Clinical applications
Three different calculations were carried out to determine the HF spectral amplitude in the invasive blood pressure signal every 90 seconds of a larger time interval. According to these three calculations, signals were formed by the resulting consecutive HF values in time. In many circumstances, these three signals show the same significant fluctuations. If the fluctuations in these signals did not correspond,
in almost all cases an explanation could be found in a change of the respiratory band width. Therefore the hypothesis can be sustained that all three methods used to calculate the HF spectral components can be applied successfully. A choice can be made whether or not to emphasise respiratory fluctuations. Of course, this hypothesis should be tested on more neonatal data sets. A relation between the spectral
amplitudes and gestational age, age after birth, and behavioural state can be investigated in the future.
The assessment of the autonomic nervous system activity by the HF values is hampered by fluctuations of the respiratory frequency and the respiratory amplitude (depth). The future developments, that allow for real-time filtering with adaptive bandwidth filters, will resolve the problem of the fluctuating respiratory frequency: the actual respiratory frequency can be followed by an adaptation of the filter
coefficients. The fluctuations of respiratory amplitude, however, will still influence the HF spectral amplitude, suggesting changes in the autonomic nervous system that do not exist. Part of the fluctuations in the HF signals, could be attributed to variations in the respiratory frequency. The impact of respiratory amplitude variations on the HF values should be investigated to be able to estimate their influence on the spectral amplitudes. The determination of cross spectra between blood pressure and heart interval as presented in section 6.2 looks promising. The quantification of mutual interactions between blood pressure and heart interval will give us more insight in the origin of the interactions and may refine the underlying physiological model. Further
measurements involving large and different patient groups will be required to be able to draw more definite conclusions on the physiological model.
