The findings of this study demonstrate that respiratory rate in neonates varies considerably over a 5-minute period of time. Furthermore, measuring respiratory rate over periods of more than one minute had poor agreement with the standard one-minute rate. Measuring the respiratory rate over 15 and 30 seconds also reduced agreement with the one-minute rate but this was a relatively small effect compared to within individual variability.
This study provides important information for device developers looking to validate new methods and techniques for measuring respiratory rate and also for clinicians measuring respiratory rate in neonates.
When comparing two independent measurements, lack of agreement may be due to the lack of precision of timings of the measurements but is also significantly degraded by the within subject variability in respiratory rate. To accurately compare two methods of respiratory rate measurement, the synchronization in time must be very precise – the same exact time period (and breaths) should be used.
For clinicians, this study demonstrates that the respiratory rate in neonates is highly variable, even over a short period of time, and so a single one-minute measurement may not accurately depict the true physiological status of the patient. We observed that the limits of agreement (LOA) was widest when comparing a one-minute period to a second one-minute period only three minutes later. From the start to the end of a 5-minute period the RR of a neonate can increase or decrease by 15 breaths per minute. This is within the 95% LOA of our results. This observation is important because it means that one single RR measurement in critically ill neonates may not give an adequate assessment of a neonate. Continuous or repeated RR monitoring of neonates is likely to be more beneficial than a single spot check measurement.
The optimum time period for counting the respiratory rate is yet to be determined. We found in this study that measuring RR over periods of more than one minute did not improve agreement. As the measurement length increased, the agreement to one-minute decreased. There was a difference of 6 breaths/minute within the spread of 95% LOA when comparing two-minutes with the standard one-minute rate, and a difference of 10 breaths/minute with the 5-minute comparison. Increasing the duration of respiratory rate counting for longer than one minute will likely not improve accuracy.
Measuring RR over periods of less than one-minute also showed reduced agreement, with the 15 second to one- minute comparison, for example, seeing the spread of LOA of 10 breaths per minute. Yet, this is a relatively small effect compared to the within individual variability that was demonstrated by comparing two one-minute RR counts, taken three minutes apart.
The 30 second period has previously been found to be imprecise since errors are multiplied when converting to “per minute” rates (4). In this study, our method of respiratory rate measurement (using the mean of the instantaneous (single breath) rates of all included breaths) does not include this typical multiplication – there is no increased imprecision in measurements under one minute, only that they are calculated based on less breaths. Despite this difference, in this study we still found the 30 second to one-minute comparison displayed wide variability with a spread of 7 breaths per minute falling within the 95% LOA.
In a similar study done in children under five years of age, comparing different counting periods to a synchronous capnograph recording, they found the least variability when two − 30 second measurements, done three minutes apart, were used (9). This difference in findings is possibly due to the different time periods used for comparison (30 versus 60 seconds). Moreover, our study was exclusively in neonates who have higher variability in their breathing compared to older children. It important to consider this difference between neonates and older children when determining the frequency of RR measurements.
Respiratory rate variability may be affected by agitation, hypoxia, sleep versus awake state, and fever (10–12). A previous study, for example, suggested a RR correction factor of 7–11 breaths per minute for each one degree °C elevation in temperatures above 38.5 degrees (13). Sleep state has also been shown to have a marked effect on the cardio-respiratory system with irregularities being more common in active sleep compared to quiet sleep (11). These factors were not adjusted for during our analysis. We also did not adjust for other physiological factors such as temperature, time of day, age, or gestational age that may contribute to the variability in respiration.
Considering the high variability of the respiratory rate in neonates, clinicians are advised to avoid using this measurement as a single factor in decision-making. This has been echoed in other studies. For example, one study found that hypoxia and increased work of breathing were more important than tachypnoea and auscultatory findings in diagnosing childhood pneumonia (14). A 2019 commentary suggests using a combination of signs and symptoms and biomarkers (for example, C reactive protein levels) in making a clinical diagnosis of pneumonia (15).
Similarly, manufacturers of medical devices that are used in neonates should take into consideration this variability. To accurately compare two methods of respiratory rate measurement, the synchronization in time must be very precise to ensure that the same exact time period is used. Furthermore, depending on a regular inter-breath duration, as is used by some devices, to derive the respiratory rate, may not account for the highly variable respiratory pattern of neonates.
More studies need to be done to investigate the optimum time period for evaluating respirations in the newborn and the benefit of repeat or continuous assessments in making clinical decisions.