Oxygenation indices

Regarding oxygenation indices the following statements are correct EXCEPT for which

one?

a) Calculating venous admixture requires a pulmonary artery flotation catheter

b) A PaO2:FiO2 ratio <26.6 kPa is a criterion for diagnosis of ARDS

c) P (a – a) O2 is the respiratory index

d) Ideally an oxygenation index should not vary with changes in FiO2

e) The alveolar gas equation is required for a number of oxygenation indices

Answer: c

Explanation

Maintaining a patient’s oxygenation is high on an anaesthetist’s list of priorities. It is

reasonable to expect knowledge of the quantification of adequacy of oxygenation and

gas exchange. It is also important to appreciate the limitations of each index in truly

representing the performance of the lung. Venous admixture may be defined as ‘that

degree of true right-to-left shunt that would explain the observed difference in alveolar

and arterial oxygen levels’. ‘Levels’ is used deliberately here because although the

equation uses oxygen contents in its calculation, venous admixture can be used to

quantify observed differences in tension, saturation or content. It is also widely

regarded as the gold-standard oxygenation index, even if the source of deficient gas

exchange is not primarily due to shunt. It does require true mixed venous blood

content, which is found in the pulmonary artery. Central venous oxygen content

may be substituted (with a correction factor) to give an approximation to the value.

P (a – a) O2 is the alveolar-arterial oxygen difference and may be expected to be less than 2kPa in a young healthy individual but does increase with age. (P (a – a) O2) / PaO2 is the respiratory index and attempts to eliminate the variation of the index with FIO2. These two indices and the PaO2/PaO2 ratio all rely on the assumptions of the alveolar gas equation.

The ideal oxygenation index should be a representation of the function of the lung

and not be influenced by external factors that we may manipulate (like FiO2). The

index PaO2:FiO2 attempts to accommodate this. If inspired oxygen is manipulated,

oxygen tension should change accordingly thus the index of oxygenation should

remain constant.

Reference

Armstrong J, Guleria A, Girling K. Evaluation of gas exchange deficit in the critically ill.

Contin Educ Anaesth Crit Care Pain 2007; 7(4): 131–4.