WPW syndrome is the prototype of cardiac pre- excitation . The accessory AV pathway short circuits the ventricle .Since there are two options available for the incoming atrial impulse to reach ventricle , often times the qrs is contributed by both .Hence a fusion occurs within qrs complex and stretches it wide , it also generates a delta wave and short PR interval .
The complexities of conduction properties and refractionaries of AV node and accessory pathways determine the degree of pre- excitation. When an optimally timed APD gate crashes into the accessary pathway it gets blocked , only to recover little late , unfortunately invites AV nodal impulse from below . This facilitates a re- entry circuit from ventricle to atria and result in classical AV reciprocating tachycardia .
Antegrade conduction through AV node is physiological and benign as it inherently checks the heart rate . Antegrade conduction occurring through the accessory pathway (which constitutes the pathological component ), is potentially dangerous as it lacks the electrical breaks (Technically called decremental conduction )
What is the specific ECG evidence for antegrade conduction thorough accessory pathway in ECG ?
Delta waves
So, what does it mean if there is absent delta waves in WPW syndrome ?
It can mean three things
- Concealed pathway
- Manifest pathway , but intermittently blocked pathway.
- It is not WPW syndrome at all .
We know concealed pathways are safe* as it allows only retrograde conduction. ( Safe regarding risk of sudden cardiac death , still unsafe for AVRT !)
Intermittent WPW
Intermittent pathways are equally safe as intermittent absence of pre-excitation indicate the presence of naturally occurring breaking system within accessory pathway . Are these accessory pathways blessed with some AV nodal cells ? May be ! . Histological studies do suggest that .This explains intermittent missing of delta waves which is electro-physiologically a good sign
(We also know there are exclusive slowly conducting accessory pathways like Mahim and variants )
If one is lucky to observe this phenomenon in ECG it can be termed as a poor man’s EP study . ( Which requires specialized methods to document the refractory period of accessory pathway to be < 250 msec)
Techniques to screen for or / unmask this concept.
Whenever we diagnose WPW one has to look , whether the patient harbors this phenomenon .
- Holter monitoring has a useful role in this regard .
- If there is nocturnal disappearance of pre- excitation it would suggest a safe accessory pathway.
- Similarly , if pre- excitation disappear during exercise stress testing it would indicate a type of intermittent WPW syndrome.
Final message
An astute cardiologist shall look for this intermittent nature of delta waves and help avoid a costly and potentially harmful EP study !
Read Full Post »
Dr.S.Venkatesan MD 