How do pulse oximeters work?
Here's how it works:
For patients with COVID-19, pulse oximetry helps with early detection of silent hypoxia, in which the patients still look and feel comfortable, but their SpO2 is
dangerously low. This happens to patients either in the hospital or at home. Low SpO2 may indicate severe COVID-19-related pneumonia, requiring a ventilator.
Pulse oximetry is based on the principle that O2Hb absorbs more near-IR light than HHb, and HHb absorbs more red light than O2Hb. Under optimal conditions,
pulse oximeters do not calculate SpO2 of venous blood (and other stationary tissues) but rather only arterial SpO2 by determining changes in absorbance of the
light transmitted over time; i.e., arterial blood volume changes with the cardiac cycle whereas the volumes of light absorbers in non-arterial tissues are relatively
constant. Since pulse oximeters are calibrated for SpO2 between 70 and 100%, displayed values below 70% should only be considered qualitatively accurate and
not quantitatively. Inaccurate SpO2 readings can occur with conditions that decrease arterial blood perfusion.
The SpO2 can overestimate FO2Hb content in blood that contains increased amount of CO. In sickle cell anemia-associated vasoocclusive crises, SpO2 can
overestimate the FO2Hb (due likely to the presence of increased endogenous production of CO) but could underestimate the SaO2 (due to vasoocclusion-induced
under perfusion). Falsely low SpO2 can occur with venous pulsations, excessive movement, pigmented dyes, and certain dyshemoglobins. Although severe anemia
per se does not cause a falsely low SpO2, its presence can cause the SpO2 to underestimate the SaO2 in someone with true hypoxemia. SpO2 can either overestimate
or underestimate the SaO2 with methemoglobinemia, sulfhemoglobinemia, and poor probe positioning. Sulfhemoglobinemia can mimic methemoglobinemia clinically
and by SpO2 and SaO2 measurements. Finally, it is important to emphasize that even in the most optimal patient-pulse oximeter interface and settings, a completely
normal SpO2 does not rule out gas exchange problems in the lungs or the adequacy of ventilation since the alveolar-arterial oxygen difference and PaCO2 are not
measured by pulse oximetry.
If you want to know more detail information, Please visit this link: Pulse Oximeter, Thermometer from China Manufacturers - UTECH CO., LTD. ( UTMI ) (chinautech.com)
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