BOX 1– Two versions of the alveolar gas equation
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The alveolar equation exists in several forms; the upper version is easier to remember and is used in clinical practice. The lower version is more accurate because it corrects for the difference in inspiratory and expiratory minute volume when the respiratory exchange ratio, R, does not equal 1.0. The equation is used to calculate the alveolar–arterial O2 difference (PA–aO2). In the absence of diffusion limitation, V′A/Q′ mismatch and shunt (see text), PaO2 equals PAO2. The PA–aO2 can, therefore, be used as a measure of the overall deviation from perfect gas exchange.
PIO2: partial pressure of O2 in inspired gas reaching the alveoli, calculated as PIO2=FIO2×(PB–PH2O); FIO2: fraction of O2 in inspired gas; PB: barometric pressure; PH2O: saturated vapour pressure of water at body temperature, 47 mmHg (6.3 kPa) at 37°C; R: the respiratory exchange ratio defined as the ratio between the amount of CO2 and O2 exchange.
PB must be known or estimated for the appropriate altitude; at sea level it is commonly estimated at 760 mmHg (101.3 kPa). If not measured, R is usually assumed to be 0.8. Using these values at sea level, the O2 concentration in air (FIO2 0.21) and a normal PaCO2 of 40 mmHg (5.3 kPa) the upper equation results in PAO2 of 100 mmHg (13.3 kPa).