End-tidal carbon dioxide (EtCO₂) refers to the level of carbon dioxide in the air exhaled during expiration and is measured using capnography. EtCO₂ is an important monitoring parameter in the management of critically ill patients on mechanical ventilation, providing indirect information about the patient's cardiac output, pulmonary perfusion-ventilation (V/Q) status, and metabolic activity. The Pa-EtCO₂ gradient, the difference between EtCO₂ and arterial CO₂ partial pressure (PaCO₂), changes significantly, especially in cases of hemodynamic deterioration, microcirculatory failure, and increased dead space. Therefore, the gradient is considered a potential indicator of perfusion in critically ill patients. In critically ill patients on mechanical ventilation, the usability of changes (courses) of the Pa-EtCO₂ gradient over time is being evaluated not only as an indicator of ventilation but also as a dynamic and indirect predictor of hemodynamic status and renal perfusion. Therefore, it should be investigated whether the Pa-EtCO2 gradient is a suitable parameter for early detection of AKI (acute kidney injury) development in patients with renal perfusion impairment.

Inclusion Criteria:

• Patients over 18 years of age
• patients under mechanical ventilation
• initiation of EtCO₂ monitoring within the first hour of admission to the ICU
• patients monitored in the ICU for at least 24 hours
• having undergone arterial blood gas analysis (for PaCO₂ measurement) within the first 24 hours.

Exclusion Criteria:

• Patients under 18 years of age
• intensive care unit monitoring shorter than 24 hours
• missing or inaccessible clinical data
• chronic renal failure
• acute renal failure present at admission to intensive care unit
• advanced pulmonary diseases affecting EtCO₂ measurement (e.g., severe obstruction, giant bulla, advanced emphysema)
• patients in whom PaCO₂ measurement cannot be performed

patients being monitored with mechanical ventilators in intensive care