Apnoeic Oxygenation When Intubating (LITFL)

From Life in The Fast Lane 

Reviewed and revised 10 January 2016
OVERVIEW
Apnoeic oxygenation is used to extend the ‘safe apnoea time’ beyond that which can be achieved by preoxygenation alone

• Apnoeic oxygenation is merely an adjunct, it is not a substitute for effective preoxygenation
• Apnoeic oxygenation is most commonly provided using nasal cannulae in addition to a face mask
  
  

SAFE APNOEA TIME
Safe apnoea time is the duration of time until critical arterial desaturation (SaO2 88% to 90%) occurs following cessation of breathing/ventilation

• an alternative term in use is duration of apnoea without desaturation (DAWD)
• SaO2 88% to 90% marks the upper inflection point on the oxygen-haemoglobin dissociation curve beyond which further decreases in PaO2 leads to a rapid decline in SaO2 (~ 30% every minute)
• In a healthy preoxygenated patient the safe apnea time can be up to 8 or 9 minutes, compared to ~1 min if they were breathing room air
• In critically ill patients critical desaturation can occur almost immediately despite optimal attempts at preoxygenation
  
  

Factors that decrease safe apnoea time include:

• inadequate preoxygenation
• airway occlusion (see below)
• pulmonary shunt
• increased oxygen consumption (VO2) (e.g. high metabolic rate, fasciculations from suxamethonium)
• critical illness
• obesity
• pregnancy
• small children

In patients who develop airway occlusion, desaturation will occur more rapidly due to loss of functional residual capacity (FRC)

• FRC usually decreases by 200-250 mL during the first minute after airway occlusion in a healthy adult patient
• This occurs due to resorption atelectasis as oxygen transfers from the lungs into the pulmonary circulation
• Pulmonary shunting (blood flow to the non-oxygenated, collapsed lung units) can now occur resulting in much more rapid desaturation than otherwise predicted — even when effective preoxygenation is performed
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PHYSIOLOGY OF APNOEIC OXYGENATION
Alveoli will continue to take up oxygen even without diaphragmatic movements or lung expansion

• In a healthy apnoeic patient, ~200-250 mL/min oxygen will move from the alveoli into the bloodstream
• Only ~8-20 mL/min of carbon dioxide moves into the alveoli during apnea, with the remainder being buffered in the bloodstream given the high water solubility of CO2
• This causes the net pressure in the alveoli to become subatmospheric, generating a mass flow of gas from pharynx to alveoli

In healthy people under ideal circumstances, PaO2 can be maintained at >100 mm Hg for up to 100 minutes without a single breath, although the lack of ventilation will eventually cause marked hypercapnia and significant acidosis

• this assumes effective preoxygenation, ongoing provision of high flow oxygen and maintenance of a patent airway

Nasal cannulae can be used for apnoeic oxygenation because the pharynx fills with high FiO2 gas and functions as an oxygen reservoir, even when the mouth is open (a patent airway must be maintained!)
PROCEDURE

• ensure patient is preoxygenated with nasal cannula in situ (15 L/min oxygen flow rate) (see Preoxygenation)
• administer induction agent
• maintain the nasal cannula flow rate to 15 L/min and adminster oxygen via non-rebreather mask or BVM as well
• If SpO2 <95% consider apneic oxygenation with positive pressure
  • CPAP or with BVM with PEEP valve with coexistent administration of oxygen at 15 L/min via nasal cannulae
  • alternatively, abandon apnoeic oxygenation and provide 6 gentle ventilations/minute (<15cmH20) if the risk:benefit of apnoea is unfavourable)
• maintain a patent airway until the time of intubation using:
  • 2-handed face-mask technique with jaw thrust
  • nasopharnygeal airway(s)
  • oropharyngeal airway
• remove the mask at the time of intubation, but continue to oxygenate via the nasal cannulae
  • this has been termed “NO DESAT” (nasal oxygen during efforts securing a tube) by Rich Levitan

EVIDENCE
Tracheal oxygenation

• Half a dozen observational studies with small numbers of patients showing that safe apnoea times can be prolonged up to 55 minutes with apnoeic oxygenation provided by tracheal catheters or equivalent devices
• This technique is widely used in ICU patients during the apnea test for brain death
• More recently Rudlof and Hohenhorst (2013) found that apnoeic oxygenation via a tracheal catheter was well tolerated during ENT procedures for up to 45 minutes in over 40 patients. Exceptions were 2 patients where the technique was not performed correctly and 1 obese pateint who could not be adequately oxygenated

Additional studies of apnoeic oxygenation using nasal cannulae are described below.
FELLOW trial, 2015

• Single-centre, randomised controlled trial
  • Allocation concealment, treating clinicians non-blinded
  • Data collection was by independent observers unaware of the study design
  • intention to treat analysis
• Population
  • n=150 Medical ICU patients at Vanderbilt 18+ years old intubated by a pulmonary and critical care medicine fellow
  •  Additional 46 were excluded
    • 23 required intubation too urgently, 18 were felt to require video or FO intubation, 1 was felt to require direct laryngoscopy, 1 was felt to require apneic oxygenation, 3 excluded for unknown reasons.
• Intervention/ Comparison
  • apneic oxygenation (n=73)
  • usual care (n=73)
  • patients were also randomised to video laryngoscopy or directed laryngoscopy
• Outcomes (apox vs usual care)
  • No statistically significant difference in:
    • Median lowest arterial oxygen saturation: 92% vs  90% (95% CI 1.6 – 7.5%; p = 0.16)
    • Incidence of oxygen saturation <90%: 44.7% vs 47.2% (p = 0.87)
    • Incidence of oxygen saturation <80%: 15.8% vs 25.0% (p = 0.22)
    • Incidence of decrease in oxygen saturation >3%: 53.9% vs 55.6% (p = 0.87)
    •  duration of mechanical ventilation, ICU length of stay, and in-hospital mortality
• Commentary and criticisms:
  • Study design
    • Pragmatic design with other treatments left to treating physicians
    • had 80% power to detect a mean lowest arterial oxygen saturation difference of 4.6%
    • Lack of blinding (could not be avoided)
  • High likelihood of selection bias due to difficult airways/ sicker patients being excluded from the trial:
    • patients deemed to benefit from a particular strategy (e.g. suspected difficult airways or prolonged intubation times – the precise patients expected to benefit from apnoeic oxygenation)
    • true emergencies and could not be randomised were excluded
    • 75% of intubations were rated “easy” – these patients are unlikely to benefit from apnoeic oxygenation
  • Lack of external validity
    • Most patients in both arms were not truly apnoeic prior to laryngoscopy
      • 73% of patients received either BVM or NIV up until laryngoscopy
      • this is a critical flaw – these patients clearly will not benefit ‘apnoeic oxygenation’ as they are being ventilated!
    • Airway patency must be strictly maintained during apnoeic oxygenation for it to work, specific measures on how to achieve this was not part of the protocol and data on airway patency is not provided for the ~2/3 patients who were not receiving NI
    • May not be generalisable outside of PulmCCM Fellow led medical ICUs (patient and skill mix)
    • A study powered to detect differences in reaching critical desaturation, rather than absolute difference on SpO2, may be more clinically relevant
• Conclusion
  • This study found no benefit or harm with using nasal cannulae for apnoeic oxygenation in medical ICU patients requiring intubation. Apnoeic oxygenation is only likely to benefit patients with difficult airways and prolonged intubation times who are truly apnoeic prior to intubation. These patients were under-represented in this study which greatly limits it’s external validity.

Dyett et al, 2015

• prospective observational study of intubations outside of the operating theatre
• no episodes of hypoxaemia in the 31 patients that used apnoeic oxygenation with nasal prongs, whereas 10 of 60 patients that did not have apnoeic oxygenation developed hypoxaemia
• The calculated NNT for apnoeic oxygenation is 6
• There may be important confounders, given the observational design of this study

Wimalesena et al, 2015

• Retrospective analysis of 9,901 NSW HEMS missions
• 728 rapid sequence intubations (310 pre- and 418 postapneic oxygenation)
• introduction of apneic oxygenation (with nasal prongs) was followed by a decrease in desaturation rates from 22.6% to 16.5% (ARR=6.1%; 95% CI 0.2% to 11.2%)
• Subject to multiple confounders due to retrospective study design

THRIVE, 2014

• 25 patients who underwent general anaesthesia for hypopharyngeal or laryngotracheal surgery; included 12 obese patients and 9 patients were stridulous
• patients had “Transnasal Humidified Rapid-Insufflation Ventilatory Exchange”: continuous delivery of transnasal high-flow humidified oxygen, initially to provide pre-oxygenation, and continuing as post-oxygenation during IV induction of anaesthesia and neuromuscular blockade until a definitive airway was secured. Airway was kept open with jaw thrust
• The median apnoea time was 14 (5-65) min, no patient experienced arterial desaturation < 90%

Ramachandran et al, 2010

• RCT of obese operative patients, n=30
• Nasal cannula attached to 5 L/min 100% FiO2 versus room air
• Apnoeic oxygenation group had significant prolongation of SpO2 ≥95% time (5.29 versus 3.49 min), a significant increase in patients with SpO2≥95% at the 6-min mark (8 patients versus 1 patient), and significantly higher minimum SpO2 (94.3% versus 87.7%)
• not blinded

Taha et al, 2006

• RCT, n=30
• Nasal catheters attached to 5L/min of 100% FiO2 versus room air
• No desaturation during the course of the 6-min predetermined stopping point in patients receiving apnoeic oxygenation, whereas the control group desaturated to the study cutoff of 95% in an average of 3.65 min
• not blinded

Teller et al, 1988

• RCT, blinded, crossover trial, n=12
• Nasopharyngeal catheters attached to 100% FiO2 at 3 L/min versus room air
• None of the patients in the insufflation arm desaturated below 98% during the 10 min

Evidence for apnoeic oxygenation in humans prior to 2012 is summarised here (from Levitan and Weingart, 2012)
OTHER INFORMATION
In addition to intubation, apnoeic oxygenation is also used in other circumstances:

• during brain death studies
• during bronchoscopy
• at the end of elective anestheisa cases when allowing CO2 to build up and stimulate spontaneous breathing

Nasal cannula

• Nasal cannulae at 15 L/min has no significant adverse affects in the sedated patient with short term use, and is even well tolerated in awake patients (Brainard et al, 2015). The desiccating effect is uncomfortable in the longer term in awake patients.
• Although flow meters for wall oxygen have 15 L/min as the maximum setting (gives FiO2 0.6 via non-rebreather) if over-dialed up to 30-60 L/min flow can be achieved (gives FiO2 0.8-0.9 via non-rebreather mask)
• If nasal cannulae compromise the seal of the face mask, they can be placed above the face mask until just prior to attempting laryngoscopy, at which point they are placed in the nares
• High flow nasal cannulae  have been used for apnoeic oxygenation (e.g. THRIVE study) however they are more likely to impair face mask seal during preoxygenation and some devices do not generate high enough FiO2.

Bag-Valve-Mask (BVM) with PEEP valve for apnoeic oxygenation

• A BVM with a PEEP valve will only provide PEEP when the patient expires, so positive pressure is not provided in apnoeic patients.
• If the patient is ventilated PEEP will only be transiently applied as the patient expires.
• However, continuous positive airways pressure (CPAP) (approx 6 cmH2O) is generated when 15L/min O2 via nasal cannulae are applied to an apnoeic patient in addition to a BVM with a PEEP valve set at 10 cm H20. This is shown below: