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.