Oxygen saturation (SpO2) | Monitoring

Oxygen saturation (SpO2)

To monitor the oxygen content of the blood, the patient is usually fitted with a special clamp (pulse oximeter) on one finger of one hand. This clamp emits red light of different wavelengths. Since the blood absorbs different wavelengths depending on oxygen saturation, the device can determine a saturation value from this.

The standard value is between 95 and 99%. However, the pulse oximeter can also display false high values, for example in the case of carbon monoxide poisoning. If colored nail polish is applied to the patient’s nails, the values are usually falsely low. In addition, false values occur when the patient is in a state of shock, as the blood then shifts more towards the center of the body.


Capnometry is the measurement of the carbon dioxide concentration in the exhaled air of the patient. In particular, the so-called endtidal carbon dioxide (etCO2) is measured, which is the CO2 that is in the exhaled air at the very end of the exhalation. This is the most meaningful as it best reflects the CO2 concentration in the lungs.

At the beginning of the exhalation there is still a higher proportion of oxygen in the exhaled air, since the air from the trachea, which did not participate in the gas exchange in the lungs (so-called dead space volume), is also exhaled. The derivation of capnometry in the form of a curve is called capnography. It is particularly important because the CO2 values in the exhaled air can be used to check whether the ventilation tubing has been correctly placed. For example, if the CO2 level does not rise when the patient is ventilated, this could indicate that the ventilation tubing was inadvertently placed in the esophagus instead of the trachea. In addition, a sudden and sharp increase in CO2 concentration may indicate so-called malignant hyperthermia, which is a potentially life-threatening reaction of the patient to certain narcotics.