Page 19 - Why is the apex static?
Having the apex static and the atrioventricular ring moving to and fro is quite a help to LV filling, as it minimises the volume of blood that has to be propelled into the LV - the "sock" effect. The question is. how is this achieved?
It is my opinion that the mechanism depends mostly on the left atrium. This chamber cannot move its posterior wall, which is firmly held by the posterior mediastinal tissues. As a result it moves outward in all other directions in atrial diastole, arguably more so in the line of the ventricular axis than in any other. As a result, the atrioventricular ring has to move to and fro. At the other end of the long axis, the ventricular apex is restrained by the pericardium. This restraint may not be great, but as the LV shortens its long axis, it is much easier for the left atrium to expand and move the atrioventricular ring forward than it is for the apex to pull back away from the chest wall.
Another mechanism could be the "jet" effect of the ejection of blood through the aortic valve, with the ventricle being moved in its apical direction as the equal and opposite reaction to the ejection force. This is an attractive mechanism, as the apex would be pushed apically and appears to move very little as it does not have much room to move in that direction. Perhaps this explains the palpable apex beat?. This mechanism would be entirely synergistic with the atrial expansion theory.
The movement of the atrioventricular ring has found a useful role in echocardiography as a measure of long axis function. The excursion of the ring will be diminished by reductions in long axis function, and by left atrial dilatation (the bigger it is the less its dimensions have to change to accommodate the stroke volume). Usually these two will coexist. Interestingly, in purely restrictive, non infiltrative cardiomyopathy, when the left atrium is large and systolic function of the LV is relatively unimpaired, the ring is static and the apex moves in and out, as in the image below.
