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 1990; King 1990; Rose 1994). Hypoxaemia in this context is a potentially catastrophic complication and can lead to dysrhythmia, hypoxic brain injury and death (Caplan 1990; Davis 2008; Mort 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 1984; Farmery 1996; Mort 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 2017a; Binks 2017b; Holyoak 2017; White 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 2017; Mir 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 2019; Weingart 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 2014; Lyons 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 2019; Mackenzie 1990; Rudlof 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 2008; Mort 2004). Apnoeic oxygenation may serve as a non‐invasive adjunct to endotracheal intubation to decrease the incidence of hypoxaemia, morbidity and mortality.