Altitude, the ratio of PaO2 to fraction of inspired oxygen, and shunt: impact on the assessment of acute lung injury

The ratio of PaO2 to the fraction of inspired oxygen (PaO2/FIO2) is commonly used to determine the severity of acute lung injury and acute respiratory distress syndrome (ARDS). The research presented here used computational models of the lung to analyze the effect of altitude on the PaO2/FIO2 ratio and pulmonary shunt. At a given shunt, the PaO2/FIO2 ratio is lower at higher altitudes. Therefore, when evaluating for ARDS based on a PaO2/FIO2 ratio of <200 mm Hg, patients residing at high altitudes will have less shunt and, presumably, less severe lung injury than patients at sea level. This should be taken into consideration when comparing patients from different altitudes. Shunt should more often be measured directly or be estimated assuming a constant arteriovenous oxygen content difference.