The injection of methylene blue or indigo carmine falsely lowers the value given by the pulse oximeter temporarily. Nail polish and strong neon lights can distort the measurement. On the other hand, hyperbilirubinemia has no effect on the measurement of SpO2.
Presence of abnormal hemoglobin
In carbon monoxide (CO) poisoning, the pulse oximeter does not differentiate between hemoglobin combined with oxygen (HbO2) and hemoglobin combined with CO (HbCO). The saturation read on the pulse oximeter is falsely high.
Example: a CO poisoned patient has 40% of his hemoglobin combined with CO ([HbCO] = 40%). This HbCO is read at 90% as HbO2, therefore interpreted as 36% of HbO2. If the rest of his hemoglobin is actually combined with O2, the pulse oximeter will show a false reassurance of 96%. As you make the Omron Nissei pulse oximeter buy you will need to be absolutely sure of the quality of the oxymeter.
Problems of interpretation of the result
Difference between SpO2 and PaO2
The pulse oximeter does not give PaO2 (partial pressure of O2 in arterial blood), but the SaO2. These two values are linked by a non-linear relationship (hemoglobin dissociation curve, or Barcroft’s curve, of sigmoid shape, see illustration below). A drop in SaO2 from 97 to 90% does not have the same meaning as a drop from 92 to 85%.
Since the accuracy of pulse oximeters is of the order of 2%, the difference between the actual value and the measured value can be of great importance when the saturation is in the region of the large slope of the curve (below 90%, which is the “knee” of the curve).
Therefore, the lower alarm limit should not be set at 90% but rather at 93 or 94%.
The pulse oximeter does not detect hyperoxemia: whether the PaO2 is 100 mmHg (13.3 kPa) or 600 mmHg (80 kPa), the SaO2 will be 100%. In neonatology, if the newborn receives oxygen, the high alarm must be set at 97% to avoid hyperoxemia (risk of retro-lental fibroplasia which can lead to blindness).
Difference between TaO2 and SpO2
Ideally, we should know the arterial transport of O2 (TaO2), i.e. the quantity of O2 transported per minute:
TaO2 = Qc x CaO2 â ‰ ƒ Qc x [Hb] x SaO2
Arterial O2 transport (TaO2) is equal to the product of cardiac output (Qc) times arterial O2 content (CaO2). So TaO2 is roughly equal to the product of cardiac output times hemoglobin [Hb] concentration and SaO2 (ignoring dissolved O2). Of these three values, the pulse oximeter provides only one.
In the event of anemia, the measurement can be falsely reassuring: if the hemoglobin concentration decreases, the SpO2 does not change. By caricaturing, we can say that if there are only ten well oxygenated red blood cells in the body (and they are willing to pass the sensor), the pulse oximeter will display “100% “, while the patient is going to die.
SpO2 may be different from SaO2
The pulse oximeter measures peripheral saturation, and takes an average (adjustable on some devices, usually 8 cycles). It actually displays a result corresponding to the situation of a few seconds ago. When the SaO2 drops sharply, there may be a lag between when the patient turns blue (cyanosis) and when the SpO2 given by the pulse oximeter begins to drop. The reverse occurs when saturation rises rapidly.
During intubation, if the patient has healthy lungs and has had good oxygenation, in the event of esophageal intubation, SpO2 takes several minutes to drop below 93%. Selective intubation (when the probe is pushed too far, intubation of a main bronchus, usually the right one) is not detected if the lungs are healthy and the FiO2 greater than 30%.