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)
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
c) Input variable in lung injury scores |
Limitations |
FiO2 dependent, Varies with lung pathophysiology |
a) Cannot distinguish
|
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.
Index |
Calculation |
Advantages |
Disadvantages |
A-a gradient |
|
|
|
PaO2/FiO2 ratio |
|
|
|
a/A ratio |
Arterial pO2 divided by alveolar pO2. |
|
|
Respiratory index |
|
|
|
Estimated shunt fraction (Fshunt) |
(using a CaO2-CVO2difference of around 30-50ml/L) |
|
|
Measured intrapulmonary shunt |
|
|
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.