Oximeter--- circulation

I'm curious as to how the oximeter really works - if you have poor circulation, will it not give accurate results?
My son's O2 has been checked on many machines and it goes very low, a lot. yesterday a nurse from the state came and did vitals and it was 61, but went up to 71. His Dr is ordering supplemental oxygen and he is also getting C-pap for sleep apnea. But I'm wondering if it's really that low a lot or just bad circulation? (He doesn't show distress but his Blood pressure goes very low - today 57/43 - or very high: 186/164 - and doesn't show any distress.) His hands are not cold when he has these numbers and have color, but still wondering.

Circulation can be a factor, but usually you can switch to a different finger to cross check. Most of the time a circulation problem is more local in the hand than global.

Another thing to consider is getting a reading from toes or an ear lobe.

I've done different hands and fingers and the readings are usually the same... Haven't done the toe or ear though.

The oximeter works by shining a red light, and an infrared light through the skin, the ratio of the two colors that is received varies with the oxygenation of the blood.

Better oximeters give a readout of the perfusion index, which gives you an indication of the blood flow.

Try a toe.