Oxygenation during the apnoeic phase preceding intubation in adults in prehospital, emergency department, intensive care and operating theatre environments

Description of the condition

Endotracheal intubation is an essential procedure for the establishment of a definitive airway in a number of clinical settings, including prehospital, emergency department (ED), intensive care unit (ICU) and operating theatre (OT) environments. The Royal College of Anaesthetists and Difficult Airway Society's Fourth National Audit Project (NAP4) suggests that endotracheal intubation is used in 38.4% of general anaesthetics in the UK (Woodall 2011). Endotracheal intubation involves the placement of a cuffed endotracheal tube into the trachea, typically through the mouth, to enable oxygenation, ventilation and prevention of aspiration. Prolonged attempts at endotracheal intubation are associated with hypoxaemia, unplanned admission to the ICU and death (Caplan 1990King 1990Rose 1994). Hypoxaemia in this context is a potentially catastrophic complication and can lead to dysrhythmia, hypoxic brain injury and death (Caplan 1990Davis 2008Mort 2004).

Preoxygenation is a widely accepted technique for prolonging the 'safe apnoea time' (Weingart 2012), the period during the intubation procedure during which the patient remains apnoeic without hypoxaemia. Preoxygenation can be less effective in critically ill people due to the effects of shunt physiology, increased metabolic demand, anaemia and decreased cardiac output (Drummond 1984Farmery 1996Mort 2005). In the NAP4, nearly 20% of all airway incidents occurred in the ICU, and ineffective preoxygenation in critically ill people was suggested as one possible contributor to this (Woodall 2011).

Description of the intervention

Apnoeic oxygenation aims to prevent hypoxaemia via the application of various oxygen delivery techniques during the apnoeic phase of intubation (Weingart 2012). Various protocols have been employed with nasal cannulae, including humidified or non‐humidified oxygen, as well as a range of oxygen flow rates (Binks 2017aBinks 2017bHolyoak 2017White 2017). Low‐flow nasal cannulae (LFNC) are traditionally used in people requiring minimal oxygen supplementation. These consist of lightweight tubing attached to wall oxygen and split into two prongs at the end. These nasal prongs are positioned in the nostrils and flows of 1 L/minute to 6 L/minute of dry oxygen are traditionally titrated in spontaneously breathing people (O'Driscoll 2008). Oxygen can be delivered via LFNC at up to 15 L/minute in the setting of apnoeic oxygenation (Binks 2017a). High‐flow nasal cannulae (HFNC) are used as a non‐invasive delivery system for up to 100% humidified oxygen and are capable of delivering higher oxygen flow rates of up to 70 L/minute or more (Corley 2017Mir 2017b). HFNC systems are generally comprised of a flow generator, air–oxygen blender and humidifier, with the heated humidified gas being delivered via a wide‐bore heated tube to nasal prongs.

How the intervention might work

During apnoeic oxygenation, the continuous oxygen supply is theorized to continue through the upper airways and into the alveoli via an aventilatory mass flow effect (Lyons 2019Weingart 2012). This process is dependent on patency between the oropharyngeal airspaces and the lungs. During apnoea, alveolar capillary blood continues to absorb oxygen from the alveoli in the absence of lung movement, leading to a net subatmospheric alveolar pressure. This forms a pressure gradient to facilitate movement of additional administered oxygen into the alveoli (Weingart 2012). Preoxygenation by breathing pure oxygen prior to the period of apnoea assists this process, as this eliminates alveolar nitrogen. The persistence of alveolar nitrogen in the lungs during apnoea decreases the pressure gradient for oxygen transfer, as oxygen is absorbed from the alveoli (Kolettas 2014Lyons 2019).

Other proposed mechanisms that facilitate apnoeic oxygenation include cardiogenic oscillations, dead space flushing and positive airway pressure. Cardiogenic oscillations are measurable changes to airway pressure secondary changes in intrathoracic pressure from heart motion on the lung parenchyma or pulmonary blood flow (Lyons 2019Mackenzie 1990Rudlof 2010). The use of apnoeic oxygenation may also enhance flushing and washout of gases from anatomical dead spaces (Möller 2017). Apnoeic oxygenation, particularly via HFNC, may also produce a positive distension pressure within the bronchioles and alveoli, which may reduce the extent of atelectasis on induction of anaesthesia (Humphreys 2017).

Why it is important to do this review

Respiratory complications of endotracheal intubation have the potential to lead to significant adverse events including dysrhythmia, haemodynamic decompensation, hypoxic brain injury and death (Davis 2008Mort 2004). Apnoeic oxygenation may serve as a non‐invasive adjunct to endotracheal intubation to decrease the incidence of hypoxaemia, morbidity and mortality.

Plain language summary

In adults, does oxygen therapy during the non‐breathing period before intubation prevent complications?

What is intubation?

Intubation is a medical procedure that involves insertion of a tube into the breathing passage during the course of anaesthesia before surgery or in life‐saving emergency situations such as road‐side trauma or in times of critical illness. The breathing tube serves as a portal for assisted ventilation.

Why might oxygen therapy help?

During the time when a person stops breathing after going to sleep ready for an operation before an oral breathing tube is inserted low blood oxygen levels (saturations) can occur. This potentially can lead to devastating complications such as a heart attack, stroke or death. Passively providing oxygen via the nose can provide oxygen deep into the lungs and might help prevent low oxygen saturations and the associated complications.

What did we want to find out?

Does oxygen therapy delivered by small tubes into the nose reduce the incidence of low and critically low oxygen levels in the blood during the time it takes for the medical team to insert the breathing tube whilst the person is not breathing? Does this then prevent the incidence of associated complications and change the length of stay in both the hospital and intensive care unit?

What did we do?

We searched medical databases for randomized controlled trials (a type of study where people are randomly assigned to treatment groups) of adults (aged years or older) that compared giving oxygen therapy with giving no oxygen therapy during the period between when they stopped breathing and intubation.

What did we find?

We found 23 studies with 2264 participants. The studies were conducted in intensive care units and operating theatres in countries around the world. Pharmaceutical companies contributed funding to some studies.

Main results

Compared with no oxygen therapy, oxygen therapy improved the lowest recorded blood oxygen levels slightly (by about 2%; 15 studies, 1525 participants), and reduced the duration of intensive care unit stay by about one day (5 studies, 815 people).

Oxygen therapy had no effect on the incidence of critically low oxygen levels in the blood during the non‐breathing period in predominately critically ill people (15 studies, 1802 people).

There was no effect on the occurrence of complications during intubation (10 studies, 997 participants), or in the success rate of first attempt at intubation (8 studies, 826 participants).

None of our included studies reported the effect on hospital length of stay.

What are the limitations of the evidence?

Although we did find some benefits of oxygen therapy, we have low to moderate confidence in the results. This was mainly because the doctors in many studies were aware if the participants were receiving extra oxygen and there were differences between the groups of participants that we could not account for.

What next?

It is unlikely that oxygen therapy during the non‐breathing period provides much benefit to all participants in any of the measured intubation‐related outcomes. Further studies might focus on length of intensive care stay and any plausible reasons for its effect, or on which groups of participants it is more helpful to.

How up to date is this evidence?

Our evidence is current to 4 November 2022.