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6 Applications and Methods in Biosignal Processing

Fig. 6.31: Example of a Poincaré-representation: for this, the time between the n+1-th and the n+2-

th R-prong (RRn+1) is plotted as a function of the time between the n-th and the n+1-th R-prong (RRn)

of the QRS complex.

plotted slightly shifted: HFN on the X-axis, opposite HFn+1 on the Y-axis. However,

here the "cloud", i.e. the width of the variability, is not necessarily a measure of the

patient’s health. For some diseases, it can also be the other way round. Figure 6.33

shows examples of this. In (a) the "heart rate cloud" is shown during an attack of at-

rial fibrillation, and in (b) when the atrial fibrillation is over. It can be seen that the

heart rate variation is significantly greater during atrial fibrillation than afterwards.

This is because the atrium can no longer be regularly excited by the sinus node, but

beats at a very high rate of about 200 to 300 per minute due to additional "interfer-

ing excitations" (mostly from the mouths of the pulmonary veins). However, since the

atrioventricular node (or AV node for short, cf. section 3.2) acts like a filter in such

a case and only transmits every second to third excitation to the main chamber, the

heart does not beat as frequently (at a rate of about 100). This excitation, however, is

not as regular as with excitation by the sinus node and is very clearly noticeable in the

Poincaré-diagram. During atrial fibrillation, a very broad cloud is therefore seen, and

in the phase thereafter a much smaller cloud.