6.3 Signals of the Cardiovascular System

|

247

Fig. 6.20: Electrode arrangement in leads according to Wilson [84]: The illustration on the left shows

the arrangement in the vertical plane, on the right in the horizontal plane with the projections of the

heart vector.

6.3.1.1 Analysis of the ECG

For the physician, it is not enough just to record an ECG. He must be able to recognise

from its course whether a disease is present. To be able to do this, various character-

istics must be determined from the ECG curve. These include not only the heart rate,

but also the regularity and the shape of the ECG curve. From this, for example, a dys-

rhythmia or a heart attack can be recognised.

The ECG measured in real life contains various disturbances and distortions.

These include: 50 Hz mains hum, electromagnetic influences such as those caused

by neon lights, radios, TVs or PCs, as well as short-term influences when the patient

moves during the measurement and the contact resistance between the skin and the

electrode is changed by it. Within the framework of this book, not all procedures for

recognizing the features of the ECG curve can be dealt with, but two important determ-

inations will be presented in more detail as examples, namely i) the recognition of the

QRS complex in a noisy ECG according to Pan-Tompkins and ii) the determination of

the heart rate and its variability.

Both procedures actually belong together. If the QRS complex is known, it is pos-

sible to determine the heart rate and from its temporal course –the heart never beats

at exactly the same frequency, because this is influenced by the nervous system de-

pending on the load –the frequency spectrum. It is not the person whose pulse is al-

most always uniform who is particularly healthy. On the contrary, several studies have

shown that a healthy heart has variable pulse rates (within certain limits).

6.3.1.2 Determination of the QRS Complex by the Pan-Tompkins Method

J. Pan and W. J. Tompkins, in their 1985 publication [58], introduced a method for de-

tecting the QRS complex in an ECG that is very robust and insensitive to interference.

The advantage of this method is also that it can be applied in real time and used, for ex-