What is a cannon sound and what is a cannon wave ?
Posted Feb 21 2010 8:54am
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 .