Background: Respiratory rate is difficult to measure, especially in neonates who have an irregular breathing pattern. The World Health Organisation recommends a one minute count, but there is limited data to support this length of observation. We sought to evaluate agreement between the respiratory rate (RR) derived from capnography in neonates, over 15 seconds, 30 seconds, 120 seconds and 300 seconds, against the recommended 60 seconds rate.

Methods: Neonates at two hospitals in Nairobi were recruited and had capnograph waveforms recorded using the Masimo Rad 97. A single high quality 5 minute epoch was randomly chosen from each subject. For each selected epoch, the mean RR was calculated using a breath-detection algorithm applied to the waveform. The RR in the first 60 seconds was compared to the mean RR measured over the first 15 seconds, 30 seconds, 120 seconds, full 300 seconds, and last 60 seconds. We calculated bias and limits of agreement for each comparison and used Bland-Altman plots for visual comparisons.

Results: A total of 306 capnographs were analysed from individual subjects. The subjects had a median gestation age of 39 weeks with slightly more females (52.3%) than males (47.7%). The majority of the population were term neonates (70.1%) with 39 (12.8%) having a primary respiratory pathology. There was poor agreement between all the comparisons based on the limits of agreement [confidence interval], ranging between 11.9 [-6.79 to 6.23] breaths per minute in the one versus two minutes comparison, and 34.7 [-17.59 to 20.53] breaths per minute in the first versus last minute comparison. Worsening agreement was observed in plots with higher RRs.

Conclusions: Neonates have high variability of RR, even over a short period of time. A slight degradation in the agreement is noted over periods shorter than one minute. However, this is smaller than observations done 3 minutes apart in the same subject. Longer periods of observation also reduce agreement. For device developers, precise synchronization is needed when comparing devices to reduce the impact of RR variation. For clinicians, where possible, continuous or repeated monitoring of neonates would be preferable to one time RR measurements.

The Masimo Rad-97 pulse CO-oximeter with capnography (Masimo Corporation, USA) was used to collect continuous capnograph waveforms from neonates.  Following data collection, the capnograph (carbon dioxide, CO₂) waveform data at approximately 20Hz was inputted into a custom breath detection algorithm developed in MATLAB (Math Works, USA) based on adaptive pulse segmentation. The algorithm analysed the waveform’s shape and identified the start and end of each breath (waveform trough to trough), and this breath duration was used to calculate an instantaneous respiratory rate (breaths per minute) for each breath. A mean respiratory rate was calculated by taking the mean of the instantaneous rates for all breaths within the epoch. A breath was considered to be within the epoch if its peak, as identified by the algorithm, was within the epoch. Additionally, the algorithm calculated a capnography quality score at 2Hz. The statistical program R was used to process this information and randomly select high quality 5-minutes epochs for each subject. The high quality criteria threshold was 90% of RR quality scores in the epoch meeting the minimum quality score of 2, indicating a regular capnography waveform with an appropriate shape (that is, minimal variation of amplitude, time interval, up-slope and down-slope of waveform peaks when compared).