Posts Tagged ‘cannon waves’

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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AV dissociation is  common clinical situation that can occur  during both    bradyarrhythmias  and tachyarrhythmias .


  • Complete heart block
  • During pacemaker rhythms


  • Accelerated junctional  rhythm
  • Idioventricular  rhythm
  • Ventricular  tachycardia

AV dissociation is essentially an  ECG diagnosis. But it is associated with some  clinical  signs   ,which can be detected by an astute physician in the bedside. At rapid heart rates  it may be really difficult at times to recognise theses findings, but a  cardiology fellow should look for these whenever they encounter AV dissociation  in ECG.

  1. Varying pulse volume
  2. Varying korotkoff  sounds during BP measurement.
  3. Cannon a waves in JVP
  4. Varying intensity of first heart sound on auscultation
  5. Mitral regurtitant murmur may be heard
  6. Hypotension in compromised hearts

What is the mechanism of clinical signs of AV dissociation ?

During AV dissociation , the atrial and ventricular contractions occur  out of phase  and the sequential contraction  is lost. So atrial contractions  might  occur with a closed AV valves .  This result in reflux of blood into the neck resulting in cannon waves . It may be visible only in few beats as the retrograde conduction VA conduction , is highly variable.

Further , only some atrial beats contribute for ventricular filling some do not.This results  in varying LV volumes and this  could result in changing pulse volume.Occasionally the ventricular and atrial   contraction occur simultaneously  .When this happens ,  some amount of blood  reguritates through the open tricuspid valve and mitral valve  which result in MR or TR .

Clinical utility

This could be important , in differentiating  the perennial  issue   of decoding the   wide qrs  VT from  SVT with  aberrancy .A rapid clinical assessment  here could  aid in the diagnosis  of VT  by  identifying  AV dissociation  . An experienced cardiologists will realise even in a given  ECG  with VT  identifying or ruling out  AV dissociation is not always a  pleasant excercise !

In this era of  high tech gadget  oriented cardiology is it not too much  to call for clinical   recognition of  this  entity ?

Definitely not , if  we know Wencke bach  recognised  the classical type 1 2nd degree  AV block in late 19th century even before the ECG machine was  invented ,

Simply by looking at the neck , by carefully observing progressive prolongation of  distance between a and c waves and subsequent dropping of c waves . Amazing isn’t it ?

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Cannon waves occur classically, during  ventricular ectopic beats .(Commonly irregular) regular cannon waves occur during Junctional tachycardias with 1:1 VA conduction

Cannon like wave may appear  in the jugular vein if the VPDs is timed in a such a fashion ,the atrial systole occurs with a closed AV ( Tricuspid and mitral valve ) so the atrial  contractile wave is reflected back into the veins.This not only happen in right atrium but also  in the left atrium , but the cannon waves are sent into the pulmonary veins , which is not visible. As by  tradition  cannon waves are  meant to be seen only in neck veins , we rarely realise   the importance of such waves in the pulmonary veins.

There must be some significance for this  abnormal pulmonary venous waves  which  travel  in a retrograde fashion.In fact , with  the advent of echocardiography, we realise  pulmonary flow reversal is an important contribution for raised PCWP.

The dyspnea during multiple  VPDs can be due to

1.Transient Mitral regurgitation and resultant elevation of PCWP.

2.Pulmonary venous cannon waves and  it’s effect on  J receptors.

3.Many of the intermittent  episodes of  dyspnea  (Especially paroxysmal nocturnal dyspnea ) , other wise unexplained could be due to this pulmonary venous cannon waves.

4.It also need to be studied how this pulmonary venous cannon waves distribute themself into the 4 pulmonary veins.


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