13. Interpreting Abnormal ECGs

Question 1

What is sinus rhythm?

Answer 1

If every QRS complex is preceded by a P wave

Question 2

What are atrioventricular conduction blocks?

Answer 2

Delay/failure of conduction of impulses from atria to ventricles via AVN and Bundle of His

Question 3

What are the 3 types of atrioventricular conduction blocks?

Answer 3

First degree heart block
Second degree heart block: Mobitz type 1 and 2
Third degree heart block

Question 4

What are the causes of heart block?

Answer 4

Degeneration of electrical conducting system with age - sclerosis and fibrosis
Acute myocardial ischaemia
Medications
Valvular heart disease

Question 5

What happens in first degree AV block?

Answer 5

Conduction is slowed without skipped beats

All normal P waves followed by QRS complexes but PR interval is longer than normal (>0.2 seconds)

Question 6

What happens in Mobitz type 1 second degree AV block?

Answer 6

Successively longs PR intervals until one QRS is dropped then cycle starts again
Electrical signal is not conducted through to ventricles

Question 7

What happens in Mobitz type 2 second degree AV block?

Answer 7

PR intervals do not lengthen - sudden dropped QRS complex without prior PR changes
Atrial rhythm (p waves) is regular
Ventricular rhythm is irregular
High risk progression to complete heart block

Question 8

What happens in third degree AV block?

Answer 8

Atria and ventricles depolarising independently - complete failure of AV conduction
Ventricular pacemaker takes over (20-40bpm)
Typically too slow to maintain blood pressure
Usually wide QRS complex
Urgent pacemaker required

Question 9

What is a bundle branch block?

Answer 9

Delayed conduction within the bundle branches (right or left)
P wave and PR intervals normal
Wide QRS complex because ventricular depolarisation takes longer

Question 10

Where can abnormal rhythms arise from?

Answer 10

Aria (superventricular arrhythmia) - sinus node, atrium or AV node
Ventricles (ventricular arrhythmias)

Question 11

What is the difference in ECGs between supraventricular and ventricular arrhythmias?

Answer 11

Supra - normal or narrow QRS complex

Ventricular - wide and bizarre QRS complexes

Question 12

Describe atrial fibrillation

Answer 12

Arises from multiple atrial foci
Rapid chaotic impulses
No P waves, just wavy baseline, irregular R-R intervals
Impulses reach AVN at rapid irregular rate
Not all conducted by when conducted ventricles depolarised normally so normal QRS

Question 13

What are the haemodynamic effects of atrial fibrillation?

Answer 13

Loss of atrial contraction leads to increased blood stasis
Stasis most evident in left atrium
Flow velocity markedly reduced concomitantly with impaired contractility of left atrial appendage
Leads to small clots in LA, risk for ischaemic stroke

Question 14

What is premature ventricular ectopic beats (PVCs)?

Answer 14

Ectopic focus in ventricle muscles
Impulse does not spread via fast His-purkinje system, so much slower depolarisation of ventricles do wider QRS
Premature because occurs earlier than would be expected for next sinus impulse
Can lead to ventricular tachycardia

Question 15

What is ventricular tachycardia?

Answer 15

Run of more than 3 consecutive PVCs
Persistent VTACH is dangerous rhythm requiring treatment
High risk progression to ventricular fibrillation

Question 16

What is ventricular fibrillation?

Answer 16

Abnormal, chaotic, fast ventricular depolarisation
Impulses from numerous ectopic sites in ventricle
No coordinated contraction
Ventricles quiver
No cardiac output, can lead to cardiac arrest

Question 17

What is ST segment elevation myocardial infarction (STEMI)?

Answer 17

Due to complete occlusion of coronary artery
Full thickness of myocardium involved
Sub-epicardial injury causing ST segment elevation in leads facing affected area is earliest sign

Question 18

What are the classic ECG findings with pulmonary embolism?

Answer 18

S wave in lead 1
Q wave in lead 3
Inverted T wave in lead 3

Question 19

What are the ECG changes in non-STEMI and ischaemia?

Answer 19

ST segment depression and T wave inversion

Question 20

What does hypokalaemia cause?

Answer 20

Decreased extracellular potassium causes myocardial hyperexcitability
Generalised muscle weakness
Respiratory depression
Ascending paralysis
Ileus, constipation
Palpitations, arrhythmia, cardiac arrest

Question 21

What does an ECG show in hypokalaemia?

Answer 21

Peaked P waves
T wave flattening and inversion
U waves

Question 22

What does hyperkalaemia cause?

Answer 22

Heart becomes less excitable
Conduction problems occur
Generalised muscle weakness
Respiratory depression
Ascending paralysis
Palpitations, arrhythmia, cardiac arrest

Question 23

What does an ECG show in hyperkalaemia?

Answer 23

Tall tented T waves
Loss of P wave
Widening QRS
QRS continues to widen, approaching sine wave