Evaluation of the Relationship of Oxygen Reserve Index (ORI) and End-Tidal Carbon Dioxide (ETCO₂) With Arterial Blood Gas Parameters During the Apnea Test

This observational study aims to evaluate the relationship between the Oxygen Reserve Index (ORI) and partial arterial oxygen pressure (PaO2), as well as between End-Tidal Carbon Dioxide (ETCO2) and partial arterial carbon dioxide pressure (PaCO2) in patients undergoing an apnea test for the diagnosis of brain death. ORI is a relatively new, non-invasive monitoring tool that allows continuous and real-time assessment of oxygenation status. It is a dimensionless parameter with values ranging from 0 to 1. ORI is based on a multi-wavelength pulse co-oximeter that detects light absorption in both arterial and venous blood through a probe. It is designed to estimate PaO2 values in the 100-200 mmHg range, as SpO2 typically reaches ≥97% when PaO2 exceeds 80 mmHg, making it unable to reflect higher PaO2 levels accurately. The apnea test is a critical procedure for the diagnosis of brain death, in which the presence or absence of spontaneous breathing in response to carbon dioxide accumulation is evaluated. During the test, arterial blood gas (ABG) measurements are taken to monitor the rise in PaCO2. Before the test begins, the patient is ventilated with 100% oxygen to raise PaO2 above 200 mmHg, which is confirmed with a pre-test ABG sample. The patient must also be normocapnic prior to testing.

As arterial blood sampling is an invasive procedure, this study aims to assess the predictability of PaO2 and PaCO2 using ORI and ETCO2 in the pre-test phase. The primary objective is to determine the capability of ORI to monitor oxygenation status and evaluate its potential to detect hypoxia earlier. Moreover, by comparing ORI to conventional methods, this study seeks to explore its potential routine use in clinical settings for PaO2 monitoring and its possible advantages. In patients undergoing apnea testing for suspected brain death, this study may help determine whether ORI offers benefits over current monitoring approaches and whether it can reduce dependence on invasive ABG sampling. Furthermore, hyperoxia, like hypoxia, poses various clinical risks. The clinical use of ORI alongside SpO2 may help prevent hyperoxia and enable earlier detection of hypoxia. This may enhance patient safety and improve clinical decision-making in critical care environments.