Physiologic and Monitoring Practice Test

Functional saturation measures what portion of the hemoglobin that can carry oxygen is actually bound to oxygen. It is the amount of oxyhemoglobin divided by the total amount of hemoglobin capable of binding oxygen, which is oxyhemoglobin plus deoxyhemoglobin. That makes HbO2/(HbO2 + Hb) the correct expression. Nonfunctional forms of hemoglobin, like methemoglobin and carboxyhemoglobin, don’t participate in oxygen delivery, so they’re not included in this functional denominator. The other forms would distort the sense of how much of the transport-capable Hb is actually carrying O2.