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 Ca_O2-CV_O2 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

• Ca_O2-CV_O2 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

• CCC — PaO2/FiO2 Ratio

Journal articles

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