Effect of Cricoid Pressure Compared With a Sham Procedure in the Rapid Sequence Induction of Anaesthesia: The IRIS Randomized Clinical TrialAurélie Birenbaum et al., JAMA Surg 2018 Oct 17. doi: 10.1001/jamasurg.2018.3577

“We can’t act under pressure from anybody when we think that pressure will force an injustice” – José Sulaimán

Background

• Cricoid pressure was first suggested in 1961 by Sellick to combat regurgitation during head down positioning for intubation.
  • Sellick described “tonsillectomy position” head extension to straighten oesophagus, with pressure on cricoid cartilage to occlude oesophagus against C6 vertebral body
  • “Moderate” pressure whilst patient awake, increased to “firm” once asleep
• There is no level 1 evidence for efficacy, wide international variation in adoption
• Cricoid pressure remains standard of care during RSI according to UK guidelines e.g. DAS
• There are some concerns that cricoid pressure may worsen laryngoscopic view.

Aims

• The IRIS trial aimed to demonstrate that a sham procedure mimicking cricoid pressure was not inferior to the real thing in preventing pulmonary aspiration

Methods

• Sham vs real cricoid pressure in patients presenting to theatre for emergency surgery under GA, with risk factors for aspiration
• Ten academic hospitals in France, 1:1 randomisation in blocks of 6 to cricoid or sham
• A total of 3472 patients were recruited; 1729 patients in Sellick group, 1730 in sham group
• Intubators & patients blinded to allocation
• Exclusions: pregnancy, pneumonia, “consciousness disorder”

Statistical design: Non-inferiority RCT

• Authors decided that at worst, sham pressure could have 1.5x higher risk of aspiration and still be deemed not inferior
• This meant that upper limit of 95% confidence interval needed to be ≤1.5
• 1:1 randomisation in blocks of 6
• Powered for 2.8% incidence of aspiration (based on Martin et al, Anaesthesiology 2011)

Results

• Primary outcome: pulmonary aspiration, per-protocol analysis
  • 10/1729 cricoid pressure patients aspirated (0.6%)
  • 9/1730 sham pressure patients aspirated (0.5%)
  • RR 0.90 (95% CI 0.39-2.00)
• Secondary outcomes:
  • Intubation time (seconds, IQR): 27(19-40) cricoid vs 23(15-37) sham
  • Incidence of CL view 3 or 4: cricoid 8%, sham 4%

Discussion

• Based on pre-specified threshold, authors conclude sham pressure is not “non-inferior” to cricoid pressure
• They failed to show sham pressure is no worse than cricoid pressure, according to their own definition
• The overall incidence of aspiration significantly lower than expected
• Although a secondary outcome, the IRIS trial reports that cricoid worsens laryngoscopic view
• Strengths:
  • Standardised anaesthetic technique and application of cricoid by trained operators
  • Randomisation & blinding techniques
  • No significant between group differences at baseline
• Flaws:
  • There was a large overestimation of the risk of aspiration risk which meant that the study was underpowered
  • Theatients at highest risk of desaturation, difficult airway, aspiration were excluded
    • Pregnancy, ICU/ED, cardiac arrest, low GCS, pneumonia
  • Suxamethonium was used rather than rocuronium which means the results are possibly less applicable in some centres

Should this trial change our practice?

• This trial does not inform most of the highest risk situations in which cricoid is employed
• IRIS trial does not seriously undermine either of the two camps: cricoid is dangerous (worse view, ineffective) or necessary (not dangerous, might prevent aspiration in higher risk patients, can always be removed)
• Cricoid pressure remains recommended by DAS and used routinely in UK obstetric practice (Desai IJOA 2018)

Summary by Dr Charles McFadyen