Oxygenation Indices

Reviewed and revised 6 January 2016

OVERVIEW

This page summaries different indices used to quantify pulmonary oxygenation

  • Measured intrapulmonary shunt is the gold standard test for determining pulmonary oxygen transfer
  • Indices are either tension-based or content-based (aka concentration-based)
    • tension-based indices require partial pressures of gases to be calculated from the alveolar gas equation and are subject to its limitations (e.g. estimation of the respiratory quotient)
    • content-based indices do not depend on oxygen partial pressures, but arterial oxygen content instead. unless measured directly, they are limited by variations in the arterio-venous oxygen content difference from a normal 30-50ml/L in critically ill patients

Tension-based indices include:

  • A-a gradient
  • a/A ratio
  • PaO2/FIO2 ratio
  • Respiratory index (RI) (RI = pO2(A-a)/pO2(a))

Content-based indices include:

  • F shunt

PULSE OXIMETRY OXYGEN SATURATIONS (SpO2)

Calculation

  • Derived from the difference in absorption of two infra-red light wavelengths

Pros

  • real-time monitoring
  • Non-invasive
  • requires no special expertise

Cons

  • erroneous in the presence of dyshemoglobinaemia
  • does not reflect level of oxygenation in hyperoxic patients
  • not a direct measurement of hemoglobin saturation – instead, correlates signal intensity with empirical data
  • no absolute method for calibration exists – only empirical data collected from hypoxic volunteers
  • unreliable in severely hypoxic patients
  • unreliable in poorly perfused patients
  • unreliable in arrhythmia
  • positional
  • confounded by motion artifact (unreliable in agitated patients)

PaO2

Calculation

  • Measured directly from the arterial blood by the Clark electrode in the blood gas analyser

Pros

  • accurate measure of oxygenation
  • not confounded by dyshemoglobins
  • allows accurate calculation of hemoglobin saturation

Cons

  • Invasive
  • requires arterial access expertise
  • requires blood gas analyser
  • confounded by collection error, eg. bubbles in the syringe
  • Measurement delay exists

A-a GRADIENT

Calculation

  • Alveolar gas equation

Pros

  • Simple
  • Minimally invasive
  • distinguishes alveolar hypoventilation from other causes of hypoxia (e.g. V/Q mismatch)
  • Required by APACHE II, III and IV

Cons

  • highly dependent on FiO2, especially in the presence of a large shunt
  • Age dependent (increases with age)
  • Non-specific

PF RATIO

Calculation

  • Alveolar oxygen tension (PaO2) divided by inspired oxygen fraction (FiO2)

Pros

  • Simple
  • Minimally invasive
  • Required by APACHE IV
  • Used in severity stratification of ARDS
  • Used in SMARTCOP (pneumonia severity score)

Cons

  • Cannot distinguish between alveolar hypoventilation and other causes of hypoxia (e.g. V/Q mismatch)
  • Does not account for changes in PaCO2
  • Unreliable unless FiO2 > 0.5 or PaO2 < 100
  • Not reliable in COPD because of V/Q mismatch
  • Barometric pressure dependent

a/A RATIO

Calculation

  • PaO2 divided by PAO2

Pros

  • Reasonably simple
  • Minimally invasive
  • distinguishes alveolar hypoventilation from other causes of hypoxia
  • Independent of FiO2 changes

Cons

  • Age dependent (increases with age)
  • Non-specific – influenced by numerous factors
  • Oxygen tension based index rather than oxygen content based

RESPIRATORY INDEX (R)

Calculation

  • A-a gradient divided by the PaO2

Pros

  • Reasonably simple
  • Minimally invasive
  • May distinguish alveolar hypoventilation from all other causes of hypoxia
  • Independent of FiO2 changes

Cons

  • No advantages over the a/A ratio
  • Not commonly used
  • Difficult to relate findings to management decision criteria or compare them to published studies

ESTIMATED SHUNT FRACTION

Calculation

  • Shunt equation (assuming a CaO2-CVO2 difference of ~30-50ml/L)

Pros

  • Oxygen content rather than oxygen tension based index
  • Minimally invasive – does not require mixed venous sampling
  • Independent of FiO2 and PaCO2changes

Cons

  • CaO2-CVO2 difference can be markedly deranged in critical illness resulting in invalid calculations

MEASURED INTRAPULMONARY SHUNT

Calculation

  • Shunt equation

Pros

  • Gold standard of shunt assessment
  • Empiric measurement
  • accounts for unpredictable influences on shunt

Cons

  • Maximally invasive (requires PA catheter)
  • Requires mixed venous sampling
  • Complex calculations involved

References and links

litfl.com

Journal articles

  • Wandrup JH. Quantifying pulmonary oxygen transfer deficits in critically ill patients. Acta anaesthesiologica Scandinavica. Supplementum. 107:37-44. 1995. [pubmed]

CCC 700 6

Critical Care

Compendium

Chris is an Intensivist and ECMO specialist at the Alfred ICU in Melbourne. He is also the Innovation Lead for the Australian Centre for Health Innovation at Alfred Health and Clinical Adjunct Associate Professor at Monash University. He is a co-founder of the Australia and New Zealand Clinician Educator Network (ANZCEN) and is the Lead for the ANZCEN Clinician Educator Incubator programme. He is on the Board of Directors for the Intensive Care Foundation and is a First Part Examiner for the College of Intensive Care Medicine. He is an internationally recognised Clinician Educator with a passion for helping clinicians learn and for improving the clinical performance of individuals and collectives.

After finishing his medical degree at the University of Auckland, he continued post-graduate training in New Zealand as well as Australia’s Northern Territory, Perth and Melbourne. He has completed fellowship training in both intensive care medicine and emergency medicine, as well as post-graduate training in biochemistry, clinical toxicology, clinical epidemiology, and health professional education.

He is actively involved in in using translational simulation to improve patient care and the design of processes and systems at Alfred Health. He coordinates the Alfred ICU’s education and simulation programmes and runs the unit’s education website, INTENSIVE.  He created the ‘Critically Ill Airway’ course and teaches on numerous courses around the world. He is one of the founders of the FOAM movement (Free Open-Access Medical education) and is co-creator of litfl.com, the RAGE podcast, the Resuscitology course, and the SMACC conference.

His one great achievement is being the father of two amazing children.

On Twitter, he is @precordialthump.

| INTENSIVE | RAGE | Resuscitology | SMACC

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