ECG Conduction Defects

Sinoatrial Block

First Degree Sinoatrial (SA) Block

Second Degree SA block, Type I (Wenckebach)

Second Degree SA block, Type II

Third Degree SA Block

Bundle Branch Blocks

Left Bundle Branch Block (LBBB)

Right Bundle Branch Block (RBBB)

Incomplete Right Bundle Branch Block (IRBBB)

Fascicular Blocks

Left Anterior Fascicular Block (LAFB)

– Left axis deviation (-45 to -90 degrees)
– qR complexes in lateral leads I, aVL
– rS complexes in inferior leads II, III, aVF
– QRS duratrion prolonged but < 120 ms (0.12 sec)
– Prolonged R wave peak time in aVL > 45ms

Left Posterior Fascicular Block (LPFB)

– Right axis deviation (RAD) (+90 to +180 degrees)
– rS complexes in lateral leads I and aVL
– qR complexes in inferior leads II, III and aVF
– QRS duratrion prolonged but < 120 ms (0.12 sec)
– Prolonged R wave peak time in aVF

Bifascicular block:

– RBBB + LAFB masnifest as left axis deviation
– RBBB + LPFB masnifest as right axis deviation”

AV block

First Degree AV Block:

– PR interval > 0.2 sec (> 200 ms or > 1 large square)

– Constant 1:1 P-wave-to-QRS-complex ratio is maintained

Second degree, Mobitz I AV Block (Wenckebach Phenomenon):

– Progressive PR interval prolongation with each beat until a P wave is dropped
– Irregular R-R interval
– May demonstrate feature of ‘group beating’
– Narrow QRS complexes in most cases
– Usually improves with atropine

Second degree, Fixed ratio AV block:

– Can be either Mobitz I or Mobitz II
– Regular R-R interval between conducted beats
– Can be 2:1, 3:1 or 4:1
– If Mobitz I, telemetry may show runs of PR prolongation intermittently
– Improves with atropine if Mobitz I and worsens if Mobitz II
– Another clue to differentiate is QRS duration, narrow QRS is usually seen in Mobitz I

Second degree, Mobitz II AV Block:

– Intermittent non-conducted P waves
– Fixed PR interval in conducted beats (no prolongation as seen in Mobitz I)
– P-P interval is constant
– R-R interval surrounding the dropped beat is multiple of conducted beats R-R interval
– Either no pattern of conduction block or fixed ratio block such as 2:1, 3:1 etc.
– Broad QRS complex is usually seen
– Can worsen into complete heart block

Third Degree AV Block (Complete Heart Block):

– Complete AV dissociation
– P and QRS complexes occur independent of each other
– Regular P-P interval
– Regular R-R interval
– More P waves than QRS complexes
– Ventricular rhythm maintained by either junctional escape or ventricular escape rhythm
– Can present with ventricular standstill without escape rhythm