ECG: Conduction

Question 1

Sinus pause and arrest

Answer 1

More common at older age
•	Node becomes calcified and/or fibrotic
•	Ischemia also possible cause
SAN beat pauses or stops/arrests (or never leaves node = exit block)
-Rhythm usually taken over by AV node
•	Latent pacemaker becomes true
•	“Junctional escape”
•	No P waves before QRS
•	Slow (40-60 bpm)
•	Regular
•	Narrow QRS
•	Retrograde P (after QRS) = variable 
•	Next level = “ventricular escape”
o	Symptoms relate to rate of escape beats 
ECG:
•	No P waves (may be transient) 
•	No other atrial activity
•	Easiest to see just as P waves cease

Question 2

1st Degree AV Block

Answer 2

EGC:
• Prolonged PR interval (>200 ms)
Pathology
• Degenerative fibrosis – chronic/progressive
• Acute inflammation (Lyme disease)
• Drug related (digoxin, beta-blockers, Ca2+ blockers)
o Common in athletes due to increased baseline vagal tone
o Usually asymptomatic and benign

Question 3

2nd Degree AV Block Type I

Answer 3

ECG:
•	Progressive lengthening of PR interval
•	Failure of AV conduction
•	Usually repeats in patterns: 3:2, 4:3
o	Clinically: usually benign
o	Block above the His bundle 
Causes:
•	Fibrosis or degeneration of AV junction
•	Drug effects
•	Inferior MI
•	Increased vagal tone (athletes)

Question 4

2nd Degree AV Block Type II

Answer 4

ECG:
•	P wave fails to conduct without a preceding change in PR interval
•	May be random or in regular ratios  
o	Block usually distal to His bundle
Causes: 
•	Acute infarction 
•	Degenerative or infiltrative disease
•	Drugs
•	Electrolyte and metabolic disorders 
Clinically: more ominous; may progress to 3rd degree block
•	Recommend pacemaker

Question 5

3rd Degree AV Block

Answer 5

o	Independent/dissociated atrial and ventricular rhythms
ECG:
•	Regular R-R intervals
•	Regular P-P intervals and not associated with R-R
Causes:
•	Congenital AV block
•	Degenerative fibrosis
•	Myocardial ischemia or infarction 
•	Inflammation, infiltration 
•	Drugs
•	Electrolyte imbalances 
Clinically: require temporary or permanent pacing

Question 6

Left Bundle Branch Block

Answer 6

ECG:
•	QRS >120 ms
•	QS waves in leads V1-V3
•	Notched or broad R waves in leads I, aVL, V5-V6
•	No Q waves
•	ST-T downsloping in leads I, aVL, V6
Caused:
•	Injury or degeneration from infarction, fibrosis, infiltration, LV hypertrophy
•	New onset may occur with MI

Question 7

Right Bundle Branch Block

Answer 7

ECG:
•	QRS >120 ms
•	rSr’ or rSR’ with inverted T in leads V1-V2
•	Broad S in leads I, aVL, V5-V6
Caused:
•	Normal variant
•	RVH
•	ASD (95%)
•	May occur acutely in ateroseptal MI
o	Rate dependent (due to slower recovery of right bundle)

Question 8

Define “high grade” AV block.

Answer 8

• Less precise term to describe AV blocks
• Implies block is more severe
• Often has consecutive beats that aren’t conducted
• Maintains PR interval when conducts
• “Worse” than 2nd, but “better” than 3rd degree block

Question 9

2:1 2nd Degree AV block

Answer 9

M3 trick question
• Special case pattern produced by both Type I and Type II mechanisms
• 2 P waves for every QRS
• Trick because can’t know if PR interval would be longer if there were 2 beats in a row (type I) or not (type II)