A new method to measure inter-breath intervals in infants for the assessment of apnoea and respiratory dynamics

Background Respiratory disorders, including apnoea, are common in preterm infants due to their immature respiratory control and function compared with term-born infants. However, our inability to accurately measure respiratory rate in hospitalised infants results in unreported episodes of apnoea and an incomplete picture of respiratory dynamics. Methods We develop, validate and use a novel algorithm to identify inter-breath intervals (IBIs) and apnoeas in infants. In 42 infants (a total of 1600 hours of recordings) we assess IBIs from the chest electrical impedance pneumograph using an adaptive amplitude threshold for the detection of individual breaths. The algorithm is refined by comparing its accuracy with clinically-observed breaths and pauses in breathing. We also develop an automated classifier to differentiate periods of true central apnoea from artefactually low amplitude signal. We use this algorithm to explore its ability to identify morphine-induced respiratory depression in 15 infants. Finally, in 22 infants we use the algorithm to investigate whether retinopathy of prematurity (ROP) screening alters the IBI distribution. Findings 88% of the central apnoeas identified using our algorithm were missed in the clinical notes. As expected, morphine caused a shift in the IBI distribution towards longer IBIs, with significant differences in all IBI metrics assessed. Following ROP screening, there was a shift in the IBI distribution with a significant increase in the proportion of pauses in breathing that lasted more than 10 seconds (t-statistic=1.82, p=0.023). This was not reflected by changes in the monitor- derived respiratory rate or episodes of apnoea recorded on clinical charts. Interpretation Better measurement of infant respiratory dynamics is essential to improve care for hospitalised infants. Use of the novel IBI algorithm demonstrates that following ROP screening increased instability in respiratory dynamics can be detected in the absence of clinically-significant apnoeas. Funding Wellcome Trust and Royal Society

High-frequency oscillations via the pleural surface: an alternative mode of ventilation?

We applied high-frequency oscillatory ventilation (HFOV) of low amplitude to the pleural surface of the isolated rat lung (IPL) perfused at 10 ml X min-1 with Krebs bicarbonate containing 4.5% albumin (hematocrit 34%). Lung volume was held constant by a continuous positive airways pressure (CPAP) of 5 cmH2O. Varying CPAP from 2 to 15 cmH2O did not affect O2 uptake. Tidal volume (VT) was estimated with an impedance pneumograph, and it bore a direct linear relationship to the amplitude of both the loudspeaker input signal and the pressure change in the chamber up to 30 Hz; VT was inversely proportional to the frequency (f). However, at a constant loudspeaker input of 10 V, minute expired ventilation (VE) remained constant (mean 104 ml X min-1) as f increased from 5 to 30 Hz. Hemoglobin saturation increased by more than 80% during HFOV of 5–30 Hz and amplitude of 10 V, the maximum O2 uptake being 14.6 ml O2 per 100 ml perfusate. Whereas dead space was approximately 335 microliters, a VT of less than 40 microliters could effect normal O2 uptake, suggesting that bulk flow is playing only a minor role in gas exchange. HFOV for 60 min (CPAP 5 cmH2O) did not affect the amount of alveolar surfactant compared with conventional ventilation at the same mean airway pressure. We conclude that normal O2 uptake can be maintained by applying HFOV to the pleural surface of the IPL held at constant volume.