We have demonstrated that there were no significant differences in the diaphragmatic EMG amplitude between three flow rates during HHHFNC, nor did we see any significant differences in the proportions of infants with the lowest EMG amplitudes or AEGMC between the three different flow rates. Furthermore, we also assessed the occurrence of bradycardias or desaturations and found no significant differences at the three flow rates. Our findings are consistent with that of Saslow et al. [
15] who showed no change in distending pressure between three flow rates of HHHFNC, but they used a smaller range of flow rates of three to 5 L/min.
The lack of significant differences between our three groups may reflect the variable pressure delivered during HHHFNC. Although, in one study, a linear increase in pharyngeal pressure as flow rate increased was demonstrated, independent of whether the mouth was open or closed; in another [
9], no positive pressure was generated while the oral cavity was open. We did not ensure the infant’s mouth was closed during the study as we wished to examine the effect of different flow rates as used in routine clinical practice and hence our results would be generalizable.
There are strengths and some limitations to our study. Three flow rates were assessed in each infant thus, although the infants recruited had a wide range of postnatal ages, they each acted as their own controls. The median FiO
2 immediately before recruitment to the study suggested that some of the infants had mild disease, but there was a range of FiO
2 levels. Indeed, the infants were typical of those on our NICU who receive HHHFNC and similar to those on other units [
16]. We have included a figure of the individual data which demonstrates there was no obvious trend according to the results according to flow rate amongst infants with respiratory distress of evolving or established BPD. Differences in skin contact and anatomical differences between infants can cause variability in the results obtained, yet as seen in Fig.
2, the majority of infants resulted in the same manner to the three flow levels. We have not included in our study infants with very severe respiratory failure, as in our experience, such infants are not routinely supported by HHFNC. We did not measure CO
2 levels as the infants did not have indwelling arterial lines. We recorded FiO
2 changes as highlighted by the number of desaturations and did not show any significant differences according to the flow rate. The diaphragmatic EMG was assessed transcutaneously as previously reported [
2]. This technique only assesses the electrical activity of the frontal diaphragm and not the intercostal muscles [
12]. In preterm infants, however, the intercostal muscles are not thought to have a substantial contribution to breathing effort during tidal breathing [
19]. Furthermore, this technique has demonstrated that weaning from CPAP to low flow nasal cannula led to an increase in diaphragmatic activity measured by EMG and was most prominent in preterm infants who failed the weaning attempt [
7]. We assessed the diaphragmatic EMG which acts as a surrogate of the work of breathing and has been shown to differ between infants receiving no respiratory support and those on HHHFNC at 6 L/min [
14]. The amplitude of the diaphragm EMG, however, may not be fully representative of the work of breathing, but in addition to assessing the diaphragm EMG, we also examined the occurrence of desaturations and bradycardias and demonstrated no significant differences in those outcomes between the three flow rates. Our results are consistent with those of de Waal et al. [
2] who suggested that different HHHFNC flow rates in stable infants do not influence work of breathing as assessed by the neural respiratory drive.
In conclusion, we have demonstrated that in prematurely born infants with respiratory distress or BPD, there were no statistically significant differences in the amplitude of the diaphragm EMG or the numbers of desaturations or bradycardias at three different levels of HHHFNC. We have not undertaken a sub-analysis according to underlying diagnosis, as our sample size highlights such a sub-analysis would be underpowered. Our individual results, however, show similar results in all infants. These results suggest there may be no overall advantages of using high compared to lower flow rates. We suggest practitioners should individualise the HHHFNC level taking into account the severity of the infants’ lung disease using the FiO2 level and objectively assess the effect of different flow rates by recording FiO2 changes and the number of desaturations and bradycardias.