How to read an ECG

Question 1

what is a normal adult heart rate

Answer 1

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

Question 2

how to calculate heart rate

Answer 2

Count the number of large squares present within one R-R interval.
Divide 300 by this number to calculate heart rate.

Question 3

how to calculate heart rate if there is an irregular rhythm

Answer 3

if the R-R intervals differ throughout the ECG, you should:
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)

Question 4

heart rhythm

Answer 4

a patients heart rhythm may be regular or irregular
regularly irregular (ie. recurrent pattern of irregularity)
irregularly irregular (ie. completely disorganised)

Question 5

typical ECG finding for a normal cardiac axis

Answer 5

lead II has the most positive deflection compared to leads I and III

Question 6

right axis deviation

Answer 6

lead II has the most positive deflection and lead I should be negative

Question 7

right axis deviation is associated with

Answer 7

right ventricular hypertrophy

Question 8

typical ECG finding for left axis deviation

Answer 8

Typical ECG findings for left axis deviation:

Lead I has the most positive deflection.
Leads II and III are negative.

Question 9

left axis deviation is associated with

Answer 9

conduction abnormalities

Question 10

what should you look for when looking at P waves

Answer 10

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?

Question 11

how long should the PR interval be

Answer 11

120-200ms (3-5 small squares)

Question 12

how long is a prolonged PR interval

Answer 12

> 0.2 seconds (200ms)

Question 13

what does a prolonged PR interval indicate

Answer 13

atrioventricular delay (AV block)

Question 14

first degree heart block (AV block)

Answer 14

fixed prolonged PR interval >200ms

Question 15

second degree heart block (type 1)

Answer 15

Typical ECG findings in Mobitz type 1 AV block include progressive prolongation of the PR interval until eventually the atrial impulse is not conducted and the QRS complex is dropped.
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.

Question 16

second degree heart block type 1 is also called

Answer 16

Second-degree AV block (type 1) is also known as Mobitz type 1 AV block or Wenckebach phenomenon.

Question 17

ECG findings of second degree heart block (type 2)

Answer 17

Typical ECG findings in Mobitz type 2 AV block include a consistent PR interval duration with intermittently dropped QRS 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.

Question 18

third degree heart block occurs when

Answer 18

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.
Cardiac function is maintained by a junctional or ventricular pacemaker

Question 19

typical ECG findings of third-degree heart block

Answer 19

Typical ECG findings include the presence of P waves and QRS complexes that have no association with each other, due to the atria and ventricles functioning independently.

Question 20

what is the anatomical location of first degree block

Answer 20

between the SA node and the AV node

Question 21

what is the anatomical location of second degree block

Answer 21

type 1 occurs IN the AV node
type 2 occurs AFTER the AV node in the bundle of His or pukinje fibres

Question 22

what is the anatomical location of third degree AV block

Answer 22

occurs at or after the AV node resulting in a complete blockade of distal conduction

Question 23

shortened PR interval

Answer 23

can mean one of two things:
1. the P wave is originating from somewhere closer to the AV node so conduction takes less time
2. the atrial impulse is getting to the ventricle by a faster shortcut instead of conducting slowly across the atrial wall. this is an accessory pathway and can be associated with a delta wave

Question 24

what should the width of the QRS complex be

Answer 24

it is narrow if it is <0.12 seconds
broad if it is >0.12 seconds

Question 25

how does a narrow QRS complex occur

Answer 25

when the impulse is conducted down the bundle of His and the Purkinje fibre to the ventricles
this results in a well organised synchronised ventricular depolarisation

Question 26

how does a broad QRS complex occur

Answer 26

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.

Question 27

height of the QRS complex

Answer 27

Height can be described as either SMALL or TALL:

Small complexes are defined as < 5mm in the limb leads or < 10 mm in the chest leads.
Tall complexes imply ventricular hypertrophy (although can be due to body habitus e.g. tall slim people).

Question 28

tall complexes imply

Answer 28

ventricular hypertrophy

Question 29

what is a Delta wave

Answer 29

a sign that 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 slurred upstroke of the QRS complex

Question 30

what is a delta wave associated with

Answer 30

wolff parkinson white syndrome
but it is not diagnostic of this

Question 31

Q-waves

Answer 31

isolated Q waves may be normal - a single Q wave is not a cause for concern
a pathological Q wave is >25% the size of the R wave that follows it or >2mm in height and >40ms in width
look for Q waves in an entire territory for evidence of previous myocardial infarction

Question 32

R wave progression

Answer 32

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
poor R wave progression is when the S wave continues to be larger than the R wave (ie. through to leads V5 and V6)

Question 33

what is poor R wave progression a sign of

Answer 33

can be a sign of previous MI but can also occur in very large people due to poor lead position

Question 34

where is the J point

Answer 34

where the S wave joins the ST segment

Question 35

what if the J point is elevated

Answer 35

this will mean the ST segment that follows it will also be raised
(this is called ‘high take off” or ‘benign early repolarisation’)

Question 36

what does benign early repolarisation mean

Answer 36

this is a normal variant
can cause unnecessary concern because it looks like ST elevation

Question 37

how should you assess the J point to figure out whether there is ischaemia or benign early repolarisation

Answer 37

• benign early repolarisation occurs mostly under the age of 50 (over 50, ischaemia should be considered first)
• 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 with STEMI where the T wave remains the same size and the ST segment is raised)
• the ECG abnormalities do not change - in STEMI, the changes will evolve, but in benign early repolarisation, they will remain the same

Question 38

in a healthy individual, what should the ST segment look like

Answer 38

it should be an isoelectric line (neither elevated or depressed)

Question 39

ST elevation is significant when

Answer 39

it is greater than 1mm (1 small square) in two or more contiguous limb leads
OR
>2mm in 2 or more chest leads

Question 40

ST elevation is most commonly caused by

Answer 40

acute full-thickness myocardial infarction

Question 41

what defines ST depression

Answer 41

> or equal to 0.5mm in > or equal to 2 contiguous leads

Question 42

ST depression indicates

Answer 42

myocardial ischaemia

Question 43

T waves represent

Answer 43

repolarisation of the ventricles

Question 44

T waves are considered tall if they are

Answer 44

> 5mm in the limb leads
AND
10mm in the chest leads
(the same criteria as small QRS complexes)

Question 45

tall T waves may be associated with

Answer 45

hyperkalaemia (tall tented T waves)
hyperacute STEMI

Question 46

can inverted T waves be normal

Answer 46

T waves are normally inverted in V1 and inversion in lead III is a normal variant

Question 47

when are inverted T waves not normal

Answer 47

if they are inverted anywhere except leads V1 and III this is a non-specific sign of a wide variety of conditions
- ischaemia
- bundle branch blocks
- pulmonary embolism
- left ventricular hypertrophy (in the lateral leads)
- hypertrophic cardiomyopathy
- general illness

Question 48

biphasic T waves

Answer 48

have two peaks and can be indicative of ischaemia and hyperkalaemia

Question 49

flattened T waves

Answer 49

non-specific sign, may represent ischaemia or electrolyte imbalance

Question 50

can U waves be normal?

Answer 50

U waves are not a common finding

Question 51

what is a U wave

Answer 51

> 0.5mm deflection after the T wave best seen in V2 or V3

Question 52

what causes a U wave

Answer 52

they become larger the slower the bradycardia
they are seen in various electrolyte imbalances, hypothermia, and secondary to antiarrhythmic therapy (such as digoxin, procainamide or amiodarone)