Delta waves are initial 20 ms (or is it up to 40ms ?) segment of qrs complex that is inscribed due to pre-excited depolarisation of the ventricle due to an accessory pathway .
It is more of a fusion complex with native normal qrs complex. The leads in which appear , the polarity and magnitude of these delta waves are determined by
- Site of APs
- Rapidity of conduction through this AP
- The quantum of native AV conduction
- Influence of Autonomic tone and the refractory period of these accessory pathways .
- Heart rate , distal conduction velocity , also can influence .
Can delta occur without AP ?
Like any other variation isolated delta waves are reported in routine ECG finding. It can be be present in 0.15% to 0.25% of the general population. A higher prevalence of 0.55% has been reported in first-degree relatives of patients with accessory pathways.
How do you account for delta in general population ? We know concealed pathways can not record delta . . . then it is possible some from of accelerated AV conduction with twin pathway should be quiet common . ( It is very much possible dual AV nodal pathway with grossly different conduction properties and distal insertion sites inscribe a delta wave .)
The crux of the discussion of WPW syndrome revolves around identifying delta wave and its direction . If the delta wave is well inscribed this job is easy but at times it can be really difficult .
Many moods of delta wave
- Positive delta wave inscribes above baseline. (See the above ECG showing different delta in same patient )
- Negative below baseline and iso-lectric on the baseline .
- Please note , delta wave polarity and QRS polarity need not be in the same direction . If they are in the opposite direction many time it appears as small a pathological “q” or pathological “r”
- It is likely a delta wave can also drag and change the direction of qrs depolarisation if the quantum pre-excitation is large and with a fast conduction property.
- It is also possible the combined contribution of negative delta with negative qrs together make a deep q waves . (Typical example is the LBBB type ECG in type B WPW in Ebstein anomaly )
- Rarely the entire QRS can be due to pre-excited tract and native AV conduction contribute less.(This exactly happen in anti-dromic tachycardia ) but this phenomenon is extremely rare to occur without tachycardia.
Final message
WPW syndrome is such a dynamic entity , one can realize how futile it will be to formulate fixed rules for ECG localization based on this wave .In fact, we suffer from a fundamental electrical ignorance .How often delta wave polarity is discordant with qrs polarity and what is the mechanism ? Standard text books do not discuss this issue . Many of the EPs skirt this question ! For this , we need to critically decode the mechanisms of delta wave generation . Hope our youngsters take up the job !
Dr.S.Venkatesan MD 