LAUSR

We thank Dr Parida for his interest [1] in our study [2] and for drawing attention to the potential impact of highflow nasal oxygen on gastric insufflation and functional residual capacity. First, to clarify our original study finding, we did not find that it took more than 8 min to achieve the primary outcome of end-tidal oxygen concentration ≥ 90% in 90% of parturients with high-flow nasal oxygen pre-oxygenation. We found that even when the duration of high-flow nasal oxygen pre-oxygenation was extended to 8 min with or without a simple facemask, the primary outcome could not be reliably attained; this was in contrast to standard facemask preoxygenation, which had 100% success rate when the duration was extended to 4 min. The positive airways pressure generated fromhigh-flow nasal oxygen has raised theoretical concerns that it could lead to gastric insufflation. Indeed, Parke et al. [3] found high-flow nasal oxygen at 50 and 70 l.min resulted in nasopharyngeal airway pressures as high as 6.8 and 10.1 cmH2O, respectively, in healthy volunteers when breathing with their mouth closed. However, we learned from our two recent studies that even when instructed, most parturients were unable to tolerate high-flow nasal oxygen of 50–70 l.min with closed mouth breathing 100% of the time [2, 4]. In fact, we found that only 37.5% of parturients were able to breathe high-flow nasal oxygen with their mouths closed [2]. Expiratory pharyngeal pressure is significantly lowered when breathing with the mouth open [5]. Therefore, the findings in the Parke et al. study may not be easily generalisable to the obstetric population. Although robust data are currently lacking, two published abstracts presented at the Difficult Airway Society meeting in 2018 provide some reassurance that gastric insufflation from high-flow nasal oxygen may not be a significant clinical concern. Using gastric ultrasound to perform serial cross-sectional areas of the gastric antrum in both the supine and right lateral decubitus positions, preliminary data involving 30 healthy fasted adults found no significant differences in gastric distension or volume of gastric secretions before or after breathing spontaneously via high-flow nasal oxygen at 70 l.min for 30 min [6]. This finding is consistent with a recent study that found no gastric insufflation when pre-oxygenation was performed with peak inspiratory pressures of up to 15 cmH2O [7]. In the second abstract, the effect of pre-oxygenation techniques on functional residual capacity were compared in 40 patients undergoing bariatric surgery randomly selected to facemask pre-oxygenation with a PEEP of 7 cmH2O vs. high-flow nasal oxygen at 50 l.min -1 which increased to 70 l.min during apnoea. The authors found high-flow nasal oxygen pre-oxygenation increased and maintained functional residual capacity compared with facemask and PEEP [8]. An ongoing study using high-flow nasal oxygen during the induction of obstetric general anaesthesia may provide further data to aid and enrich the discussion [9]. We echo Dr Parida’s comments that future research could incorporate gastric ultrasound to better characterise the impact and complications of high-flow nasal oxygen therapy on parturients, especially considering that gastric ultrasound is approaching validation in the obstetric population [10]. The concerns raised by Dr Parida highlight our incomplete understanding of the physiological impact of high-flow nasal oxygen, especially during pregnancy. The role of high-flow nasal oxygen in obstetric general anaesthesia remains unclear but there is increasing awareness and ongoing momentum to acquire the evidence needed to improve our understanding of this oxygenation adjunct [11].