High-flow nasal cannula versus nasal cannula for advanced bronchoscopy

Extract

A recent article by Pikman Gavriely et al. [1] entitled “Laryngeal mask airway or high-flow nasal cannula versus nasal cannula for advanced bronchoscopy: a randomised controlled trial” presents data and results that warrant further clarification to ensure the accuracy of the study's conclusions.

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Discussing a recent article by Pikman Gavriely et al. entitled “Laryngeal mask airway or high-flow nasal cannula versus nasal cannula for advanced bronchoscopy: a randomised controlled trial” https://bit.ly/4nJhyyw

To the Editor:

A recent article by Pikman Gavriely et al. [1] entitled “Laryngeal mask airway or high-flow nasal cannula versus nasal cannula for advanced bronchoscopy: a randomised controlled trial” presents data and results that warrant further clarification to ensure the accuracy of the study's conclusions.

Firstly, the pre-oxygenation protocol adopted in this trial may undermine the potential anti-hypoxic benefits of high-flow nasal cannula (HFNC). All patients received pre-oxygenation at 4 L·min⁻¹ for 10 min via a standard nasal cannula before achieving the targeted sedation level (Ramsay score 5–6), and then HFNC (100% O₂, 45 L·min⁻¹) or oxygen delivery via a laryngeal mask was initiated. However, HFNC can provide constant inspiratory oxygen fraction (up to nearly 100%) and generate positive airway pressure, and its denitrogenating effect and oxygen reservoir expansion are far superior to conventional oxygen supply at 4 L·min⁻¹ via a standard nasal cannula. It is obvious that both pre-oxygenation and apnoeic oxygenation of HFNC are important for respiratory depression or apnoea, as well as for preventing desaturation during painless bronchoscopy, where apnoea is quite common. A study by Minami et al. [2] confirmed that apnoea episodes were recorded in all patients (100%) under fentanyl and midazolam sedation during bronchoscopy. Clinically, the mainstay method of increasing the apnoeic window during general anaesthesia induction is through pre-oxygenation. In this trial, the incidence of hypoxaemia in the HFNC group was as high as 26%, which was significantly higher than those reported in other literature, and could be attributed to the unique pre-oxygenation protocol adopted in this trial. Coincidentally, in our clinical practice of routine flexible bronchoscopy or transbronchial needle aspiration, a similar sedation regime (a combination of alfentanil and midazolam given initially, and an addition of propofol in appropriate dosing to reach deep sedation) was adopted, but the incidence of hypoxaemia was usually less than 10% [3], which was also consistent with other literature cited by authors in their discussion section [4, 5]. However, it is worth noting that this point has not been emphasised or fully discussed in the article, which may lead to readers’ misunderstanding or miscitation.

Secondly, the CO₂ measurements in this trial may be biased. The results showed that the incidence of hypoxaemia in the HFNC group (26%) was significantly lower than that in the NC (standard nasal cannula) group (90%), as well as intervention times for hypoxaemia. However, the interventions (including crossover to a different respiratory support method, chin lift and oxygen enrichment) mentioned in the article were not described in detail, such as whether assisted facemask ventilation was used. As is well known, facemask ventilation itself can significantly affect the CO₂ values. Moreover, the more times intervention with facemask ventilation is required, the greater the impact on the CO₂ values will be, which will lead to a significant bias in the results. Therefore, the true CO₂ value in the NC group with more intervention times may be significantly underestimated. In fact, there is still controversy and dispute about the impact of HFNC on CO₂ values at present. Several studies have shown that HFNC not only improved oxygenation, but also can alleviate CO₂ accumulation [6, 7]. However, Irfan et al. [4] and Douglas et al. [5] reported that compared with the conventional oxygen delivery via a standard nasal cannula, the effect of HFNC on end-tidal CO₂ at the end of the surgery was not significant. Therefore, the CO₂ measurements obtained in this trial need to be interpreted with caution. However, the authors gave contradictory explanations, such as attributing the highest intraoperative CO₂ in the HFNC group to a respiratory drive depression caused by HFNC, and attributing the fastest CO₂ recovery to baseline within 30 min after surgery to the CO₂ washout effect of HFNC. In fact, both effects play the same role during and after surgery. Furthermore, the authors apparently ignored the potential impact or bias of hypoxic interventions on the CO₂ measurements, as well as other physiological advantages of HFNC, including reduced anatomical dead space in the upper respiratory tract [8].

Therefore, we hope that the authors can provide more detailed explanations to help readers better understand or cite their data results accurately.