Hi all, I'm hoping I can get some help working through some physiology of anemia and SpO2 here.
I had a 4 month old patient with a condition that results lots of slow blood loss. He came in with tachypnea and increased work of breathing and was found to have an SpO2 of 80%. We put him on oxygen and he came right up to 100% with improvement in his respiratory work. He didn't have any cough or runny nose that would make me think he had bronchiolitis. His chest x-ray was completely normal, so no pulmonary edema due to low oncotic pressure and no pneumonia/atelectasis. His hemoglobin was 2 (!). My attending said that the low SpO2 in this scenario was a spurious reading due to vasoconstriction with the severe anemia, but I don't quite understand how this would explain the improvement in SpO2 and his respiratory distress with supplemental O2… He didn't have a great explanation when I asked him about the response specifically.
I was wondering if perhaps so much of the oxygen from his hemoglobin was extracted in the capillaries that his lungs could only oxygenate up to 80%, then extra PAO2 via supplementation would allow for oxygenation up to 100%? Similar to relative anemia in R-to-L shunts or sepsis. Or maybe he just has some element of pulmonary edema that isn't enough to be seen on x-ray but enough to cause V/Q mismatch? I had also read that sometimes with vasoconstriction the pulse ox will read venous pulsations, similar to what my attending said, or that the low concentration of hemoglobin changes the wavelengths that the oximeter reads, but again I don't understand how the respiratory distress and response to supplemental O2 would fit in to those explanations.
Thanks for the help!
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