What is the normal PaCO2 range (mmHg) on room air in neonates?

• A. 30-40
• B. 39-56
• C. 60-75
• D. 80-95

Answer: (B)

PaCO2 reflects how effectively a neonate is ventilating. In newborns breathing room air, the arterial CO2 tension is normally a bit higher than in adults, due to factors like higher metabolic rate and relatively larger dead space relative to tidal volume. The accepted normal range is about 39–56 mmHg, which represents adequate alveolar ventilation for most healthy neonates at rest.

If PaCO2 rises toward the upper end (approaching 56 mmHg or higher), it suggests hypoventilation or increased dead space leading to CO2 retention. If PaCO2 falls toward the lower end (near 39 mmHg or below), it suggests hyperventilation. Keep in mind that PaCO2 is just one piece of the puzzle; acid-base balance and oxygenation (PaO2) also need to be considered, and values can shift with age, illness, or changes in ventilation.