ECG: Heart block

Question 1

What is the ecg morphology in 1st degree heart block?

Answer 1

• Prolonged delay in conduction at the AV node or bundle of His
• PR interval longer than 0.2s
• Every QRS precede by P wave

Question 2

What is the ecg morphology in mobitz type 1 2nd degree heart block?

Answer 2

• Progressive lengthening of PR interval from beat to beat
• Until QRS complex is dropped
• PR interval following non conducted P wave is always shorter than PR interval of beat just before non conducted P wave
• Group beating (clustering of QRS complexes separated by pause)

Question 3

What is the ecg morphology in mobitz type II 2nd degree heart block?

Answer 3

Block within Bundle of His

Occasional dropped beats without progressive PR interval lengthening

More serious, signifying serious heart disease; capable of progressing to 3rd degree heart block 🡪 hence is an indication for pacemaker

Question 4

What is the ecg morphology in mobitz type II heart block?

Answer 4

Complete heart block – no atrial impulses transmit to ventricles

AV dissociation: P and QRS complexes independent

Question 5

How do you tell the difference between a congenital heart block from a Junctional Rhythm?

Answer 5

CHB

• P waves are SINUS in origin and hence have NORMAL morphology despite dissociation
• With the exception of 2:1 AV block, CHB will result in DESYNCHRONY

Junctional Tachy / Brady / Rhythm

• P waves are due to retrograde impulse from AV node 🡪 hence will either be MISSING or RETROGRADE
• P waves (if present) are SYNCHRONOUS to the QRS complexes

Confounding factor – if there is AF + Junctional Rhythm ☹

Question 6

What is the morphology of RBBB?

Answer 6

Broad QRS (> 120 ms)

V1, V2, V3 – RSR’ : R’ wave results from depolarization of right ventricle which comes after the left ventricle is depolarized (M shaped)

I, aVL, V5, V6: late deep S waves (for the same reason as above)

Broad QRS > 120ms

Question 7

What is the morphology of LBBB?

Answer 7

Broad QRS (> 120 ms)

I, aVL, V5, V6 – broad monophasic R waves; prolonged R peak time (>0.06s)

• Also absence of Q waves in I, V5, V6
• • appropriate discordance: ST depression, T inversion in lateral leads

V1: Broad negative deflection (‘W’ shaped)

V6: Broad positive deflection with a slurred upstroke in lead V6 (‘M’ shaped)

Deep S waves inferiorly

Inverted T waves laterally (I, aVL, V5, V6)

Classic teaching - chest pain + new LBBB = AMI

Paced rhythm – may have LBBB appearance 🡪 NORMAL

Question 8

What is the morphology of third degree AV block?

Answer 8

• No stimuli transmitted from atrial to ventricles
• Atria and ventricles paced independently
• Ventricles paced by nodal or infra-nodal escape pacemaker located beyond point of block

Question 9

What is the morphology of AF with complete heart block?

Answer 9

• Fibrillatory baseline, absence of P waves
• Slow rate of 50
• R-R intervals regular
• QRS narrow suggestive of supra-Hisian escape rhythm