Math
Spreadsheet arithmetic for a simplified alveolar gas equation and P/F ratio
Use fictional values to practice pressure units and ratios; this example does not diagnose respiratory disease or recommend treatment.
Fictional physiology arithmetic with fixed assumptions
This teaching example uses the simplified alveolar gas equation with fixed water-vapor pressure47mmHg at37°C and respiratory quotient0.8, approximating alveolar carbon dioxide by the arterial value. It is not the complete gas equation and becomes less accurate in some conditions, including high inspired oxygen fractions.
FiO2 is a fraction, not a percentage number; pressures use mmHg. An A-a difference or P/F ratio alone does not diagnose ARDS or establish that a patient is healthy. Clinical interpretation needs additional findings, support settings and timing. The age-based automatic normal/elevated labels have been removed. Do not use this spreadsheet to select oxygen or treatment.
| ABG Parameter | Cell | Fictional inputs | Pulmonary Result |
|---|---|---|---|
| FiO2 & Barometric P_atm | B2 = 0.21, B3 = 760 mmHg | 0.21 * (760 - 47) = 149.73 | Inspired dry O2 pressure |
| Arterial PaCO2 & RQ (0.8) | B4 = 40 mmHg | 40 / 0.8 = 50.00 mmHg | PAO2 = 99.73 mmHg |
| Arterial oxygen | B5=88mmHg | 99.73-88 | Difference11.73mmHg |
| PaO2 / FiO2 (P/F Ratio) | 88 / 0.21 | 419.05 mmHg | Ratio only; no health classification |
Alveolar Oxygen Tension PAO2 & A-a Gradient (mmHg)
Use an appropriate measured barometric pressure if evaluating this mathematical expression; altitude alone does not supply all physiological assumptions. Negative modeled alveolar pressure is flagged as inconsistent inputs. Negative A-a differences are left visible, without a diagnostic interpretation.
=IF(OR(COUNT(B2:B5)<>4,B2<=0,B2>1,B3<=47,B4<0,B5<0),"Invalid inputs",IF(B2*(B3-47)-B4/0.8<0,"Inconsistent inputs",ROUND((B2 * (B3 - 47) - (B4 / 0.8)) - B5, 2)))PaO2 divided by inspired oxygen fraction
The ratio retains mmHg when PaO2 is entered in mmHg. No diagnostic severity categories are assigned.
=IF(OR(COUNT(B2:B5)<>4,B2<=0,B2>1,B3<=47,B4<0,B5<0),"Invalid inputs",ROUND(B5 / B2, 2))Try the example
Use the original values or change them to test your understanding.