The goal of this observational study is to learn about the Association of the PaO₂/PaCO₂ Ratio 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.

This study included adult patients admitted to intensive care units (ICUs) represented in the Medical Information Mart for Intensive Care II (MIMIC-II) database, a large, publicly available critical care database developed by the Massachusetts Institute of Technology (MIT) in collaboration with Beth Israel Deaconess Medical Center (BIDMC). The database contains detailed, de-identified clinical data from patients admitted to the BIDMC ICUs between 2001 and 2008.

All adult (≥18 years) ICU patients with at least one arterial blood gas measurement containing both arterial oxygen tension (PaO₂) and arterial carbon dioxide tension (PaCO₂) were eligible for inclusion. For each included ICU stay, corresponding measurements of oxygen saturation (SpO₂) and mean arterial pressure (MAP) were also extracted to allow adjustment for oxygenation and hemodynamic status.

Patients were excluded if they lacked valid PaO₂ or PaCO₂ measurements, had missing ICU mortality data, or had physiologically impla