Lead aVL usually looks similar to left-sided leads I and V6 (ie, with a monophasic = all upright R wave).For example, excessively rapid anterograde conduction predisposes to development of AFib rates that may attain ≥220-250/minute ( CLICK HERE for Review of WPW-associated arrhythmias). It is the presence of one ( and sometimes of more than one) AP - and, the ability of the AP to conduct rapidly either anterograde ( forward ) to the ventricles or retrograde ( backward ) to the atria or both - that predisposes to development of certain tachyarrhythmias.If the AP in such a patient never conducts anterograde (ie, in the forward direction ) - then this is said to be an “occult” AP. As a result, the ECG in sinus rhythm will look normal - BUT - because there is an AP present capable of conducting retrograde - a ready-made “reentry pathway” exists - and the patient may be predisposed to developing the reentry SVT rhythm known as AVRT ( Atrio Ventricular Reentrant Tachycardia ). Some patients may only be able to conduct over the AP in retrograde fashion.The relative amount of preexcitation may vary from one occasion to another - such that a patient’s ECG may look entirely normal on one day - and may manifest classic WPW on another.This explains why sometimes it may be extremely difficult to recognize WPW in a patient who is manifesting only partial preexcitation. Partially over the AP, and partiallyover the normal AV nodal pathway - in which case, depending on the relative amount of preexcitation - the PR interval may be only slightly ( or more than that) shortened - and the QRS complex only slightly ( or more than that) widened.Entirely over the normal AV nodal pathway - in which case the PR interval is normal, and the QRS complex is not widened.Entirely over the AP - in which case all 3 features ( short PR interval delta waves QRS widening ) will be seen on the ECG.Not all patients with WPW manifesting on the ECG have clinical WPW. When it does not, it has sometimes been called "concealed conduction". He had had episodes of tachycardia in the past, so he has clinical WPW. I looked into the chart to find more information: it turns out that this patient had a known diagnosis of WPW and had never had it ablated. All of the repolarization abnormalities are due to the depolarization abnormality of WPW. It looks like LBBB because the accessory pathway is on the right side of the heart, so the right heart is pre-excited and then depolarizes the left heart from right to left, just like in LBBB. The upstroke of the QRS is delayed (delta wave). The Veritas computer algorithm called it Left Bundle Branch Block without ischemia. Patients with RBBB and IVCD derive less reverse cardiac remodeling and symptomatic benefit from CRT compared with those with a native LBBB.I was reading through a stack of ECGs and saw this one: There is no difference in 3-year survival in patients undergoing CRT based on baseline native QRS morphology. Patients with LBBB had greater improvements in most echocardiographic endpoints and NYHA functional class than those with IVCD and RBBB. In multivariate analysis, no mortality difference amongst the three groups was noted. There were 32 deaths in the LBBB group, 10 in the RBBB, and 27 in the IVCD group over a mean follow up of 3.4 +/- 1.2 years. Three hundred and thirty-five patients met inclusion criteria of which 204 had LBBB, 38 RBBB, and 93 IVCD.
Secondary endpoints were changes in EF, left ventricular end-diastolic and systolic diameter, mitral regurgitation, and New York Heart Association (NYHA) functional class. The primary endpoint was long-term survival. Patients with a narrow QRS or paced ventricular rhythm were excluded. Patients were placed into one of three groups based on the preimplantation electrocardiogram morphology: LBBB, RBBB, or IVCD. We assessed 542 consecutive patients presenting for the new implantation of a CRT device. Little is known about the response to CRT in patients with right bundle branch block (RBBB) or non-specific intraventricular conduction delay (IVCD) compared with traditionally studied patients with left bundle branch block (LBBB). In select patients with systolic heart failure, cardiac resynchronization therapy (CRT) has been shown to improve quality of life, exercise capacity, ejection fraction (EF), and survival.
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