Posts Tagged ‘pr interval’

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

  1. Concealed pathway
  2. Manifest pathway , but intermittently  blocked pathway.
  3. 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 !

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Cannon Sound

A loud first heart sound (S 1)   which is  heard intermittently in patients with complete heart block (CHB)  is  often referred to as  cannon sound .

What is the mechanism of loud S1 in CHB ?

We know , the intensity of S 1 is  mainly determined by the  relative position of mitral leaflet (To be precise, the  anterior mitral leaflet(AML) )   at the onset of systole.  We also know the  PR interval  has an intricate relationship to  mitral leaflet  position .

The shorter it is ,  wider the leaflet separation  and a longer PR interval makes a mitral leaflet assume a almost closed position   by the time the ventricle contracts.this happens because  a long drawn PR interval fills the ventricle more completely and LVEDV  reaches the maximal levels and LV blood column lifts up the mitral leaflets , and hence the LV  contraction  which follows does not close it with a  bang. In a short PR interval the opposite happens and hence a loud S1 .

In CHB we have variety of PR intervals ranging between  very short to long   ( falling just before the qrs complex) It is not difficult to understand this , as P waves are totally dissociated with the QRS complex  in CHB.In fact p waves have a liberty to fall any where in the ECG tracing , some call this as marching through the qrs complex !.

Hence typically the S1 is variable in intensity , varying between loud to soft.  When  P wave falls just behind a QRS complex , it generates a very  loud S 1  that is called cannon sound .This happens intermittently.

Cannon wave

This is entirely different phenomenon except that it shares the word cannon . Cannon a wave is  a visual finding on the jugular venous pulse.(JVP) .It is a systolic event . It is also seen in CHB as like a cannon sound

This is a giant a wave  in  JVP  when the right atrium contracts against a closed tricuspid valve. In physiological situations atrium contracts with an open AV valves , so that ventricle gets  filled . So atrial contraction  does not does not cause any reflux of blood back into vena cava.

But, when the atrium  contracts and  finds , the AV valve closed  there is no other option   for the incoming blood  to reflux  back into  the neck veins. This is seen as giant a waves called as cannon ” a “waves

With reference to ECG  location ,  this cannon”  a” wave occurs   whenever p wave falls within the ventricular systole ie  the QT interval .The cannon waves also occur intermittently like the  cannon sounds.

What is the  peculiar relationship between cannon a wave and   sound ?

In fact , it is  a non- relationship.  Though  , both the sound and wave   can occur in a given  patient with CHB ,   they can not occur simultaneously .This is because ,  for cannon sounds   to occur  the  P  wave has to fall before  QRS  and for cannon waves to occur the  p   waves must fall after QRS  ie with QT interval .

Clinical significance  of  cannon wave

Complete heart block is the most common situation for cannon waves to occur.

Ironically ,the VVI pacemaker which is used  to treat CHB does not prevent the cannon waves , and atrial contractions continue to occur at random , causing various degrees of intermittent venous reflux into the veins .This may produce, worrisome venous palpitation in some (Usually settles down after few weeks !)

Some attribute , the so called pacemaker syndrome ie giddiness, dizziness to this abnormal venous waves triggering the carotid baroreceptors (Venous -artery spillover )

Will DDD pacemakers  eliminate venous cannon waves ?

We hoped so , it does in fact . But,  it really happens only if the A sense V pace mode . A pace V pace mode with programmed PR interval is not a realiable way to produce AV synchrony. It is  common ,  many of the DDD pacemakers fall back to VVI mode either intentionally or by mode switching  for various reasons.


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