Normal Conduction

Normal ECG

Electrical Activation of the Heart

Conduction Times Across the Heart

40 ms

80 ms

Conduction with LBBB

RBBB

S

R’

Rapid activation

Slow muscle to muscle activation producing a wider deflection

Activation in LBBB

r s

LBBB Criteria

• QRS duration ≥120 ms

• Q wave absent in lateral leads (exception - aVL)

• R peak time prolonged by >60 ms in V5, V6

• R peak time normal in V1

LBBB

LBBB

Why is the QRS Complex Wide in LBBB?

• Is it because of interventricular septum?

• Or, because of delay of impulse within the left ventricle?

QRS axis in the Frontal Plane

• Usually no QRS shift

• Superior axis indicates pre-existing LAFB, advanced heart disease and higher mortality

• Right axis is less common (RVH or MI)

LBBB and Normal Axis

LBBB and L Axis

LBBB With Leftward Axis

LBBB with Right Axis Deviation

LBBB and Diagnosis of MI

©2016 MFMER | 3580791-17

Results of Univariate Analysis of Electrocardiographic Criteria

Sgarbossa et al: NEJM 334(8): 481, 1996

Criterion Sensitivity (95% CI)

Specificity (95% CI)

Positive Likelihood ratio

(95% CI)

Negative Likelihood ratio

(95% CI) ST-segment elevation ≥1 mm and concordant with QRS complex

73 (64–80) 92 (86–96) 9.54 (3.1–17.3) 0.30 (0.22–0.39)

ST-segment depression ≥1 mm in lead V1, V2, or V3

25 (18–34) 96 (91–99) 6.58 (2.6–16.1) 0.78 (0.7–0.87)

ST-segment elevation ≥5 mm and discordant with QRS complex

31 (23–39) 92 (85–96) 3.63 (2.0–6.8) 0.75 (0.67–0.86)

Positive T wave in lead V5 or V6

26 (19–34) 92 (86–96) 3.42 (0.18–65) 0.80 (0.72–0.9)

Left-axis deviation 72 (63–79) 48 (39–57) 1.38 (1.13–9.8) 0.59 (0.25–1.39)

LBBB With Upright T

LBBB with Upright T (L Axis)

LVH and LBBB

• S in V2 + R in V6 >4.5 mV

• LAE

• QRS >160 ms

Criteria for LAFB

• Frontal plane QRS axis -45º to 90º

• QRS ≤120 ms (or 110 ms)

• rS patterns in leads II, III and aVf

Activation in LAFB

LAFB and aVR

• QRS complex in aVL and aVR, each end in an R wave

• Peak of the terminal R occurs later in aVR than in aVL (against aVR earlier in inferior MI)

AJC 51:723, 1983

LAFB - aVR Delay

Inferior Infarct

Inferior MI and LAFB

LAFB and Pseudo infarct

• Commonest cause for pseudoinfarction secondary to poor R wave progression

• A less common is the erroneous diagnosis of septal infarction secondary to small q waves in the right precordial leads

• Record one space below

LAFB and PRWP

LAFB

LAFB

LAFB, Delay in aVR and PRWP

LAFB and LVH

• R in aVL may cause false positive LVH

• Highly sensitive and specific criteria is the sum of S in LII and of the largest QRS complex in precordial leads is ≥3.0 mV

Criteria for LPFB

• Frontal Plane QRS axis ≥120°

• RS in leads I and aVL with qR patterns in inferior leads

• QRS duration <120 ms

• Exclusion of other factors causing right axis deviation (eg, RV overload, lateral infarction)

LPFB

LPFB

RBBB

r

S

R

Rapid activation

Slow muscle to muscle activation producing a wider deflection

Activation in RBBB

RBBB

RBBB and LAFB

RBBB and LAFB

RBBB and LAFB

RBBB and LPFB

RBBB, LAFB and RVH

When R’ is taller than 15mm

Incomplete RBBB (<120 ms)

• Delayed activation of RVOT

• R1 voltage may correlate with severity of PS

• R1 voltage may disappear after corrective surgery

QRS 116 msec

Intermittent LBBB

Anterior MI/RBBB/LAFB

LAFB