Analysis of tidal volume changes in preterm infants

Assessment of tidal volume using the volume waveform relies on detection of the maximum peak and minimum trough. A possible alternative could be the calculation of a function of the mean of the squared amplitude such as the root of the mean squared amplitude (rms); this type of calculation may be less dependent on the extreme points of the volume waveform. The aim of this study was to compare assessment of rms with tidal volume. A FloRight system (Volusense AS, Oslo, Norway) was used to assess respiration volume (Pickerd et al. J Pediatr 2014). Recordings of 15 minutes or less on seven preterm infants of post-conceptional age 29 to 39 weeks (median 30 weeks) were analysed; in these recordings the babies were receiving SiPaP (Carefusion Corporation, UK) respiratory support at pressures of 5 or 9/5 cmH2O. Raw data were low pass and high pass filtered and analysed using software that we developed using MATLAB (The MathWorks, Inc., USA). In each recording, one epoch of 60 seconds duration was analysed. The epoch chosen was the first where there were no respiratory pauses and the respiratory pattern visually appeared regular; epochs with clear artefact were excluded. For six of the seven infants there was an increase in rms volume amplitude associated with change in pressure to 9/5 cmH2O although the degree was variable. Tidal volume was assessed visually and compared with calculation of rms volume. The data were tested for consistency with a normal distribution. Using the Pearson's correlation coefficient there was a significant relation between tidal volume and rms volume (r= 0.98, p<0.001, n= 14). These results suggest that rms amplitude may help to assess changes in tidal volume.