[The cardiotocography instrument test]

This paper reports on a comparative test of eight cardiotocographic units which compute fetal heart rate on the basis of an ultrasonic signal. In order to ensure comparable test conditions for each unit, consisting of the monitor and the signal recorder, a simple measuring system was built. From outside the system, via a controlled electromagnet, a diaphragm in a water tank was put into defined motion. With individual recorders and by exploiting the Doppler effect it was possible to register and process each movement of the diaphragm. The frequencies and amplitudes of the artificially generated signals had been matched to the in vivo signals. The individual test graphs are compared, as a standard comparison, to the graphs obtained when the generation signal is fed to the direct ECG input of a cardiotocograph. In view of their high clinical information value, attention was focused in particular on the number of oscillations per minute and the oscillation amplitude. In this connection the electronic distortion of these two factors is as disregardable as portions of them which are incompletely or not at all recorded. Applying stringent criteria, the test shows that none of the units tested satisfies the desires of the obstetrician wanting to determine the fetal heart rate on a beat-to-beat basis.