Cardiotocographic distortion of fetal heart rate variability: Theoretical considerations and examplification by computerized simulation

The error arising when evaluating FHR variability on CTG curves is demonstrated. Visual estimation is based on the appreciation of oscillation amplitude (O.A.) which is necessarily dependent on basic heart frequency and the changes therein. It reflects variability in a distorted way – the distortion can be as great as 33% even when the basic heart frequency remains within the normal range of 120- 160 beats per minute.