Is the PaO₂/PaCO₂ Ratio Associated With ICU Mortality Independent of Oxygen Saturation and Mean Arterial Pressure?

Inclusion criteria Adult patients (≥ 18 years old) at time of ICU admission. Rationale: Restrict to adult physiology and consent/ethics comparability. First ICU stay during hospital admission (index ICU admission). Rationale: Avoid within-hospital dependency and repeated measures bias. ICU admission captured in the MIMIC-II dataset between the database start and end dates. Rationale: Use all eligible records within the dataset timeframe. At least one arterial blood gas (ABG) measurement with both PaO₂ and PaCO₂ available during the ICU stay. Rationale: Primary exposure is PaO₂/PaCO₂ ratio; require both values to compute it. Available outcome data for ICU mortality (discharge status from ICU). Rationale: Necessary to ascertain primary endpoint. Available SpO₂ and mean arterial pressure (MAP) measurements in the same ICU stay (for confounder adjustment). Rationale: Study aims to evaluate independence from SpO₂ and MAP; require at least one measurement of each during the stay. (If either is missing, see handling below.) Exclusion criteria Age \< 18 years. Rationale: Exclude pediatric physiology. Missing or unusable arterial blood gas values (PaO₂ or PaCO₂ absent or non-numeric) for the ICU stay. Rationale: Cannot compute exposure. No recorded ICU outcome (missing ICU discharge status) or lost/invalid mortality data. Rationale: Cannot determine primary outcome. Extreme or physiologically implausible ABG values that suggest data error (e.g., PaO₂ ≤ 0 mmHg, PaCO₂ ≤ 0 mmHg, or PaO₂ \> 1000 mmHg or PaCO₂ \> 300 mmHg - thresholds may be tightened after exploratory data review). Rationale: Remove likely data-entry errors. Document thresholds and perform sensitivity analysis. Patients on extracorporeal life support (ECMO) during ABG measurement (if identifiable in MIMIC-II). Rationale: ECMO profoundly alters gas exchange and PaO₂/PaCO₂ physiology; consider exclusion or separate subgroup. Repeat ICU admissions within the same hospital stay beyond the index admission (if you choose to restrict to single index admission). Rationale: Prevent correlated observations - include only the first ICU stay per hospital admission. ICU stays with no recorded SpO₂ or MAP at any time (if you insist on requiring these for adjustment). Rationale: If primary models must adjust for SpO₂ and MAP, exclude records without those covariates or plan appropriate imputation. Do-not-resuscitate (DNR)/comfort-care only status recorded before ABG measurement (optional exclusion depending on study question). Rationale: Treatment limitation orders can confound mortality associations; you may choose instead to adjust or analyze separately.