Page 179 - Visions of the Future Chemistry and Life Science

160 P. J. KOLSTON

A number of factors, including electric shock, deprivation of oxygen,
or abnormally high levels of potassium or low levels of calcium in the
blood, can cause malfunction of the conduction system. The resulting
irregular contraction of the heart wall can be stopped by applying con-
trolled electric shocks to the heart, either internally or externally. Patients
at risk may be fitted with internal devices for supplying such shocks when
they are needed. One very important design aim is to maximise battery life,
thereby reducing the frequency of invasive surgery. Purely electrical finite-
element models of the heart can aid the optimisation of the type of stimu-
lation and positioning of such devices, so as to minimise the energy
required to arrest irregular contractions.
Whole-body finite-element models can be used to optimise the deliv-
ery of external electric shocks to the control of irregular beating. These
models can also be used in reverse, to aid in the interpretation of the skin-
surface voltages induced by heart activity. Unfortunately, the body does
not behave simply as a salty solution in a leathery container since, for
example, the resistivity of bone is 100 times greater than that of blood.
This huge variation in the resistance of the intervening tissues greatly
influences how energy passes between the heart and the skin. Performing
subject-specific analyses could reduce existing discrepancies between
models and experiments in both types of whole-body model.
Purely electrical models of the heart are only a start. Combined
electromechanical finite-element models of the heart take into account
the close relationship that exists between the electrical and mechanical
properties of individual heart cells. The mechanical operation of the heart
is also influenced by the fluid–structure interactions between the blood
and the blood vessels, heart walls, and valves. All of these interactions
would need to be included in a complete description of heart contraction.

9.7 An ear model

Whilst finite-element modelling of gap junctions occurs at a sub-cellular
level, these models do not consider the operation of intact organs.
Conversely, in models of the complete heart the discretisation is usually
on a millimetre scale. However, the cochlea (see Figure 9.3) is already being
simulated on a 0.01mm, or cellular, scale. Although cochlear malfunction
is not life threatening, damage to it does adversely affect the ability of
almost 1000000000 people to communicate.