Tension-based and Content-based Indices of Oxygenation

Question 17 from the second paper of 2007 and  Question 3 from the first  paper of 2006 interrogated the candidate's understanding of oxygenation variables. Specific issues under review were the advantages and limitations of the A-a gradient, PaO2/FiO2 ratio, PaO2 and SpO2. The college have never asked about any of the other indices (eg. the A/a ratio, or the respiratory index), nor mentioned the shunt equation.  Local digressions on these topics include the chapter on oxygen tension - based indices of oxygenation as well as the shunt equation and content - based indices of oxygenation. There is probably too much material in those.

In order to simplify revision, the indices are presented here in a tabulated format:

An alternative tabulated answer would resemble the following:

 

PaO2

SpO2

Advantages

  • accurate impression of oxygenation
  • not confounded by dyshemoglobins
  • allows accurate calculation of hemoglobin saturation
  • real-time monitoring
  • Non-invasive
  • requires no special expertise

Disadvantages

  • Invasive
  • requires arterial access expertise
  • requires blood gas analyser
  • confounded by collection error, eg. bubbles in the syringe
  • Measurement delay exists
  • confused by dyshemoglobins
  • does not reflect level of oxygenation in hyperoxic patients
  • not a direct measurement of hemoglobin saturation - instead, uses a signal intensity and a look-up table derived from 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
  • unreliable in confused patients, confounded by mostion artifact
Indices of Pulmonary Oxygen Transfer: Advantages and Limitations

Index

Calculation

Advantages

Disadvantages

SpO2 Derived from the difference in absoption of two infra-red light wavelengths
  • real-time monitoring
  • Non-invasive
  • requires no special expertise
  • confused by dyshemoglobins
  • does not reflect level of oxygenation in hyperoxic patients
  • not a direct measurement of hemoglobin saturation - instead, uses a signal intensity and a look-up table derived from 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
  • unreliable in confused patients, confounded by mostion artifact
PaO2

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

  • accurate impression of oxygenation
  • not confounded by dyshemoglobins
  • allows accurate calculation of hemoglobin saturation
  • Invasive
  • requires arterial access expertise
  • requires blood gas analyser
  • confounded by collection error, eg. bubbles in the syringe
  • Measurement delay exists

A-a gradient

Alveolar gas equation

  • Simple
  • Minimally invasive
  • May distinguish alveolar hypoventilation from all other causes of hypoxia
  • Required by APACHE II, III and IV
  • The magnitude of the A-a gradient is highly dependent on FiO2, especially in the presence of a large shunt
  • Age dependent (increases with age)
  • Non-specific - influenced by numerous factors

PaO2/FiO2 ratio

Divison of inspired O2fraction by alveolar tension

  • Simple
  • Minimally invasive
  • Required by APACHE IV
  • Used in severity stratification of ARDS
  • Cannot distinguish between alveolar hypoventilation and other causes of hypoxia
  • Makes no attempt to incorporate 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

Arterial pO2 divided by alveolar pO2.

  • Reasonably simple
  • Minimally invasive
  • May distinguish alveolar hypoventilation from all other causes of hypoxia
  • Independent of FiO2 changes
  • Age dependent (increases with age)
  • Non-specific - influenced by numerous factors
  • Oxygen tension based index;
Respiratory index

A-a gradient divided by the PaO2

  • Reasonably simple
  • Minimally invasive
  • May distinguish alveolar hypoventilation from all other causes of hypoxia
  • Independent of FiO2 changes
  • No addiitonal 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 (Fshunt)

Shunt equation

(using a CaO2-CVO2difference of around 30-50ml/L)

  • Oxygen content rather than oxygen tension based index
  • Minimally invasive- does not require mixed venous sampling
  • Independent of FiO2 and PaCO2changes
  • Assigned CaO2-CVO2 difference can be completely incorrect in critical illness, completely invalidating the calculations.
Measured intrapulmonary shunt

Shunt equation

  • Gold standard of shunt assesment
  • Empiric measurement; accounts for unpredictable influences on shunt.
  • Maximally invasive (requires PA catheter)
  • Requires mixed venous sampling
  • Complex calculations involved

 

References

Curran-Everett, Douglas. "A classic learning opportunity from Fenn, Rahn, and Otis (1946): the alveolar gas equation." Advances in physiology education 30.2 (2006): 58-62.

Rice, Todd W., et al. "Comparison of the SpO2/FIO2 ratio and the PaO2/FIO2 ratio in patients with acute lung injury or ARDS." CHEST Journal 132.2 (2007): 410-417.

Hess, D., and C. Maxwell. "Which is the best index of oxygenation: P (Aa) O2, PaO2/PAO2, or PaO2/FIO2?." Respiratory Care 30.11 (1985): 961-963. - this is not available even as an abstract; Respiratory Care dont seem to care about online back-issues beyond 2003.

Cane, Roy D., et al. "Unreliability of oxygen tension-based indices in reflecting intrapulmonary shunting in critically ill patients." Critical care medicine 16.12 (1988): 1243-1245.

Wandrup, J. H. "Quantifying pulmonary oxygen transfer deficits in critically ill patients." Acta Anaesthesiologica Scandinavica 39.s107 (1995): 37-44.

Hahn, C. E. W. "Editorial I KISS and indices of pulmonary oxygen transfer."British journal of anaesthesia 86.4 (2001): 465-466.

Zander R, Mertzlufft F, eds. The Oxygen Status of Arterial Blood. Würzburg, Germany: Bonitas‐Bauer, 1991

Nirmalan, M., et al. "Effect of changes in arterial‐mixed venous oxygen content difference (C (a–v̄) O2) on indices of pulmonary oxygen transfer in a model ARDS lung†,††." British journal of anaesthesia 86.4 (2001): 477-485.

LAGHI, FRANCO, et al. "Respiratory index/pulmonary shunt relationship: Quantification of severity and prognosis in the post-traumatic adult respiratory distress syndrome." Critical care medicine 17.11 (1989): 1121-1128.

Zetterström, H. "Assessment of the efficiency of pulmonary oxygenation. The choice of oxygenation index." Acta anaesthesiologica scandinavica 32.7 (1988): 579-584.

Liliethal JL, Riley RL, Prommel DD, et al: "An experimental analysis in man of the oxygen pressure gradient from alveolar air to arterial blood" Am J Physiol 1946; 147:199-216

Gilbert, R., and J. F. Keighley. "The arterial-alveolar oxygen tension ratio. An index of gas exchange applicable to varying inspired oxygen concentrations."The American review of respiratory disease 109.1 (1974): 142.

Viale, JEAN-PAUL, et al. "Arterial-alveolar oxygen partial pressure ratio: a theoretical reappraisal." Critical care medicine 14.2 (1986): 153-154.

PERIS, LUIS V., et al. "Clinical use of the arterial/alveolar oxygen tension ratio." Critical care medicine 11.11 (1983): 888-891.

Gowda, Madhu S., and Robert A. Klocke. "Variability of indices of hypoxemia in adult respiratory distress syndrome." Critical care medicine 25.1 (1997): 41-45.

Doyle, D. John. "Arterial/alveolar oxygen tension ratio: a critical appraisal."Canadian Anaesthetists’ Society Journal 33.4 (1986): 471-474.

McFarlane, Michael J., and Thomas F. Imperiale. "Use of the alveolar-arterial oxygen gradient in the diagnosis of pulmonary embolism." The American journal of medicine 96.1 (1994): 57-62.

Jones, Jeffrey S., Timothy L. Neff, and Scott A. Carlson. "Use of the alveolar-arterial oxygen gradient in the assessment of acute pulmonary embolism." The American journal of emergency medicine 16.4 (1998): 333-337.