MCQ 3:

A patient has a pulse oximeter saturation (spO2) of 85%, where as his saturation on ABG (saO2) is 93%, what is the likely cause???

Possible diagnosis:

a – Carboxyhemoglobin

b – Methhemoglobin

c – Fetal hemoglobin

d – sulfhemoglobin

Ans: b – Methhemoglobinemia.

Because methemoglobin absorbs more light at 660 nm than at 990 nm, it affects pulse oximetry readings when methemoglobin levels exceed 6%. Moreover, higher levels of methemoglobin tend to bias the reading toward 85% to 90%. (Ref: Irwin and Rippe.)

In the presence of high HbMet concentrations, the measured SpO2 approaches 85%, independently of the actual arterial oxygenation. (Ref: Miller)