“Incidence and risk factors of hypoxaemia after preoxygenation at induction of anaesthesia” Baillard et al. British Journal of Anaesthesia, 122 (3): 388e394 (2019)

Background

• Hypoxaemia most common form cause of death at induction of anaesthesia
• 2010 retrospective study demonstrated hypoxaemia (SpO₂≤ 90%) in 7.4% of patients at induction
• Hypoxaemia known to occur more quickly if preoxygenation omitted

Aim

To determine incidence of hypoxaemia during induction of anaesthesia, and its contributory factors in a large population after a standardised preoxygenation procedure

Methods

• Multicentre prospective analysis (France; 2 teaching hospitals and one DGH)
• 6 month inclusion period
• Orthopaedic, vascular, gynaecological, ENT and abdominal surgery; GI endoscopy
• Omitted: thoracic surgery, haemodynamic instability, O₂ requirement

Preoxygenation technique:

• Supine EXCEPT obese patients (head-up 10-25°)
• O₂flow 12L/min
• TV breathing 3-5 mins until FeO₂≥ 90%
• Face mask adjusted according to capnograph

Outcomes

PRIMARY – Incidence of hypoxaemia (SpO₂≤ 95%) 

SECONDARY – Incidence of difficult preoxygenation (FeO₂< 90%)

Results

2398 patients included in study

Hypoxaemia observed in 158 patients (6.6%)

Severe hypoxaemia observed in 34 patients (1.4%)

At 3 min, difficult preoxygenation in 1202 patients (50%)

At 5 min, difficult preoxygenation in 723 patients (30%)

5 independent risk factors for hypoxaemia: COPD, HTN, anticipated difficult FMV, anticipated difficult intubation, emergency

5 independent risk factors for difficult preoxygenation: male, COPD, HTN, emergency, anticipated difficult FMV

Can anticipate difficult MV/intubation with a high negative predictive value

Discussion

• Large patient cohort, representative of our patient population
• Significant incidence of difficult preoxygenation (30%)
• Significant incidence of hypoxaemia given strict preoxygenation regimen
• Anaesthetists need to become familiar with techniques to improve preoxygenation

Limitations

• Exclusion of haemodynamically unstable/O₂requiring patients
• Likely underestimate of hypoxaemia due to preoxygenation
• Hypoxaemia definition SpO₂≤ 95% – not powered for hypoxaemia ≤ 90%
• Vague definitions of difficult FM ventilation/intubation 
• Data collected by anaesthetist
• No differentiation for starting SpO₂of COPD patients
• Emergency surgery as a risk factor for hypoxaemia likely related to RSI
• Definition of RSI and variability of techniques unclear 

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Summary by Dr Adam Patrick