When the "terminal force" of the QRS in V1 is below the baseline (i.e. QS wave), a LBBB is the most likely diagnosis.
When the "terminal force" of the QRS in V1 is above the baseline (i.e. RSR' wave), it's a RBBB.
If the QRS > 0.12 sec. but the morphological criteria of LBBB or RBBB do not apply, it is called '
interventriculair conduction delay', a general term.
[[Image:LAHB.png|thumb|Left anterior hemiblock]]
In ''left anterior fascicular block'' the anterior part (fascicle) of the left bundle is slow. This results in delayed
depolarisation of the upper anterior part of the left ventricle. On the ECG this results in left axis deviation. The QRS width is <0 ,12 seconds in isolated LAFB.
;Criteria for posterior fascicular block:
right [[heart axis|axis devation]] >+120°; : deep S in I; : small q in III; : no or very few QRS widening;
:Right ventricular [[hypertrophy]] and previous [[Ischemia#Lateral|lateral myocardial infarction]] have been excluded
===Retrograde Concealed Conduction===
Phase 3 aberration is often the cause of the first wide QRS complex. However at a regular rate retrograde concealed conduction is often the sustaining mechanism. The sequence of QRS widening that is often observed is phase 3 aberration in the first premature beat. This can leave the left bundle (
as an example) refractory for the next beat. This next beat is conducted by the right bundle and once it reaches the apex, it is conducted retrograde by the left bundle. This can continue until a new premature ventricular beat causes a compensatory pause and 'resets' the system.
===Phase 4 Aberration===
Phase 4 aberration only occurs after prolonged pause. During such a pause (e.g. in second degree AV block) the fibers
of the Purkinje system can 'hyper'-depolarize spontaneously. As their membrane potential becomes more and more negative the conduction velocity reduces and they can even block altogether. This also requires an upwards shift of the threshold membrane potential and a change in membrane responsiveness, so it is rarely seen in normal hearts.