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.
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.