What is the preferred initial diagnostic test to evaluate gas exchange after intubation?

• A. Chest X-ray
• B. Arterial blood gas (ABG) to assess PaO2 and PaCO2
• C. Venous blood gas
• D. Pulse oximetry alone

Answer: (B)

Evaluating gas exchange after intubation requires direct data on oxygenation and ventilation, along with acid–base status. An arterial blood gas provides PaO2 and PaCO2, plus pH and bicarbonate, giving a precise snapshot of how well oxygen is moving into the blood and how effectively CO2 is being removed. This information is essential to judge ventilator performance and to make timely adjustments. While a chest radiograph helps confirm endotracheal tube placement and reveal structural issues, it does not quantify gas exchange. Pulse oximetry offers continuous oxygen saturation but can miss discrepancies in ventilation and cannot replace PaO2 or PaCO2 data. Venous blood gas reflects venous blood properties and isn’t a reliable stand-in for arterial gas exchange.