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 Ca_O₂-CV_O2difference 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 Ca_O₂-CV_O2 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.

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Tension-based and content-based indices of oxygenation

Index

Calculation

Advantages

Disadvantages

References