We thank Drs Loo and Chrimes for their interesting and important letter [1] in response to our study [2]. The purpose of pre-oxygenation is to extend safe apnoea time by the provision of additional oxygen to the patient. This is of particular importance during rapid sequence induction where manual lung ventilation should be avoided. Much of the discussion in the letter from Drs Loo and Chrimes relates to whether pre-oxygenation should be defined solely as the denitrogenation process before apnoea or the entire process of extending safe apnoea time, including both denitrogenation and peri-oxygenation/apnoeic oxygenation by the addition of high- or low-flow oxygen. When designing the study, we aimed to describe pre-oxygenation from a safe apnoea perspective that we and others have described before [3, 4]. However, based on the discussion arising from Drs Loo and Chrimes, perhaps it is time to define what we mean by pre-oxygenation, and which components are included. The primary aim of our study was not to compare the effects of denitrogenation itself between the two methods, because this has been done before [5], but rather to compare the techniques in a clinical setting the way they are usually implemented to ensure safe apnoea. The value of etO₂ could have been the primary endpoint if we were only interested in denitrogenation. However, S_pO₂ is the parameter most often used to reflect the level of oxygenation in a patient, taking oxygen content in the lungs and blood, as well as oxygen consumption, into account. This was a multicentre study taking place throughout the day, performed by all anaesthetists, and we strived for a safe and feasible protocol and therefore did not include patients at higher risk, that is, the pregnant and obese, with BMI > 35 kg.m^-2. Nevertheless, these patients should be studied in future trials.

In our study, we found higher etO₂ levels in the first breath after tracheal intubation in the facemask group compared with the high-flow nasal oxygen group, which is in line with the volunteer study by Hanouz et al. [5]. We agree that it is possible to measure etO₂ as a measure of denitrogenation, but then without subsequent tracheal intubation, as in the volunteer study by Hanouz et al. [5]; however, our goal was to evaluate whether the incidence of oxygen desaturation during tracheal intubation can be reduced by maintaining oxygenation delivery rather than creating an oxygen store that, sooner or later, will be consumed. In that sense, peri-oxygenation might describe the high-flow nasal oxygen technique more accurately than pre-oxygenation. We agree that having air, that is, 21% oxygen, in the anaesthetic circuit could be troublesome in patients desaturating during induction of anaesthesia where a rapid increase of oxygen levels is of utmost importance. Although not part of our study protocol, we suggest that when starting pre-oxygenation, also start the ventilator and allow the anaesthetic circuit, including the tubing, to be flushed with 100% oxygen until tracheal intubation is complete.

The power calculation for our study was based on numbers from a single-centre rapid sequence induction study with the same protocol [4] but, obviously, the incidence of desaturation in this study was different.

We agree that pre-oxygenation with a facemask has several advantages and our study aimed to objectively describe the outcome from two different techniques of maintaining oxygenation. Increased knowledge regarding the advantages and disadvantages of different pre-/peri-oxygenation techniques can help us decide which technique to use in an individual situation, based on patient factors, local traditions and clinician experience. Further randomised trials, focusing on the topics mentioned by Drs Loo and Chrimes, are required.

我们感谢 Loo 博士和 Chrimes 博士对我们的研究 [ 2 ]的有趣而重要的来信 [ 1 ]]。预充氧的目的是通过向患者提供额外的氧气来延长安全呼吸暂停时间。这在应避免手动肺通气的快速序列诱导期间尤为重要。Loo 博士和 Chrimes 博士的信中的大部分讨论都涉及是否应将预给氧定义为呼吸暂停前的脱氮过程，还是延长安全呼吸暂停时间的整个过程，包括脱氮和围氧合/呼吸暂停氧合，添加高流量或低流量的氧气。在设计该研究时，我们旨在从我们和其他人之前描述的安全呼吸暂停的角度来描述预充氧 [ 3, 4]。然而，根据 Loo 博士和 Chrimes 博士的讨论，也许是时候定义我们所说的预充氧的含义，以及包括哪些成分。我们研究的主要目的不是比较两种方法之间脱氮本身的效果，因为这之前已经做过 [ 5 ]，而是比较临床环境中的技术，它们通常以确保安全呼吸暂停的方式实施. 如果我们只对脱氮感兴趣，则etO ₂的值可能是主要终点。然而，S _p O ₂是最常用于反映患者氧合水平的参数，考虑到肺部和血液中的氧含量以及耗氧量。这是一项全天进行的多中心研究，由所有麻醉师进行，我们力求制定安全可行的方案，因此不包括高危患者，即孕妇和肥胖者，BMI > 35 kg.m ^-2 . 尽管如此，这些患者仍应在未来的试验中进行研究。

在我们的研究中，我们发现面罩组气管插管后第一次呼吸的etO ₂水平高于高流量鼻氧组，这与 Hanouz 等人的志愿者研究一致。[ 5 ]。我们同意可以测量 etO ₂作为脱氮的一种措施，但随后无需气管插管，如 Hanouz 等人的志愿者研究。[ 5]; 然而，我们的目标是评估是否可以通过维持氧合输送而不是建立迟早会消耗的氧气储备来降低气管插管期间氧饱和度下降的发生率。从这个意义上说，旁充氧可能比预充氧更准确地描述高流量鼻氧技术。我们同意，麻醉回路中有空气，即 21% 的氧气，对于在麻醉诱导过程中去饱和的患者来说可能会带来麻烦，此时氧气水平的快速增加至关重要。虽然不是我们研究方案的一部分，但我们建议在开始预充氧时，同时启动呼吸机并允许使用 100% 氧气冲洗麻醉回路（包括管道），直到气管插管完成。

我们研究的功效计算基于使用相同方案的单中心快速序列诱导研究的数字 [ 4 ]，但显然，本研究中去饱和的发生率不同。

我们同意使用面罩进行预充氧有几个优点，我们的研究旨在客观地描述两种不同的维持充氧技术的结果。增加对不同预/围氧合技术的优缺点的了解可以帮助我们根据患者因素、当地传统和临床医生经验，决定在个人情况下使用哪种技术。需要进一步的随机试验，重点关注 Loo 博士和 Chrimes 博士提到的主题。