Correlation between alveolar-arterial oxygen tension difference (AaPO2) on room air and while breathing 100% O2 : implications for screening for shunting

Abstract

Hypoxemia on room air with increased AaPO2 can be due to alveolar ventilation/blood flow (V’A/Q’) mismatch, diffusion limitation or right-to-left (RL) shunting. AaPO2 while breathing 100% O2 (“hyperoxia test”) greater than normal value suggests the presence of RL shunting. Contrarily to V’A/Q’ mismatch and diffusion limitation, RL shunting is probably diagnosed for higher levels of hypoxemia on room air. Therefore, the aim of this study was to assess how blood gas results on room air would predict a positive hyperoxia test in favour of RL shunting. All patients with hereditary haemorrhagic telangiectasia admitted to our department over a 21-year period were analysed. Patients had blood gases on room air and while breathing 100% O2. Room air AaPO2 was calculated using the equation : ((Barometric pressure - 47) * 0.21) - (PaCO2/R) - PaO2. AaPO2 while breathing 100% O2 was obtained using the equation : PiO2 - (PaCO2 /R) - PaO2 (R : respiratory exchange ratio; PiO2 : partial pressure of O2 in the mask). 192 patients (82 male) were included. A significant correlation was found between AaPO2 calculated on room air (X) and while breathing 100% O2 (Y) in upright position (Y : 8.2 * X - 36.2; R2 : 0.765) and in supine position (Y : 7 * X - 40.6; R2 : 0.599). A threshold of AaPO2 equal to twice the normal value in upright position had an excellent negative predictive value (96.7%) for the diagnosis of RL shunting. In summary, AaPO2 calculated on room air in upright position is predictive of the results of the hyperoxia test. Indication to measure blood gases while breathing 100% O2 may be refined according to the level of AaPO2 on room air.