The oximeter also switches both diodes off to allow ambient light to be detected and compensated for. Superimposed on the constant absorption is a pulsatile arterial absorption. This absorption follows the Beer-Lambert Law. The reading is averaged over a few seconds to give the number which is ultimately displayed. The pulse oximeter has several sources of error Abnormal hemoglobins and dyes.
Fetal hemoglobin gives an essentially normal reading.
Intravenous dyes may also give a false reading as may bilirubin. Co-oximeters in the blood gas machine are capable of determining the quantities of these other hemoglobins by using multiple wavelengths. The pulse oximeter requires adequate pulsatile blood flow though the tissue being monitored - which may be a problem in shocked or cold patients. Movement may interfere with the reading. Sometimes seen during positive pressure ventilation or tricuspid regurgitation.