Which is a late sign of respiratory failure?

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Cyanosis is a late sign of respiratory failure because it only becomes noticeable after oxygen levels have fallen quite low. When the body first struggles to oxygenate, you’ll typically see signs like rapid breathing and increased work of breathing, along with the heart rate rising as a compensatory response. As hypoxemia worsens, the body's tissues start to suffer, fatigue sets in, and in severe cases the heart rate may eventually slow (bradycardia) once decompensation progresses. It’s only when there is a substantial amount of deoxygenated hemoglobin in the blood that the bluish discoloration of skin, lips, and mucous membranes appears. Keep in mind that cyanosis isn’t perfect—it can be hard to see in some patients (anemia, peripheral poor perfusion, or dark skin)—so don’t rely on it alone, but within the typical progression, it signals a late, advanced stage of respiratory failure.