Page 24 - Formative and Adaptive Growth

I think a distinction can be made between "formative" growth, when an organ adopts the general topology that its genetics require, and "adaptive" growth, which is the extra modelling secondary to the stresses of use. For example, a foetal leg is clearly a leg but is not a good shape for weight bearing. When it becomes a useful, weight bearing structure its shape is influenced not only by its general increase in size but also by the mechanical stresses it begins to experience.

The myocardium experiences adaptive growth earlier than most structures of the body, having a useful function within a week. The heart must surely grow and shape itself to minimise wall stresses, and to adapt to its changing role after birth. I feel that the heart works best if it minimises any unnecessary motion. To this end it grows and adapts within the pericardium such that the only movements of note are the indrawing of the walls in systole and the to and fro motion of the atrioventricular ring. If the pericardium is opened, this fine balance is ruined: imaging shows us that the heart is very stable in the intact chest, and the rotating/"twisting" motion seen when the pericardium is opened at surgery is not its natural motion (as seen in the videos of page 21).

After growth has ceased, disturbances of anatomy and/or function cannot be easily corrected. For example, I understand that at birth the right ventricle has a layer of circular myocytes as a legacy of contraction at systemic pressure in utero. This layer is lost as the RV involutes to a lower pressure chamber. I am not sure (and would be pleased to hear) but I gather that if the RV becomes higher pressure later in life, it does not regrow a circular layer.

Similarly, if a left ventricle looses part of its myocardium, unbalanced forces may well alter the degree of twist identified on speckle tracking. However, correction of the problem must still be the restoration of normal circumferential contractility, and measures to enforce a correction of twist for its sake alone may not be worthwhile.