Page 25 - How we measure the axes
I have an observation or two to make about the measured left ventricular axes, which enlarge on points made ain various preceding pages.
The axes can be drawn from endocardial border to endocardial border, as is the case in routine imaging.
There is effectively no thickness to the basal end of the long axis, as it is the mitral valve leaflets for the most part. At the apex, the myocardium is quite thin. As a result, the endocardial long axis is very little different to the "epicardial" long axis.
In contrast, the endo- and epi-cardial short axis dimensions are different and we have seen how they vary more during contraction than the long axis, by virtue of the hydraulic effect. In the model, for ventricles of reasonably normal wall thickness, the shortening of the epicardial circumference is two thirds of the shortening of the mid-wall, which is in turn a reasonable surrogate for cell shortening. Perhaps we should be using the epicardial fractional shortening as a useful index of cell contractility?