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142 P. KOHL ET AL.

8.4.4 Simulating the ECG
To date, the most common tool for clinical assessment of cardiac electri-
cal function is the electro-cardiogram (ECG). It is a dynamic representa-
tion, usually obtained from the body surface, of the changes in cardiac
electrical behaviour.
While the ECG is an invaluable tool for the observation of heart rate
and rhythm, as well as for the diagnosis of conduction abnormalities,
ischaemia, and infarcts, its detailed interpretation is not without pitfalls.
One reason for this is that different changes in cardiac cellular behaviour
may give rise to very similar effects on the ECG. This makes it difficult to
draw conclusions from a patient’s ECG to the underlying (sub-)cellular
mechanisms. This issue is usually referred to as the ‘inverse problem’.
Today’s heart models do not yet possess the power to solve the inverse
problem. They do, however, aid the understanding and interpretation of
the ECG by repeatedly solving ‘forward problems’ to study the effects of
cellular modifications on the calculated ECG. Model reconstruction of a
normal ECG is therefore a necessary first step towards developing a better
understanding of the information ‘hidden’ in it. Figure 8.3(a) illustrates
this.
The same may be applied to simulations of the ECG in pathologies.
Here, we illustrate work on simulating the typical ECG of patients with
congestive heart failure (CHF), a disease that affects roughly 1 per cent of
the population in Western countries and causes a reduction in cardiac
output. While therapeutic advances have reduced mortality from pump
failure, they have been relatively ineffective in reducing the incidence of
sudden cardiac death caused by disturbances in the heart’s electrical activ-
ity. The virtual heart can be used to identify promising targets for pharmac-
ological interventions in CHF.
CHF is accompanied, at the cellular level, by changes in the content of
proteins that govern electrical repolarisation and cellular calcium han-
dling. This threatens orderly repolarisation of cardiac tissue by increasing
the likelihood of spontaneous ‘early after-depolarisations’ that can initiate
an irregular heartbeat. Figure 8.3(b) illustrates the effect of CHF-typical cel-
lular changes on the computed electrical activity of the heart and the ECG.
The virtual heart shows the characteristic pattern of rhythm disturbance
observed in CHF patients, together with the distinctive saw-tooth like
ECG. The circulating waves of electrical excitation prevent the heart from