Question 2

College answer

Syllabus topic/section: 2.1.5 Respiratory Intensive Care / Oxygen delivery systems and mechanical ventilatory support:L1

Discussion: Candidate answers demonstrated a sound knowledge of the evidence in general. The more successful answers focussed on a broad range of specific oxygen toxicities and demonstrated a knowledge of different patient subgroups at risk of oxygen toxicity. Examples of these subgroups include OOHCA, TBI, Sepsis, Bleomycin exposure and sickle cell disease. With regards to evidence, candidates were not required to recite details or the names of multiple trials to pass, however discussion of at least one trial OR knowledge of summated results of trials were expected. Answers highlighting differing results from various relevant trials and for specific sub- groups were rewarded.

Discussion

• Definitions
  • Oxygen saturation is usually the parameter used to define this, but PaO₂ or P/F ratio are occasionally used.
  • "Conservative" is usually SpO₂ 90-92% or 88-92%
  • "Liberal" is usually 96-100% or 98-100%
  • Conventional oxygenation targets for most patients fall into the range of 94-98% (BTS 2017 guidelines) 
• Advantages of liberal oxygenation targets
  • More likely to meet unrecognised oxygen requirements
  • Buffer against respiratory accidents (eg. self-extubation) by maintaining an oxygen-rich FRC
  • Buffer against undershot targets (i.e. if the high goals are not met the oxygenation would still be acceptable)
  • Decreased work of breathing
  • Decreased staff cognitive workload in titrating oxygen
  • Protection of marginally perfused organs (eg. myocardium, renal medulla)
  • Accidentally therapeutic effects (eg. where it leads to improved reabsorption of pneumnothoraces or treats carbon monoxide toxicity)
  • Some patient populations may have a mortality benefit with higher targets 
• Advantages of conservative oxygenation targets
  • Reduced risk of oxygen toxicity and free radical mediated organ damage
  • Prevention of hyperoxic vasoconstriction, improved microcirculatory function
  • Prevention of absorption atelectasis
  • Observation of respiratory disease progress through titration of FiO₂
  • Reduced risk of CO₂ accumulation in unrecognised COPD
  • Reduced cost of healthcare
  • Some patient populations and some rare toxidromes (paraquat, bleomycin) may have a mortality benefit with lower targets 
• Evidence to describe these practices:
  • Early data from studies like de Jonge et al (2008) produced the impression that the relationship between oxygen targets and mortality was U-shaped, i.e. both high and low targets worsened mortality
  • Specific subgroups (cardiac arrest survivors, TBI, sepsis patients, myocardial infarction) appeared to have unique requirements
  • Multiple trials have produced equivocal results
  • Those that favour higher targets:  LOCO2 (ARDS), ICU-ROX sepsis cohort
  • Those that favour lower targets: OXYGEN-ICU, ICU-ROX post-arrest cohort 
  • Those that demonstrated no difference: HOT-ICU, PILOT, BOX, EXACT, ICONIC
  • In summary, most guidelines still recommend 94-98%, an 88-92% for patients with COPD.