A 4-year-old with pneumonia develops severe hypoxemia and is intubated for acute respiratory distress syndrome (ARDS). On the ventilator, her PaO2 is 58 mmHg with an FiO2 of 0.8. The pediatric resident wants to calculate the patient’s Oxygenation Index to stratify the severity of her ARDS. In addition to the PaO2 and FiO2, what other value is essential for this calculation?