Orthodromic Reciprocating Tachycardia
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Localization of ORT using the P wave
- Left Free Wall has a negative p wave in lead I. One of three P waves in inferior leads are isoelectric or biphasic.
- Right free wall has a positive or isoelectric P wave in V1. If positive in inferior leads then anterior. If negative then posterior. The presence of a positive retrograde p wave in lead I suggests a right free wall AP.
- Septal AP Posteroseptal pathways have negative in inferior leads, positive in leads aVR and aVL. Isoelectric or biphasic in Lead 1.
- For either right of left sided AP the presence of negative P waves in all three inferior leads indicates an inferior location, whereas positive P waves indicates a superior location.
- Isoelectric or biphasic p waves in the inferior leads suggest a middle free wall location
Criteria for ORT
- Increase in VA time with Bundle Branch Block is diagnostic
- Presence of Ventricular preexcitation is strongly predictive
- Extranodal reponse with Parahisian Pacing is strongly predictive
- Development of LBBB is strongly predictive
- Absence of VA conduction at baseline is near exclusion
Criteria for Left Lateral Pathway
- Earliest VA on lateral wall
- Shortest VA time (QRS-to-atrium) > 60 ms
- Constant VA times despite cycle length variations
- Advance atrial activation during his refractoriness
- Pre-Excitation Index > 70 ms (TCL - coupling interval of PVC)
- Ipsilateral Bundle Branch Block prolongs his (or V) to A time by > 35ms
- Terminate tachycardia with PVC during his refractoriness without a
Differential of ORT using Free Wall from Atrial Tachycardia
The use of Ventricular Overdrive pacing will result in a VAV response in ORT or AVNRT and a VAAV response in Atrial Tachycardia
Differential of Atypical AVNRT from Free Wall Pathway
Approximately 6% of cases of AVNRT are associated with eccentric atrial activation, with the shortest VA > 60 msec. The keys to diagnosis of an atypical AVNRT are:
- Demonstration of Dual AV Nodal Physiology
- The Ability to induce typical AVNRT with concentric or variable patterns of retrograde atrial activation
- Absence of VA conductionwithout Isuprel
- Inability to advance the atrium without a PVC on His
- Demonstration of only decremental VA conduction
- The ability to dissociate the atrium from the ventricle during Tachycardia
