Intraventricular Conduction: Difference between revisions

Intraventricular Conduction (view source)

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 :QRS widening due to delay or block in bundle branch or intramyocardial conduction<cite>wellens</cite>
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-Aberrancy can result from:
+Mechanisms of aberration:
-#A sudden increase in frequency of the heart rate that the bundles cannot conduct (phase 3 aberration)
+#Phase 3 aberration
-#Retrograde concealed conduction
+#Acceleration dependant
-#A slow heart rate (phase 4 aberration)
+#Deceleration dependant
+#Retrograde invasion
 Right bundle branch block is more common, because the right bundle has the longer refractory period. Left bundle branch block accounts for about 1/3rd of cases.
 ===Phase 3 Aberration===
-Phase 3 aberration occurs when conduction fibers receive a new impulse, before they have fully repolarized. This can sometimes be observed at the start of paroxysmal supraventricular tachycardias or in a long-short sequence in which the refractory period of the long sequence is prolonged.
+Phase 3 aberration occurs when conduction fibers receive a new impulse, before they have fully repolarized. A premature impulse is encroaching on the refractory period of the bundle branch. This is a physiological phenomenon. This can sometimes be observed at the start of paroxysmal supraventricular tachycardias or in a long-short sequence in which the refractory period of the long sequence is prolonged.
-===Retrograde Concealed Conduction===
+===Acceleration dependant===
-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 (for example) refractory for the next complex. 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 complex causes a compensatory pause and 'resets' the system.
+A small increase in rhythm resulting in aberrancy due to an abnormal response of tissue that has diminished excitability.
-===Phase 4 Aberration===
+===Deceleration dependant: Phase 4 Aberration===
-Phase 4 aberration only occurs after prolonged pause. During such a pause (e.g. in second degree AV block) the Purkinje fibers can 'hyper'-depolarize spontaneously. As their membrane potential becomes more and more negative, the conduction velocity decreases and can even be blocked altogether. This also requires an upward shift of the threshold membrane potential and a change in membrane responsiveness, so it is rarely seen in normal hearts.
+Phase 4 aberration only occurs after prolonged pause. During such a pause (e.g. in second degree AV block) the Purkinje fibers can depolarize spontaneously. As their membrane potential becomes more positive, the conduction velocity decreases and can even be blocked altogether. This is usually a pathological response, but can be normal at very low heart rates (e.g. 40 bpm)
+===Retrograde Concealed Conduction===
+This is the most common mechanism for sustained aberrancy during tachycardia. The sequence of QRS widening that is often observed is phase 3 aberration in the first premature beat. This can leave the left bundle (for example) refractory for the next complex. 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 complex causes a compensatory pause and 'resets' the system.
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 ==References==