82

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3 Fundamentals of the Formation of Biosignals

approximately in one direction, i.e. from the SA to the AV and then to the HP oscillator,

as shown in the lower part of Figure 3.13.

A mathematical description of the three oscillators is obtained by extending the

two-oscillator system according to Equation 3.26 with an equation for the third oscil-

lator and adding couplings:

­xSA = −αSA(xSA −vSA1)(xSA −vSA2)^_xSA

−xSAfSA(xSA + dSA)(xSA −eSA)

­xAV = −αAV(xAV −vAV1)(xAV −vAV2)^_xAV

−xAVfAV(xAV + dAV)(xAV −eSA)

+ kSA−AVx^τSA−AV

SA

−kAV−AVxAV

­xHP = −αHP(xHP −vHP1)(xHP −vHP2)^_xHP

−xHPfHP(xHP + dHP)(xHP −eHP)

+ kAV−HPx^τAV−HP

SA

−kHP−HPxHP.

(3.28)

The corresponding model shows Figure 3.29. Disturbances caused by fluctuations in

the ECG baseline, e.g. due to changes in skin contact, and interspersed noise, e.g. due

to 50 Hz mains hum, neon tube influences or radio waves, were taken into account.

An ECG signal disturbed by a noise signal and a fluctuating baseline is shown in Fig-

ure 3.31.

Figure 3.30 shows one below the other the signals generated by this system for

the SA node, the AV node and the HP complex, and the resulting ECG signal (without

interference). It is easy to see where the individual components in the ECG signal come

from. The P wave comes from the signal of the sinus node, and the QRS complex from

the interaction of the AV node and the HP complex. The T wave, on the other hand, is

predominantly generated by the HP complex.

3.3 Taxonomy of Biosignals

In the previous section on cardiac excitation, the origin and modelling of the electrical

biosignals was described in detail. In conclusion to this chapter, this view will now be

extended again to arbitrary biosignals and the different types will be classified accord-

ing to their properties. Life forms – from the single cell to complex organisms such as

mammals – generate biosignals with different properties, function and meaning. Ba-

sically, these signals can be divided into:

1.

autonomous signals, i.e. signals of the body produced by the life form without

external influences, such as the electrical activity of the heart and