV block), map out the atrial rate and the ventricular rhythm separately
(i.e. mark the P waves and R waves). As you move along the rhythm strip, you can see if the PR interval changes, if QRS
complexes are missing or if there is complete dissociation between the two.
Measure the R-R intervals to assess if the rhythm is regular or irregular

Cardiac axis

Cardiac axis describes the overall direction of electrical spread within the heart.
In a healthy individual, the axis should spread from 11 o'clock to 5 o'clock.
To determine the cardiac axis, you must look at leads I, II and III.
Read our cardiac axis guide to learn more.

Normal cardiac axis

Typical ECG
Lead II has the most positive de
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Normal cardiac axis

Right axis deviation

Typical ECG
Lead III has the most positive de
Right axis deviation is associated with right ventricular hypertrophy.
Right axis deviation [2]

Left axis deviation

Typical ECG
Lead I has the most positive de
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Leads II and III are negative.
Left axis deviation is associated with heart conduction abnormalities.
Left axis deviation [2]

P-waves

The next step is to look at the P waves and answer the following questions\:
1. Are P waves present?
2. If so, is each P wave followed by a QRS complex?
3. Do the P waves look normal? - check duration, direction and shape
4. If P waves are absent, is there any atrial activity?
Sawtooth baseline →
Chaotic baseline →
Flat line → no atrial activity at all
Hint
If P waves are absent and there is an irregular rhythm, it may suggest a diagnosis of atrial .
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P waves [1]

P-R interval

The PR interval should be between 120-200 ms (3-5 small squares).

Prolonged PR interval (>0.2 seconds)

A prolonged PR interval suggests the presence of atrioventricular delay (AV block).
First-degree heart block (AV block)
First-degree heart block involves a
First-degree heart block (AV block)
Second-degree heart block (type 1)
Second-degree AV block (type 1) is also known as Mobitz type 1 AV block or Wenckebach phenomenon.
Typical ECG
impulse is not conducted and the QRS complex is dropped.
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AV nodal conduction resumes with the next beat, and the sequence of progressive PR interval prolongation and the eventual
dropping of a QRS complex repeats itself.
Second-degree AV block (Mobitz Type 1 – Wenckebach)
Second-degree heart block (type 2)
Second-degree AV block (type 2) is also known as Mobitz type 2 AV block.
Typical ECG
complexes due to a failure of conduction.
The intermittent dropping of the QRS complexes typically follows a repeating cycle of every 3rd (3\:1 block) or 4th (4\:1 block) P
wave.
Second-degree AV block (Mobitz type 2 AV block)
Third-degree heart block (complete heart block)
Third-degree (complete) AV block occurs when there is no electrical communication between the atria and ventricles due
to a complete failure of conduction.
Typical ECG
the atria and ventricles functioning independently.
Cardiac function is maintained by a junctional or ventricular pacemaker.
Narrow-complex escape rhythms (QRS complexes of \<0.12 seconds duration) originate above the bifurcation of the bundle
of His.
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Broad-complex escape rhythms (QRS complexes >0.12 seconds duration) originate from below the bifurcation of the bundle
of His.
Complete heart block (3rd degree)
Tips for remembering types of heart block
To help remember the various types of AV block, it is useful to know the anatomical location of the block within the
conducting system.
First-degree AV block\:
Occurs between the SA node and the AV node (i.e. within the atrium).
Second-degree AV block\:
Mobitz I AV block (Wenckebach) occurs IN the AV node (this is the only piece of conductive tissue in the heart which
exhibits the ability to conduct at di
Mobitz II AV block occurs AFTER the AV node in the bundle of His or Purkinje
Third-degree AV block\:
Occurs at or after the AV node resulting in a complete blockade of distal conduction.

Shortened PR interval

If the PR interval is shortened, this can mean one of two things\:
Simply, the P wave originates from somewhere closer to the AV node, so the conduction takes less time (the SA node is not
in a
The atrial impulse is getting to the ventricle by a faster shortcut instead of conducting slowly across the atrial wall. This
accessory pathway can be associated with a delta wave (see below).
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Delta wave

QRS complex

When assessing a QRS complex, you need to pay attention to the following characteristics\:
Width
Height
Morphology
QRS complex

Width

The width can be described as NARROW (\< 0.12 seconds) or BROAD (> 0.12 seconds)\:
A narrow QRS complex occurs when the impulse is conducted down the bundle of His and the Purkinje
organised synchronised ventricular depolarisation.
A broad QRS complex occurs if there is an abnormal depolarisation sequence – for example, a ventricular ectopic where the impulse spreads slowly
across the myocardium from the focus in the ventricle. In contrast, an atrial ectopic would result in a narrow QRS complex because it would conduct
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down the heart's normal conduction system. Similarly, a bundle branch block results in a broad QRS complex because the impulse gets to one
ventricle rapidly down the intrinsic conduction system and then spreads slowly across the myocardium to the other ventricle.
Bundle branch block
Broad QRS complexes are a hallmark feature of bundle branch block, which includes left bundle branch
block (LBBB) and right bundle branch block (RBBB).
The WiLLiaM MaRRoW mnemonic can be used to quickly recognise left and right bundle branch blocks by
looking at V1 and V6.
The middle letters of the names help you remember which bundle branch block each name is referring two (two
Ls in WiLLiaM = left bundle branch block, two Rs in MaRRoW = right bundle branch block).
Each name’s
LBBB\: deep S wave in V1 which may be notched ("W") and broad "M" shaped R wave in V6
RBBB\: RSR' pattern in V1 ("M") and broad S wave in V6 ("W")
Left bundle branch block

Height

Height can be described as either SMALL or TALL\:
Small complexes are de
Tall complexes imply ventricular hypertrophy (although can be due to body habitus e.g. tall slim people). There are numerous algorithms for
measuring LVH, such as the Sokolow-Lyon index or the Cornell index.

Morphology

To assess morphology, you need to assess the individual waves of the QRS complex.
Delta wave
The mythical 'delta wave' indicates that the ventricles are being activated earlier than normal from a point distant
from the AV node. The early activation then spreads slowly across the myocardium, causing the QRS complex's
slurred upstroke.
The presence of a delta wave does NOT diagnose Wol This requires evidence of
tachyarrhythmias AND a delta wave.
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Delta wave
Q-waves
Isolated Q waves can be normal.
A pathological Q wave is > 25% the size of the R wave that follows it or > 2mm in height and > 40ms in width.
A single Q wave is not a cause for concern – look for Q waves in an entire territory (e.g. anterior/inferior) for
evidence of previous myocardial infarction.
Q wave
R and S waves
Assess the R wave progression across the chest leads (from small in V1 to large in V6).
The transition from S > R wave to R > S wave should occur in V3 or V4.
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Poor progression (i.e. S > R through to leads V5 and V6) can be a sign of previous MI but can also occur in very
large people due to poor lead position.
Poor R wave progression
J point segment
The J point is where the S wave joins the ST segment.
This point can be elevated, resulting in the ST segment that follows it being raised (this is known as "high take-o
High take-o
LOOKS like ST elevation.
Key points for assessing the J point segment\:
Benign early repolarisation occurs mostly under the age of 50 (over the age of 50, ischaemia is more common and should be suspected
Typically, the J point is raised with widespread ST elevation in multiple territories making ischaemia less likely.
The T waves are also raised (in contrast to a STEMI, where the T wave remains the same size and the ST segment is raised).
The ECG abnormalities do not change! During a STEMI, the changes will evolve – in benign early repolarisation, they will remain the same.

ST segment

The ST segment is the part of the ECG between the end of the S wave and the start of the T wave.
In a healthy individual, it should be an isoelectric line (neither elevated nor depressed).
Abnormalities of the ST segment should be investigated to rule out pathology.
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Elements of an ECG

ST-elevation

ST-elevation is signi
>2mm in 2 or more chest leads.
It is most commonly caused by acute full-thickness myocardial infarction.
ST elevation

ST depression

ST depression ≥ 0.5 mm in ≥ 2 contiguous leads indicates myocardial ischaemia.
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ST depression

T waves

T waves represent the repolarisation of the ventricles.

Tall T waves

T waves are considered tall if they are\:
> 5mm in the limb leads AND
> 10mm in the chest leads (the same criteria as 'small' QRS complexes)
Tall T waves can be associated with\:
Hyperkalaemia ("tall tented T waves")
Hyperacute STEMI
Tall tented T waves
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Inverted T waves

T waves are normally inverted in V1, and inversion in lead III is a normal variant.
Inverted T waves in other leads are a nonspeci
Ischaemia
Bundle branch blocks (V4-6 in LBBB and V1-V3 in RBBB)
Pulmonary embolism
Left ventricular hypertrophy (in the lateral leads)
Hypertrophic cardiomyopathy (widespread)
General illness
Around 50% of patients admitted to ITU have some evidence of T wave inversion during their stay.
Observe the distribution of the T wave inversion (e.g. anterior/lateral/posterior leads). You must take this ECG

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