The normal QRS duration in adults ranges from 67-114 msec (Glasgow cohort). If the QRS duration is > 120 msec (three small boxes or more on the ECG paper), there is usually an abnormality of conduction of the ventricular impulse. The most common causes are either RBBB or LBBB, shown below. However, other conditions may also prolong the QRS duration.

RBBB is defined by delayed, terminal QRS forces that are directed to the right and anteriorly, producing broad terminal positive waves in leads V1 and aVR and a broad, terminal negative wave in lead I. LBBB is defined by delayed terminal QRS forces that are directed to the left and posteriorly, producing wide R waves in leads that face the left ventricular freewall and wide S waves in the right precordial leads.

RBBB:

Lead I must have a terminal broad S wave, but the R/S ratio may be < 1 in cases of right axis deviation.

In lead V1, the typical QRS is usually triphasic, but sometimes notched and monophasic. The latter must have notching on the ascending limb of the R wave, usually at the lower left. The right-hand peak must be the taller of the positive components of the QRS complex.

LBBB:

Lead I must have a monophasic; usually notched R wave and cannot have Q or S waves.

Leads V1-2 must have a dominant S wave, usually with a small, narrow R wave. The S descent must be rapid and smooth, without notching.

The diagnosis of incomplete bundle branch blocks:
Incomplete LBBB:The waveforms are similar to those in LBBB, but the QRS duration is < 120 msec. Septal Q waves are absent in 1 and V6. Incomplete LBBB is synonymous with LVH, and commonly mimics a delta wave in leads V5 and V6.
Incomplete RBBB: The waveforms are similar to those in RBBB, but the QRS duration is < 120 msec. This diagnosis suggests RVH. Occasionally, there is an rSr' waveform in lead V1 because of a normal variant pattern. In this case, the r' is usually smaller than the initial r wave, and is not indicative of incomplete RBBB.

Intraventricular conduction delay or defect (IVCD):
If the QRS duration is > 120 msec, but typical waveforms of either RBBB or LBBB are not present, there is an intraventricular conduction delay or defect (IVCD). This pattern is common in dilated cardiomyopathy. The wider the QRS becomes, the poorer is the ventricular function. QRS durations of > 170 msec with an IVCD are highly predictive of dilated cardiomyopathy.

Masking of the S wave in RBBB by concomitant LAFB:
Occasionally, LAFB may mask the terminal S wave in lead I in RBBB, producing an apparent IVCD pattern. In this case, the RBBB is diagnosed by the presence of terminal broad positive waves in both leads V1 & aVR.

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