S7) ECG Abnormalities

Question 1

What are the causes for abnormal rhythms?

Answer 1

• Abnormal impulse formation
• Abnormal conduction

Question 2

What are the two types of abnormal rhythms?

Answer 2

• Supraventricular rhythms (SAN, Atrium, AVN)
• Ventricular rhythms

Question 3

Describe 3 features of supraventricular rhythms

Answer 3

• Conducts impulse into and within ventricles by His-Purkinje system
• Normal ventricular depolarisation
• Normal QRS complexes (narrow)

Question 4

Describe 4 features of ventricular rhythms

Answer 4

• Impulses arise from a focus/foci in ventricle
• Conduction not via usual His-Purkinje system
• Depolarisation takes longer
• Wide/bizarre QRS complexes

Question 5

What is the best way to interpret rhythm from an ECG?

Answer 5

• Look at the ‘rhythm strip’ at the bottom of 12 lead ECG
• Some machines record Lead II, V1 and V5 rhythm strips

Question 6

What is atrial fibrillation?

Answer 6

• Atrial fibrillation is a condition where impulse arise from multiple atrial foci, leading to chaotic atrial depolarisation wherein atria quiver rather than contract
• It carries risk of thrombosis

Question 7

In 4 steps, explain the electrical activity in atrial fibrillation

Answer 7

⇒ Chaotic impulses from multiple atrial foci

⇒ Impulses arrive at AVN at rapid irregular rate

⇒ Only some conducted to ventricles (at regular intervals)

⇒ Ventricles depolarise and contract normally

Question 8

What are two characteristic features of atrial fibrillation?

Answer 8

• No p waves (wavy baseline)
• Pulse and heart rate irregularly irregular

Question 9

What are AV conduction blocks?

Answer 9

A heart block is a delay/ failure of conduction impulses from the atrium to the ventricles via the AVN and bundle of His

Question 10

What are the causes of AV conduction blocks?

Answer 10

• Acute myocardial infarction (commonest)
• Degenerative changes

Question 11

What are the three different types of AV conduction blocks?

Answer 11

• First degree heart block
• Second degree heart block
• Complete Heart Block

Question 12

What are the characteristic features of first degree heart block (1^o HB)?

Answer 12

• P wave normal
• QRS normal
• PR interval prolong > 5 small squares (slow conduction in AV and Bundle of His)

Question 13

What are the characteristic features of Mobitz Type I (2^o HB)?

Answer 13

• Progressive lengthening of PR interval
• Until one P is not conducted (this allows time for AVN to recover),
• Cycle begins again

Question 14

What are the characteristic features of Mobitz Type II (2^o HB)?

Answer 14

• PR interval normal
• Sudden non-conduction of a beat
• Dropped QRS
• High risk of progression to complete heart block

Question 15

What causes complete heart block?

Answer 15

• Normal atrial depolarisation but impulses not conducted to ventricle
• Ventricular pacemaker takes over (ventricular escape rhythm)

Question 16

What are the characteristic features of third degree heart block (CHB)

Answer 16

• Rate is very slow (30-40 bpm)
• Wide QRS complexes
• HR often too slow to maintain BP and perfusion

Question 17

Identify three causes of ventricular rhythms

Answer 17

• Ventricular premature beats
• Ventricular tachycardia
• Ventricular fibrillation

Question 18

What are ventricular ectopic beats?

Answer 18

• Ventricular ectopic beats are when an ectopic focus in the ventricle muscle prevents the spread of impulse via the fast His -purkinje system
• Hence, much slower depolarisation of ventricle (wide & bizzare QRS complex)

Question 19

What is ventricular tachycardia?

Answer 19

• Ventricular tachycardia involves a run of ≥ 3 consecutive ventricular ectopics producing broad QRS complexes
• Persistent VT is a dangerous rhythm, requires urgent treatment and has a high risk of ventricular fibrillation

Question 20

What is ventricular fibrillation?

Answer 20

• Ventricular fibrillation is the abnormal, chaotic and fast ventricular depolarisation due to impulses from multiple ventricular ectopic foci
• The ventricles quiver as there is no co-ordinated contraction, no cardiac output and is a state of cardiac arrest

Question 21

Compare and contrast ventricular and atrial fibrillation

Answer 21

Question 22

Why are there ECG changes in ischaemia and myocardial infarction?

Answer 22

• Due to reduced perfusion of myocardium
• Changes seen in leads facing affected area

Question 23

Explain how reduced myocardial perfusion due to coronary atherosclerosis impacts the heart

Answer 23

• Major coronary arteries lie on epicardial surface hence sub-endocardial muscle is furthest away & most vulnerable
• Flow is during diastole, so if diastole is short (rapid HR) there is less time for blood flow e.g. exercise

Question 24

The sub endocardial region is the most vulnerable.

What are the ischaemic changes observed in an ECG?

Answer 24

Leads facing affected area show:

• ST segment depression
• T wave inversion

Question 25

What is STEMI?

Answer 25

• ST segment elevation myocardial infarction is a condition occurring due to complete occlusion of lumen by thrombus
• Muscle injury extends ‘full thickness’ from endocardium to epicardium and ST segment elevation is observed in leads facing area

Question 26

Describe the evolving changes in a STEMI (6 stages)

Answer 26

Question 27

How do we identify pathological Q waves?

Answer 27

• > 1 small square (width)
• > 2 small squares (length)

Question 28

Describe the ECG changes observed in hyperkalaemia

Answer 28

• In hyperkalaemia, the RMP less negative
• Heart becomes less excitable as hyperkalaemia worsens

Question 29

Describe the ECG changes observed in hypokalaemia

Answer 29

In hypokalaemia, the RMP more negative: