Titrate FiO2 to achieve adequate oxygenation with the lowest feasible oxygen level. In most adults, aim for SpO2 around 92–96% and, once you reach that target, reduce FiO2 to the smallest value that keeps SpO2 in range. This approach avoids unnecessary oxygen exposure and potential oxygen toxicity while ensuring tissues stay well-oxygenated. Increasing FiO2 to push SpO2 above 98% can promote hyperoxia without added benefit and can be harmful over time. Keeping FiO2 fixed (e.g., at 0.5) ignores changing patient needs and may leave hypoxemia or unnecessary oxygen exposure if lung conditions improve. Waiting for distress to raise FiO2 risks prolonged periods of inadequate oxygenation. Special populations (like COPD) may have a lower SpO2 target (e.g., 88–92%), but the general strategy remains: titrate to the lowest FiO2 that maintains the appropriate saturation.

PaCO2 is the partial pressure of carbon dioxide in arterial blood and is obtained from an arterial blood gas. To reflect gas exchange in the lungs accurately, the sample must come from an artery; venous blood carries CO2 that has been picked up from tissues and its CO2 level (PvCO2) does not equal the arterial PaCO2. Capillary samples can be used when arterial access isn’t feasible, but the standard measurement labeled PaCO2 refers to arterial blood. Mixed venous samples measure venous CO2 and are used for different assessments. So the described measurement corresponds to an arterial blood sample.

Airway protection ability is checked because, when you start spontaneous breathing, the patient must not only move air on their own but also guard the airway and clear secretions without the help of the ventilator. This means protective reflexes like coughing and swallowing, as well as the ability to close the glottis and manage secretions, need to be intact. If airway protection is poor, the risk of aspiration, desaturation, or the need for reintubation rises during a spontaneous breathing trial. So evaluating airway protection is part of determining readiness for spontaneous breathing trials to ensure a safe transition off support.

Capnography measures end-tidal CO2—the CO2 concentration in the gas at the end of an exhaled breath. This reflects the CO2 content of alveolar gas, which in turn tracks the arterial CO2 under normal conditions, making it a practical, noninvasive proxy for PaCO2. It is not a direct measure of arterial CO2, nor does it represent mixed venous CO2 or inspired CO2. In healthy lungs the PaCO2 is only slightly higher than the end-tidal value (a small gradient), but this gradient can widen with dead space ventilation or ventilation–perfusion mismatch. Clinically, end-tidal CO2 is used to assess ventilation adequacy, detect changes in perfusion, and confirm airway placement among other dynamic monitoring cues.