Pulse oximetry is the measurement of both the pulse and the haemoglobin saturation. Pulse is easily measured doing a wrist, or radial, measurement so the important part of the measurement is the haemoglobin saturation level. The amount of haemoglobin saturated with oxygen is an important indicator for patients, especially in intensive care and emergency situations.
To understand pulse oximetry and what a normal or abnormal reading indicates, it is first important to understand how oxygen is transported in the blood. The blood comprises red blood cells, white blood cells and plasma. The red blood cells contain haemoglobin, a compound formed by a globin chain and iron. Each of these molecules has the ability to carry eight atoms of oxygen. Since oxygen is essential to life, this function of haemoglobin is essential as well.
Haemoglobin has such a high affinity for oxygen that full saturation (100 per cent) could be possible, but only in the presence of pure oxygen. In the presence of normal air, which is only 21 per cent oxygen, it is rare to ever reach full oxygenation. Therefore normal and healthy pulse oximetry levels range between 95 to 98 per cent, but a level as low as 90 per cent can be common.
A pulse oximeter consists of a two-sided probe that emits red and infrared light through body tissue, usually a finger. Although most of the light is absorbed in the tissue, the small amount that is not is used to calculate the haemoglobin saturation, since the absorption level of oxygen-rich haemoglobin is different from oxygen-poor haemoglobin.
A pulse oximetry reading between 85 and 90 per cent can indicate respiratory failure. This is a very small margin considering that normal levels can reach as low as 90 per cent. However, when the level of haemoglobin oxygen saturation dips lower than 90 per cent, there is not enough oxygen present in the body for cells to function normally. Patients with this level must receive oxygen to raise the level and relieve them of respiratory distress.
A pulse oximeter can only work when a strong detectable peripheral pulse is available. If the pulse is weak, the obtained pulse oximetry reading is not reliable. Patients with irregular heartbeats will also give false readings. Avoid the presence of bright light when taking a reading because this can interfere with the light transmitted through the meter. Movement can also affect the light so patients must hold still for an accurate reading to be obtained. Because of these limitations, a pulse oximetry level should only be considered normal after several readings.