Dr.S.Venkatesan MD

Every one knows VT  presents as a wide qrs tachycardia  .Few of us ,   know VT can be narrow or even a normal qrs complex . But .none of us know what is the exact reason , why the width of  qrs complex  in VT swings from narrow  to wide.

Factors determining qrs width in VT.

• Origin of VT :Septal, freewall , apex etc
• Epicardial exit points make it wider
• Rate dependent Intraventricular conduction (VT with aberrancy a possibility ?)
• Drugged VT(Cumulative dose of amiodarone widens the  VT  )
• Associated LV dysfunction/Myopathy
• Electrolytic milieu (K + etc )
• Preexisting bundle branch blocks (Surprisingly common and still more a surprise some BBB  may convert  a VT from wide qrs to narrow qrs ) 

* Another unique , but common observation is  variation in qrs width between different leads.This again points to different exit points and tachycardia circuits traversing and looping around the epicardium to endocardium at different depths  from the chest wall leads.

The commonest explanation of VT being narrow is it’s origin near the septum and travel down the fairly physiological his purkinje tracts and resultant uniform depolarisation.While myocardial,free wall apical VT are bizarre and wide. We now know , many factors come into play in determining the  qrs width.In fact , we are only beginning to understand the complex conduction pattern of VT.