Uses and mechanisms of apnoeic oxygenation: a narrative review

Apnoeic oxygenation refers to oxygenation in the absence of spontaneous respiration or mechanical ventilation. It has been described in humans for over half a century and has seen a resurgence in interest given its potential to delay oxygen desaturation during airway management, especially with the advent of high-flow nasal cannulae. This narrative review summarises our current understanding of the mechanisms of gas exchange during apnoeic oxygenation and its diverse range of clinical applications, including its use at induction of anaesthesia and for the facilitation of ‘tubeless anaesthesia’. Additional discussion covers use in critical care, obese, obstetric and paediatric sub-populations. The article also highlights current research efforts aiming to enhance the evidence base for the use of this technique.     

Exploring the limits of prolonged apnoea with high-flow nasal oxygen: an observational study

High-flow nasal oxygen is increasingly used for oxygenation during apnoea. Extending apnoea duration using this technique has mainly been investigated during minor laryngeal surgery, but it is unclear how long it can be administered for before it should be discontinued due to acidosis. We aimed to describe the dynamics of arterial blood gases during apnoeic oxygenation with high-flow nasal oxygen with jaw thrust only, to explore the limits of this technique. We included adult orthopaedic patients scheduled for general anaesthesia. After pre-oxygenation, anaesthesia with neuromuscular blockade was induced and high-flow nasal oxygen (70 l.min⁻¹) was continued with jaw thrust as the only means of airway management, with monitoring of vital signs and arterial blood gas sampling every 5 minutes. Apnoeic oxygenation with high-flow nasal oxygen was discontinued when arterial carbon dioxide tension (PaCO₂) exceeded 12 kPa or pH fell to 7.15. This technique was used in 35 patients and median (IQR [range]) apnoea time was 25 (20–30 [20–45]) min and was discontinued in all patients when pH fell to 7.15. The mean (SD) PaCO₂ increase was 0.25 (0.06) kPa.min⁻¹ but it varied substantially (range 0.13–0.35 kPa.min⁻¹). Mean (SD) arterial oxygen tension was 48.6 (11.8) kPa when high-flow nasal oxygen was stopped. Patients with apnoea time > 25 minutes were significantly older (p = 0.025). We conclude that apnoeic oxygenation with high-flow nasal oxygen resulted in a significant respiratory acidosis that varies substantially on the individual level, but oxygenation was maintained.     

Transcutaneous carbon dioxide monitoring during prolonged apnoea with high-flow nasal oxygen

Background

The duration of apnoeic oxygenation with high-flow nasal oxygen is limited by hypercapnia and acidosis and monitoring of arterial carbon dioxide level is therefore essential. We have performed a study in patients undergoing prolonged apnoeic oxygenation where we monitored the progressive hypercapnia with transcutaneous carbon dioxide. In this paper, we compared the transcutaneous carbon dioxide level with arterial carbon dioxide tension.

Methods

This is a secondary publication based on data from a study exploring the limits of apnoeic oxygenation. We compared transcutaneous carbon dioxide monitoring with arterial carbon dioxide tension using Bland–Altman analyses in anaesthetised and paralysed patients undergoing prolonged apnoeic oxygenation until a predefined limit of pH 7.15 or PCO₂ of 12 kPa was reached.

Results

We included 35 patients with a median apnoea duration of 25 min. Mean pH was 7.14 and mean arterial carbon dioxide tension was 11.2 kPa at the termination of apnoeic oxygenation. Transcutaneous carbon dioxide monitoring initially slightly underestimated the arterial tension but at carbon dioxide levels above 10 kPa it overestimated the value. Bias ranged from −0.55 to 0.81 kPa with limits of agreement between −1.25 and 2.11 kPa.

Conclusion

Transcutaneous carbon dioxide monitoring provided a clinically acceptable substitute for arterial blood gases but as hypercapnia developed to considerable levels, we observed overestimation at high carbon dioxide tensions in patients undergoing apnoeic oxygenation with high-flow nasal oxygen.     

Changes in blood-gas tensions during apnoeic oxygenation in paediatric patients

We report changes in arterial blood-gas tensions for up to 5 min of apnoeic oxygenation in 26 anaesthetized paediatric patients (21 children, five infants). Changes in oxygen and carbon dioxide tension were greatest in the first minute of apnoeic oxygenation. In subsequent minutes, rates of change in gas tension were approximately constant. The rate of decline in oxygen tension (31 (95% confidence interval (CI) 20.1-42.2) mm Hg min-1) was more than three times that reported in studies in adults. The rate of increase in carbon dioxide tension (4.2 (95% CI 3.7-4.7) mm Hg min-1) was similar to that reported in adults. After successful preoxygenation, oxygen tension remained greater than 290 mm Hg in all children (age > 1 yr) throughout the study. This was not the case in infants. We found no correlation between changes in blood-gas tensions and age or weight of patients. The small number of infants studied showed rapid decreases in oxygen tension which if sustained would be expected to limit the safe duration of apnoeic oxygenation, unlike adults where apnoeic oxygenation is limited by hypercapnia. Extrapolation of our results suggests that when preoxygenation has been successful, apnoeic oxygenation could continue safely in children for at least 10 min. Infants may become hypoxic after only 2 min.      

A comparison of gastric gas volumes measured by computed tomography after high-flow nasal oxygen therapy or conventional facemask ventilation*

High-flow nasal oxygen therapy is increasingly used to improve peri-intubation oxygenation. However, it is unknown whether it may cause or exacerbate insufflation of gas into the stomach. High-flow nasal oxygen therapy is now standard practice in our hospital for adult patients undergoing percutaneous thermal ablation of liver cancer under general anaesthesia with tracheal intubation. We compared gastric gas volumes measured from computed tomography images that had been acquired immediately after intubation in two series of patients: 50 received peri-intubation high-flow nasal oxygen therapy and another 50 received conventional facemask pre-oxygenation and ventilation before intubation and before high-flow nasal oxygen therapy became standard practice in our unit. Median (IQR [range]) gastric gas volume was 24.0 (14.2–59.9 [3–167]) cm³ in the high-flow nasal oxygen therapy group and 23.8 (12.6–38.8 [0–185]) cm³ in the facemask group. There was no difference between the two groups in the volume of gastric gas measured by computed tomography imaging (Mann–Whitney U-test, U = 1136, p = 0.432, n₁ = n₂ = 50). Our results demonstrate that a small volume of gastric gas is commonly present after induction of anaesthesia, but that the use of peri-intubation high-flow nasal oxygen therapy for pre-oxygenation and during apnoea does not increase this volume compared with conventional facemask pre-oxygenation and ventilation. This is clinically relevant, as high-flow nasal oxygen therapy is increasingly being used in a peri-intubation context and in patients at higher risk of aspiration.     

High-flow nasal oxygen therapy in intensive care and anaesthesia

Oxygen therapy is first-line treatment for hypoxaemic acute respiratory failure (ARF). High-flow nasal oxygen therapy (HFNO) represents an alternative to conventional oxygen therapy. HFNO provides humidified, titrated oxygen therapy matching or even exceeding the patients' inspiratory demand. The application of HFNO is becoming widespread in Intensive Care Units (ICUs), favoured by increasing evidence based on numerous studies supporting its efficacy. The mechanisms of action and physiological effects of HFNO are not yet fully understood. Pharyngeal dead space washout, decrease in airway resistance, generation of a positive end-expiratory pressure, and enhanced delivery of oxygen are all alleged to be potential mechanisms. The emerging evidence suggests that HFNO is effective in improving oxygenation in most patients with hypoxaemic ARF of different aetiologies. Notwithstanding the potential benefit of HFNO in the management of hypoxaemia, further large cohort studies are necessary to clarify the indications, contraindications and factors associated with HFNO failure. HFNO may also be valuable in reducing the need for tracheal intubation in the management of post-extubation ARF. In addition, HFNO has been proposed to limit oxygen desaturation by prolonging apnoeic oxygenation during intubation both in ICUs and operating theatres.     

A physiological study to determine the mechanism of carbon dioxide clearance during apnoea when using transnasal humidified rapid insufflation ventilatory exchange (THRIVE)

Clinical observations suggest that compared with standard apnoeic oxygenation, transnasal humidified rapid-insufflation ventilatory exchange using high-flow nasal oxygenation reduces the rate of carbon dioxide accumulation in patients who are anaesthetised and apnoeic. This suggests that active gas exchange takes place, but the mechanisms by which it may occur have not been described. We used three laboratory airway models to investigate mechanisms of carbon dioxide clearance in apnoeic patients. We determined flow patterns using particle image velocimetry in a two-dimensional model using particle-seeded fluorescent solution; visualised gas clearance in a three-dimensional printed trachea model in air; and measured intra-tracheal turbulence levels and carbon dioxide clearance rates using a three-dimensional printed model in air mounted on a lung simulator. Cardiogenic oscillations were simulated in all experiments. The visualisation experiments indicated that gaseous mixing was occurring in the trachea. With no cardiogenic oscillations applied, mean (SD) carbon dioxide clearance increased from 0.29 (0.04) ml.min⁻¹ to 1.34 (0.14) ml.min⁻¹ as the transnasal humidified rapid-insufflation ventilatory exchange flow rate was increased from 20 l.min⁻¹ to 70 l.min⁻¹ (p = 0.0001). With a cardiogenic oscillation of 20 ml.beat⁻¹ applied, carbon dioxide clearance increased from 11.9 (0.50) ml.min⁻¹ to 17.4 (1.2) ml.min⁻¹ as the transnasal humidified rapid-insufflation ventilatory exchange flow rate was increased from 20 l.min⁻¹ to 70 l.min⁻¹ (p = 0.0014). These findings suggest that enhanced carbon dioxide clearance observed under apnoeic conditions with transnasal humidified rapid-insufflation ventilatory exchange, as compared with classical apnoeic oxygenation, may be explained by an interaction between entrained and highly turbulent supraglottic flow vortices created by high-flow nasal oxygen and cardiogenic oscillations.     

Measurement of airway pressure during high-flow nasal therapy in apnoeic oxygenation: a randomised controlled crossover trial*

It is recognised that high-flow nasal therapy can prevent desaturation during airway management. Studies in spontaneously breathing patients show an almost linear relationship between flow rate and positive airway pressure in the nasopharynx. Positive airway pressure has been suggested as one of the possible mechanisms explaining how high-flow nasal therapy works. However, data on pressures generated by high-flow nasal therapy in apnoeic adults under general anaesthesia are absent. This randomised controlled crossover trial investigated airway pressures generated by different flow rates during high-flow nasal therapy in anaesthetised and paralysed apnoeic patients, comparing pressures with closed and open mouths. Following induction of anaesthesia and neuromuscular blockade, a continuous jaw thrust was used to enable airway patency. Airway pressure was measured in the right main bronchus, the middle of the trachea and the pharynx, using a fibreoptically-placed catheter connected to a pressure transducer. Each measurement was randomised with respect to closed or open mouth and different flow rates. Twenty patients undergoing elective surgery were included (mean (SD) age 38 (18) years, BMI 25.0 (3.3) kg.m^-2, nine women, ASA physical status 1 (35%), 2 (55%), 3 (10%). While closed mouths and increasing flow rates demonstrated non-linear increases in pressure, the pressure increase was negligible with an open mouth. Airway pressures remained below 10 cmH₂O even with closed mouths and flow rates up to 80 l.min⁻¹; they were not influenced by catheter position. This study shows an increase in airway pressures with closed mouths that depends on flow rate. The generated pressure is negligible with an open mouth. These data question positive airway pressure as an important mechanism for maintenance of oxygenation during apnoea.     

Pre-oxygenation using high-flow nasal oxygen vs. tight facemask during rapid sequence induction

Pre-oxygenation using high-flow nasal oxygen can decrease the risk of desaturation during rapid sequence induction in patients undergoing emergency surgery. Previous studies were single-centre and often in limited settings. This randomised, international, multicentre trial compared high-flow nasal oxygen with standard facemask pre-oxygenation for rapid sequence induction in emergency surgery at all hours of the day and night. A total of 350 adult patients from six centres in Sweden and one in Switzerland undergoing emergency surgery where rapid sequence induction was required were included and randomly allocated to pre-oxygenation with 100% oxygen using high-flow nasal oxygen or a standard tight-fitting facemask. The primary outcome was the number of patients developing oxygen saturations <93% from the start of pre-oxygenation until 1 min after tracheal intubation. Data from 349 of 350 patients who entered the study were analysed (174 in the high-flow nasal oxygen group and 175 in the facemask group). No difference was detected in the number of patients desaturating <93%, five (2.9%) vs. six (3.4%) patients in the high-flow nasal oxygen and facemask group, respectively (p = 0.77). The risk of desaturation was not increased during on-call hours. No difference was seen in end-tidal carbon dioxide levels in the first breath after tracheal intubation or in the number of patients with signs of regurgitation between groups. These results confirm that high-flow nasal oxygen maintains adequate oxygen levels during pre-oxygenation for rapid sequence induction.     

Pre-oxygenation using high-flow nasal oxygen in parturients undergoing caesarean section in general anaesthesia: A prospective,multi-centre,pilot study

Background

Parturients undergoing caesarean section in general anaesthesia have an increased risk of desaturating during anaesthesia induction. Pre- and peri-oxygenation with high-flow nasal oxygen prolong the safe apnoea time but data on parturients undergoing caesarean section under general anaesthesia are limited. This pilot study aimed to investigate the clinical effects and frequency of desaturation in parturients undergoing caesarean section in general anaesthesia pre- and peri-oxygenated with high-flow nasal oxygen and compare this to traditional pre-oxygenation using a facemask.

Methods

In this prospective, non-randomised, multi-centre study we included pregnant women with a gestational age ≥30 weeks undergoing caesarean section under general anaesthesia. All parturients were asked to participate in the intervention group consisting of pre-oxygenation using high-flow nasal oxygen. Parturients declining participation were pre-oxygenated with a traditional facemask. Primary outcome was the proportion of parturients desaturating below 93% from start of pre-oxygenation until 1 min after tracheal intubation. Secondary outcomes investigated end-tidal oxygen concentrations after tracheal intubation and the proportion of parturients with signs of regurgitation.

Results

A total of 34 parturients were included, 25 pre- and peri-oxygenated with high-flow nasal oxygen and 9 pre-oxygenated with facemask. No difference in patient or airway characteristics could be seen except for a higher BMI in the high-flow nasal oxygen group (31.4 kg m⁻² [4.7] vs. 27.7 kg m⁻² [3.1]; p = .034). No woman in any of the two groups desaturated below 93%. The lowest peripheral oxygen saturation observed, in any parturient, was 97%. There was no difference detected in end-tidal oxygen concentration after tracheal intubation, 87% (6) in the high-flow nasal oxygen group vs 80% (15) in the facemask group (p = .308). No signs of regurgitation, in any parturient, were seen.

Conclusion

Pre- and peri-oxygenation with high-flow nasal oxygen maintain adequate oxygen saturation levels during induction of anaesthesia also in parturients. Regurgitation of gastric content did not occur in any parturient and no other safety concerns were observed in this pilot study.     

High-flow nasal oxygen vs. standard flow-rate facemask pre-oxygenation in pregnant patients: a randomised physiological study

High-flow nasal oxygen has been shown to provide effective pre-oxygenation and prolong apnoeic time during intubation attempts in non-pregnant patients. We aimed to compare pre-oxygenation using high-flow nasal oxygen (30–70 l.min⁻¹ oxygen flow) via nasal prongs with standard 15 l.min⁻¹ oxygen breathing via a tight-fitting facemask. Forty healthy parturients were randomly allocated to these two groups, and furthermore each patient underwent the selected pre-oxygenation method with both 3-min tidal volume breathing and 30s tidal breathing followed by eight vital capacity breaths. With 3-min tidal volume breathing, the respective estimated marginal means for high-flow nasal oxygen and standard flow rate facemask pre-oxygenation were 87.4% (95%CI 85.5–89.2%) and 91.0% (95%CI 89.3–92.7%), p = 0.02; with eight vital capacity breaths the estimated marginal means were 85.9% (95%CI 84.1–87.7%) and 91.8% (95%CI 90.1–93.4%, p < 0.0001). Furthermore, high-flow nasal oxygen did not reliably achieve a mean end-tidal oxygen concentration ≥ 90% compared with the standard flow rate facemask. In this physiological study, high-flow nasal oxygen pre-oxygenation performed worse than standard flow rate facemask pre-oxygenation in healthy term parturients.     

Gum elastic bougie, capnography and apnoeic oxygenation

BACKGROUND AND AIM: This study assessed the accuracy of using capnography with a modified, hollow gum elastic bougie in predicting tracheal intubation, and its effectiveness as a method of apnoeic oxygenation. METHODS: Patients were randomly allocated to having the gum elastic bougie inserted, under anaesthesia, in the trachea or the oesophagus. End-tidal carbon dioxide measurements were made at 10 and 20 s. The position of the gum elastic bougie was correctly predicted in 89.2% of patients. We tested the apnoeic oxygenation on an anaesthetic simulator model, which is housed in the Scottish Anaesthesia Simulator Centre, Stirling, UK. RESULTS: The time taken for the oxygen saturation to fall to 90% was significantly prolonged when the gum elastic bougie was used for apnoeic oxygenation. CONCLUSION: The modification of the gum elastic bougie allows a more objective assessment of correct placement than the previous tactile method. The current design of bougie is unsuitable but can be modified.     

Effect of high-flow nasal oxygen on hypoxaemia during procedural sedation: a systematic review and meta-analysis

We conducted a systematic review to evaluate the effect of high-flow nasal oxygen and conventional oxygen therapy during procedural sedation amongst adults and children. We searched MEDLINE, EMBASE and CINAHL for randomised controlled trials that reported the effects of high-flow nasal oxygen during procedural sedation. The primary outcome measure was hypoxaemia and the secondary outcomes were minimum oxygen saturation; hypercarbia; requirement for airway manoeuvres; and procedure interruptions. The quality of evidence was assessed using the revised Cochrane risk-of bias tool and grading of recommendations, assessment, development and evaluation (GRADE). Nineteen randomised controlled trials (4121 patients) including three in children were included. Administration of high-flow nasal oxygen reduced hypoxaemia, risk ratio (95%CI) 0.37 (0.24–0.56), p < 0.001; minor airway manoeuvre requirements, risk ratio (95%CI) 0.26 (0.11–0.59), p < 0.001; procedural interruptions, risk ratio (95%CI) 0.17 (0.05–0.53), p = 0.002; and increased minimum oxygen saturation, mean difference (95%CI) 4.1 (2.70–5.50), p < 0.001; as compared with the control group. High-flow nasal oxygen had no impact on hypercarbia, risk ratio (95%CI) 1.24 (0.97–1.58), p = 0.09, I² = 0%. High-flow nasal oxygen reduced the incidence of hypoxaemia regardless of the procedure involved, degree of fractional inspired oxygen, risk-profile of patients and mode of propofol administration. The evidence was ascertained as moderate for all outcomes except for procedure interruptions. In summary, high-flow nasal oxygen compared with conventional oxygenation techniques reduced the risk of hypoxaemia, increased minimum oxygen saturation and reduced the requirement for airway manoeuvres. High-flow nasal oxygen should be considered in patients at risk of hypoxaemia during procedural sedation.     

Equipment and monitoring in paediatric anaesthesia

Advances in paediatric anaesthetic equipment and monitoring continue to be made. While the mainstay of airway intubating equipment currently is the direct laryngoscope, video laryngoscopes and endoscopes are increasing in their use. These continue to evolve, generating better quality pictures, with more sophisticated yet easier to use equipment. Vascular access in paediatric anaesthesia can be challenging with an increasing number of patients presenting with difficult intravenous access. Ultrasound has become an integral piece of equipment in the management of these children. As the population increases in weight, so the management of the obese paediatric patient is now a reality. This requires thought and careful planning of their perioperative care. Newer techniques such as high-flow nasal oxygen are useful both to prevent hypoxia at induction, but also to facilitate open airway surgery. Neurological monitoring in the form of near infrared spectroscopy and bispectral index are discussed with evidence relevant to paediatric practice. This article will discuss all of these devices and techniques with particular emphasis on paediatric anaesthetic practice.     

Evaluating the ventilatory effect of transnasal humidified rapid insufflation ventilatory exchange in apnoeic small children with two different oxygen flow rates: a randomised controlled trial*

Transnasal humidified rapid insufflation ventilatory exchange prolongs safe apnoeic oxygenation time in children. In adults, transnasal humidified rapid insufflation ventilatory exchange is reported to have a ventilatory effect with PaCO₂ levels increasing less rapidly than without it. This ventilatory effect has yet to be reproduced in children. In this non-inferiority study, we tested the hypothesis that children weighing 10–15 kg exhibit no difference in carbon dioxide clearance when comparing two different high-flow nasal therapy flow rates during a 10-min apnoea period. Following standardised induction of anaesthesia including neuromuscular blockade, patients were randomly allocated to high-flow nasal therapy of 100% oxygen at 2 or 4 l.kg⁻¹.min⁻¹. Airway patency was ensured by continuous jaw thrust. The study intervention was terminated for safety reasons when SpO₂ values dropped < 95%, or transcutaneous carbon dioxide levels rose > 9.3 kPa, or near-infrared spectroscopy values dropped > 20% from their baseline values, or after an apnoeic period of 10 min. Fifteen patients were included in each group. In the 2 l.kg⁻¹.min⁻¹ group, mean (SD) transcutaneous carbon dioxide increase was 0.46 (0.11) kPa.min⁻¹, while in the 4 l.kg⁻¹.min⁻¹ group it was 0.46 (0.12) kPa.min⁻¹. The upper limit of a one-sided 95%CI for the difference between groups was 0.07 kPa.min⁻¹, lower than the predefined non-inferiority margin of 0.147 kPa.min⁻¹ (p = 0.001). The lower flow rate of 2 l.kg⁻¹.min⁻¹ was non-inferior to 4 l.kg⁻¹.min⁻¹ relative to the transcutaneous carbon dioxide increase. In conclusion, an additional ventilatory effect of either 2 or 4 l.kg⁻¹.min⁻¹ high-flow nasal therapy in apnoeic children weighing 10–15 kg appears to be absent.     

Use of a gum elastic bougie to facilitate blind nasotracheal intubation in children: a series of three cases

Management of a difficult paediatric airway is challenging, and the unavailability of a paediatric fibreoptic bronchoscope, a common limitation in developing countries, adds to these difficulties. Children with bilateral temporomandibular joint ankylosis have limited mouth opening and therefore direct laryngoscopy and intubation is not usually possible. In the absence of sophisticated fibreoptic equipment, blind nasal intubation remains the only non-surgical option for control of the airway. Blind nasal intubation in paediatric anaesthesia is difficult. We describe a novel method of blind nasal intubation in paediatric patients using a gum elastic bougie. We have used this method successfully in three patients in whom tracheal intubation using a conventional blind nasal approach was unsuccessful. In view of its reliability and the absence of any soft tissue injury, we propose the use of this novel technique as an alternative to conventional blind nasal intubation, when more sophisticated fibreoptic equipment is not available.     

Determining the effective pre-oxygenation interval in obstetric patients using high-flow nasal oxygen and standard flow rate facemask: a biased-coin up–down sequential allocation trial

Using biased-coin sequential allocation, we sought to determine the effective time interval in 90% of healthy parturients to achieve a target endpoint end-tidal oxygen of ≥ 90% using standard flow rate facemask and high-flow nasal oxygen. Eighty healthy parturients were randomly assigned to standard facemask (n = 40) or high-flow nasal oxygen (n = 40) groups; half of the parturients in the high-flow nasal oxygen group also used a simple no-flow facemask to minimise air entrainment. The effective time interval for 90% of parturients to achieve the target endpoint for standard facemask was 3.6 min (95%CI 3.3–6.7 min), but could not be estimated for the high-flow nasal oxygen groups with or without an additional simple facemask, as eight minutes was insufficient to achieve the target endpoint for 55% and 92% of parturients, respectively. Furthermore, after three minutes, the target endpoint was reached by 71% in the standard facemask group vs. 0% in the high-flow nasal oxygen groups. After four minutes, the target endpoint was reached by 100% in the standard facemask, 80% in the high-flow nasal oxygen with simple facemask and 67% in the high-flow nasal oxygen groups. Beyond four minutes, there was no improvement in pre-oxygenation success using high-flow nasal oxygen. In conclusion, under the conditions of our study, the effective time interval for 90% of parturients to achieve an end-tidal oxygen ≥ 90% for standard flow rate facemask was estimated to be 3.6 min, but could not be estimated for high-flow nasal oxygen groups even after eight minutes.     

Transtracheal jet ventilation

Transtracheal jet ventilation (TTJV) is a method of lung ventilation via a narrow-bore catheter placed percutaneously into the trachea, bypassing the upper airway and glottis. A pressurized jet of oxygen that entrains air is delivered to the lungs. This technique can be utilized in both elective and emergency situations. It is no longer recommended by the Difficult Airway Society (DAS) in the ‘Can't Intubate, Can't Oxygenate’ (CICO) situation, as a surgical technique is now recommended in this scenario (unless the practitioner is familiar with TTJV). In the non-CICO emergency situation, TTJV may be utilized to preoxygenate the patient and allow time to secure a definitive airway. Humidified high-flow nasal oxygenation is a more recently described technique that may allow preoxygenation and time for a transtracheal catheter to be placed, and even replace the need for TTJV. However, a jet ventilator should be immediately available in the event of failure of the nasal high-flow system. This article will examine the indications for TTJV and physiology behind its mechanism of action. We will also describe the equipment required, technique and potential complications.     

The theoretical basis for using apnoeic oxygenation via the non-ventilated lung during one-lung ventilation to delay the onset of arterial hypoxaemia

At the time one-lung ventilation is initiated, nitrogen from the atmosphere may enter the non-ventilated lung via a double-lumen tube connector that has been left open to air, even momentarily. Ongoing oxygen uptake from the non-ventilated lung raises the partial pressure of nitrogen. This should lead to activation of hypoxic pulmonary vasoconstriction and a reduction in intra-pulmonary shunting. However, in spite of this, some patients still become hypoxaemic. In such cases, it may be advantageous to have excluded nitrogen from the non-ventilated lung by connecting it to an oxygen source at ambient pressure. Ongoing apnoeic oxygenation, while the airways are patent, and as the lung collapses, should delay the onset of arterial desaturation. In this paper we review the theoretical basis for apnoeic oxygenation during one-lung ventilation, and in particular on oxygen uptake by the non-ventilated lung prior to and during its subsequent collapse.     

Low-flow apnoeic oxygenation for laryngeal surgery: a prospective observational study

Laryngeal surgery requires a shared airway and close collaboration between surgeon and anaesthetist in order to optimise operating conditions. Apnoeic oxygenation uses the principle of aventilatory mass flow to maintain oxygenation of pulmonary capillary blood under apnoeic conditions while minimising laryngeal movement. Concerns regarding accumulation of carbon dioxide and resultant acidaemia have limited the use of the technique. We performed a prospective study of low-flow apnoeic oxygenation for patients undergoing microlaryngoscopy under general anaesthesia in order to evaluate the ability of the technique to maintain oxygenation and determine the resultant rate of carbon dioxide accumulation. Sixty-four patients undergoing microlaryngoscopy under general anaesthesia were studied between November 2016 and December 2018. Intra-operative oxygenation was provided via a 10-French oxygen catheter placed into the trachea delivering oxygen at 0.5–1.0 l.min⁻¹. Data regarding apnoea time, peripheral oxygen saturation and venous blood gas concentrations were recorded. The mean (SD) duration of apnoea was 18.7 (7.2) min. Apnoeic oxygenation allowed successful completion of the surgical procedure in 62/64 patients. Mean (SD) rate of rise of the venous partial pressure of carbon dioxide was 0.15 (0.10) kPa.min⁻¹. Operating conditions were recorded qualitatively as being adequate in all cases. No adverse effects were reported. Low-flow intra-tracheal apnoeic oxygenation is a simple, effective and inexpensive technique to maintain oxygenation for laryngeal surgery.