Which of the following is the most reliable indicator of effective respiration in a patient?

Mental status reflects perfusion and oxygen delivery to the brain, which is highly sensitive to hypoxia and hypercapnia. Even if respiratory rate and SpO₂ are normal, altered mental status suggests ineffective gas exchange or hypoperfusion.

Pulse oximetry may be falsely normal in CO poisoning or poor perfusion. Skin color is subjective and not as sensitive or specific as neurological status.