Common ECG Abnormalities

Question 1

When is a PR interval prolonged?

Answer 1

When it is more than 1 large box

Question 2

When is the QRS complex prolonged?

Answer 2

When it is more than 3 small boxes

Question 3

When is the Corrected QT interval (QTc) prolonged?

Answer 3

When more than 44 small boxes

Question 4

What criteria must be met for the ECG to be in sinus rhythm?

Answer 4

• Rhythm must be regular
• Heart rate between 60 - 100 bpm
• P waves are present
• P waves must be upright in leads I,II
• PR interval should be 3-5 boxes (normal)
• QRS complex should be under or equal to 3 small boxes
• Every P wave must be followed by a QRS complex
• Every QRS is preceded by a P wave.

Question 5

What is an atrioventricular conduction block?

Answer 5

This is a delay / failure of conduction of impulses from atrium to ventricles via AV node and bungle of His.

Question 6

What causes AV conduction block?

Answer 6

Acute MI

Degenerative change (fibrosis as people age - changes electrical conduction)

Question 7

What are the types of AV conduction block?

Answer 7

First degree heart block

Second degree heart block (Mobitz type 1 and Mobitz type 2)

Thrid degree heart block

Question 8

What is 1st degree heart block?

Answer 8

Impulses get through but takes longer.

PR interval is prolonged to over 0.2 seconds (5 small boxes)

Question 9

What is 2nd degree heart block - Mobitz type 1?

Answer 9

Also called Wenkebach type.

Successively longer PR intervals until one QRS is dropped then the cycle starts again.

Question 10

What is 2nd degree heart block - Mobitz type II?

Answer 10

PR intervals do not lengthen but, suddenly dropped QRS complex.

This is dangerous as there is a high risk of progression to complete heart block.

Question 11

What is 3rd degree heart block?

Answer 11

This is complete failure of AV conduction.

This means Atria and Ventricles are depolarising independently.

Ventricular pacemaker takes over (Ventricular escape rhythm).

Usually wide QRS complexes.

Ventricular rate is very slow (30-40bpm), often too slow to maintain BP.

Urgent pacemaker insertion usually required.

Ventricle doesn’t conduct through His Purkinje system so slower conduction.

In 3rd degree heart block, P-P intervals are regular and R-R intervals are regular but, there is no relationship between P waves and QRS complexes (The PR interval is completely variable from beat to beat)

Question 12

What is a bundle branch block?

Answer 12

• Delayed conduction in the branches of the bundle of His (right or left bundle)
• P waves and PR interval normal
• Wide QRS complex (>3 small squares) - since ventricular depolarisation takes longer.

Question 13

Where can abnormal impulses form from?

Answer 13

• These are also called supraventricular rhythms as they arise above the ventricles.
  
  • Sinus node
  • Atrium
  • AV node
• Ventricular rhythms.
  
  • Ventricle

Question 14

What happens in a supraventricular rhythm?

Answer 14

SVT Normal (narrow) QRS complex.

Question 15

What happens in ventricular rhythm?

Answer 15

Wide and bizarre QRS complexes.

Question 16

What is atrial fibrillation?

Answer 16

• This is a supraventricular rhythm.
• Rhythm arises from multiple atrial foci
• Rapid, chaotic impulses
• No P waves, just a wavy baseline (as chaotic depolarisation)
• Impulses reach AV node at a rapid, irregular rate.
• Not all are conducted (because of AV node refractory period)
• When conducted, ventricles will depolarise normal.
• Therefore narrow QRS complexes with irregular R-R intervals

Question 17

What are the haemodynamic effects of AF?

Answer 17

• Atrial contraction lost (they just quiver)
• Ventricles contract normally (but irregular rate)
• Heart rate and pulse is irregular

Question 18

What are ventricular ectopic beats?

Answer 18

• Ectopic focus in ventricle muscle
• Impulses does not spread via the fast His-purkinje system.
• Therefore much slower depolarisation of ventricular muscle
• S0, wide QRS complex, different in shape to usual QRS

Question 19

What is ventricular tachycardia?

Answer 19

A run of 3 or more consecutive ventricular ectopics

VT is a broad complex tachycardia

Persistent VT is a dangerous rhythm - it needs urgent treatment.

High risk of VF

Question 20

What is ventricular fibrillation?

Answer 20

• Abnormal chaotic fast ventricular depolarisation
• Impulses from numerous ectopic sites in ventricular muscle
• No co-ordinated contraction
• Ventricles quiver
• No cardiac output
• Cardiac arrest

Question 21

What are the ECG changes in Ischaemia and MI?

Answer 21

• Coronary artery occlusion -ischaemia or infarction (necrosis) in area supplied by artery
• Changes are seen in the leads facing affected area of ventricles
• Need to look at P-QRST in all 12 leads.
• Need to know which groups of leads look at different parts of the heart

Question 22

MI and Ischaemia?

Answer 22

Reduced perfusion due to narrowed coronary artery/ies

Partial narrowing of the lumen causes:

• Sub endocardial ischaemia / injury - This area is furthest away from coronary arteries which lie on the surface of heart - hence most vulnerable region, involved first

Complete occlusion of lumen causes:

• Full thickness (trans mural) injury
• Including sub-epicardial region

Question 23

What is a STEMI?

Answer 23

This is due to complete occlusion of coronary artery by a thrombus

‘Full thickness’ of myocardium involved

Subepicardal injury causes ST segment elevation in leads facing the affected area.

ST elevation - Behave as if abnormal current coming towards injured epicardium during repolarisation.

ST elevation is the earliest sign of a STEMI

Diagnosis of STEMI - indication for urgent re-perfusion to prevent / minimise muscle necrosis.

Question 24

What are the evolving changes in an ST segment elevation Mi (STEMI)?

Answer 24

Acute: ST elevation

Hours: ST elevation , Decrease R wave, Q wave begins

Day 1-2: T wave inversion, Q wave deeper

Days later: ST normalizes, T wave inverted

Weeks later: ST and T normal, Q wave persists

Question 25

Why does necrosis cause Q waves?

Answer 25

In MI, dead muscle tissue (necrosis) produces no action potential and ECG “looks through” the infarcted area to pick up electrical forces from opposite side of heart which are directly opposite.

Question 26

What makes a Q wave pathological?

Answer 26

Over 1 small square wide More than 2 small squares deep Depth more than 1/4 of the height of the subsequent R wave.

Question 27

Non STEMI and Ischaemia?

Answer 27

ECG changes are due to the subendocardial injury: ST segment depression and T wave inversion.

Question 28

What are the changes present in a resting ECG of severe ischaemia?

Answer 28

Same ECG changes seen in both unstable angina and NSTEMI. Acute: ST depressions and T inversion Weeks later: ST and T waves are normal but there are no Q waves. UA and NSTEMI are differentiated by a blood test for evidence of myocyte necrosis (e.g. cardiac troponin)

Question 29

What happens in hyperkalcaemia?

Answer 29

Resting membrane potential is less negative This inactivates some Voltage Gates Na+ Channels Heart becomes less excitable Conduction problems occur

Question 30

What is seen on an ECG during Hyperkalaemia?

Answer 30

1. Tall, peaked T wave - earliest sign 2. Tall, peaked T wave and flattened p wave; prolonged PR interval. 3. Tall peaked T wave, absent p waves - atrial standstill and widened QRS 4. Widened QRS, ST segment merges with T wave to give a sine wave pattern.

Question 31

What is the U wave on an ECG?

Answer 31

The U wave is a waveform that preceeds the T wave. It is thought to be present as a result of repolarisation of the purkinje fibres. However, it is not always present due to its small size.

Question 32

What ECG changes occur in hypokalaemia?

Answer 32

1. Low T wave 2. Low T wave and high U wave 3. Low T wave, high U wave and low ST segement.