Page 4 - The Assertive Short Axis (part 1 - lessons from an ASD)
This is a CT study of a large atrial septal defect (superior sinus venosus ASD) with no pulmonary hypertension. The top row is the short axis view and the four chamber view, at end diastole. The ventricular walls are flaccid and with their end diastolic pressures equalised by virtue of the ASD communication, the interventricular septum is straight and the LV cavity is "D" shaped in the short axis view.
The lower images are in the phase of isovolumetric contraction. In the short axis the dominant shape of the mid-wall circumferential myocytes is asserted, giving the LV a circular cross section. This makes mechanical sense, as the ventricle will minimise its wall tension (and thereby minimise oxygen demand).
The interesting thing is that, in the corresponding four chamber view alongside, the tensioning of the myocardium does not lead to any overt change in outline of the cavity. Why does it not take up a more circular shape? It cannot fully do so as the inflexible atrioventricular ring forms part of the outline, but you would think it would try.
I think the dissection of page 3 helps with this. Firstly, the myocytes that would naturally try to form a circle are those in the plane of the long axis, the subendocardial longitudinal cells. We have already noted that there are not many of them.
Secondly, much of the power of long axis contractility must therefore come from the contribution that the angled myocytes can make as a vector of force in the long axis direction. Each layer will try to twist the ventricle along its long axis, but the two obliquities oppose each other and this will supress the tendancy to form a circle.