Question 17

Outline the advantages and limitations of the A-a gradient and PaO2/FiO2 ratio as indices of pulmonary oxygen transfer. (You may tabulate your answer)

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College Answer

A-a gradient

PaO2/FiO2 ratio

Advantages

a) Bedside index,

b) easily calculated,

c) may allow the distinction between hypoventilation (normal gradient) and V/Q mismatch (raised gradient) as causes of hypoxemia

a) Bedside index

b) Easily calculated

c) Input variable in lung injury scores

Limitations

FiO2 dependent,
Age dependent

Varies with lung pathophysiology

a) Cannot distinguish
between hypoventilation and V/Q mismatch

b) P/F ratio unreliable unless FiO2 > 0.5 or PaO2 < 100

c) Not reliable in COPD because of V/Q mismatch

d) Barometric pressure dependent

Discussion

The tabulated college answer is comprehensive. Essentialy, the two indices differ slightly. Both assess the gradient of oxygen exchange. A-a gradient gives one some sort of impression of whether hypoventilation or diffusion are to blame for one's hypoxia. The PaO2/FiO2 ratio does not tell you anything about the diffusion, but is useful as a risk stratification tool for comparing the severities of hypoxic states. A-a gradient will change with age, and becomes confused by shunts. P/F ratio is also confused by shunts, and only works while the atmospheric pressure is normal (it breaks down at altitude and in hyperbaric oxygen chambers) A more detailed discussion of tension-based indices of oxygenation is carried out elsewhere. There, a table of indices is available, which closely resembles the college answer, and I reproduce it below to simplify revision.

Additionally, this whole thing seems heavily based on Chapter 18 from Oh's Manual ("Monitoring oxygenation") by Thomas J Morgan and Balasubramanian Venkatesh, where precisely this question is answered on pages 148-149.

Indices of Pulmonary Oxygen Transfer: Advantages and Limitations
Index	Calculation	Advantages	Disadvantages
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 alveolar tension by inspired O₂ fraction	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

Hess, D., and C. Maxwell. "Which is the best index of oxygenation: P (Aa) o2, Pao2/Pao2, or Pao2/Fio2." Respir Care 30 (1985): 961-963.

Chapter 18 from Oh's Manual ("Monitoring oxygenation") by Thomas J Morgan and Balasubramanian Venkatesh (p. 148-149)

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.

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Question 17

College Answer

Discussion

Index

Calculation

Advantages

Disadvantages

References