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British Journal of Anaesthesia, 2002, Vol. 88, No. 6 771-778
© 2002 The Board of Management and Trustees of the British Journal of Anaesthesia

Clinical Investigations

Variation of venous admixture, SF6 shunt, PaO2, and the PaO2/FIO2 ratio with FIO2

J. P. Whiteley1,2, D. J. Gavaghan1,2 and C. E. W. Hahn*,1

1Nuffield Department of Anaesthetics, University of Oxford, Radcliffe Infirmary, Woodstock Road, Oxford OX2 6HE, UK. 2Oxford University Computing Laboratory, Wolfson Building, Parks Road, Oxford OX1 3QD, UK*Corresponding author

Background. Measures of impairment of oxygenation can be affected by the inspired oxygen fraction.

Methods. We used a mathematical model of an inhomogenous lung to predict the effect of increasing inspired oxygen concentration (FIO2) on: (1) venous admixture (Q·va/Q·t); (2) arterial oxygen partial pressure (PaO2); (3) the PaO2/FIO2 index of hypoxaemia; and (4) sulphur hexafluoride (SF6) retention (often taken to be true right-to-left shunt). This model predicts whether or not atelectasis will occur.

Results. For lungs with regions of low V·/Q·, increasing the inspired oxygen concentration can cause these regions to collapse. In the absence of atelectasis, the model predicts that Q·va/Q·t will decrease and arterial oxygen partial pressure increase as FIO2 is increased. However, when atelectasis occurs, Q·va/Q·t rises to a constant value, whilst PaO2 falls at first, but then begins to rise again, with increasing FIO2. The SF6 retention increased markedly in some cases at high FIO2.

Conclusions. Venous admixture will estimate true right-to-left shunt at high FIO2, even when oxygen consumption is raised. This model can explain the way that the Pa/FI ratio changes with increasing inspired oxygen concentration.

Br J Anaesth 2002; 88: 771–8


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