A pulse oximeter is placed onto the end of a patient's finger or ear lobe. The medical device measures the saturation of oxygen in the blood. These devices are useful in determining specific conditions that affect the heart and lungs. Although pulse oximeters are accurate devices, some mechanisms cause pulse oximeters to be inaccurate.
If a patient inhales too much carbon monoxide, the molecules fight for haemoglobin spots -- usually taken by oxygen molecules -- on red blood cells. There are only so many spots available for oxygen to attach to red blood cells, and if carbon monoxide takes up all of these spots, the blood becomes oxygen-deprived. Pulse oximeters are unable to tell the difference between carbon monoxide and oxygen; they just read that the blood cell is carrying a molecule. Readings sometimes seem normal when the patient is really starving for oxygen. Patients with smoke inhalation or carbon monoxide poisoning -- whether from excess cigarette smoking or inhaling smoke from a fire -- are not to be given a pulse oximeter to measure oxygen saturation as the reading is likely to be inaccurate.
Blood disorders, such as anaemia, hypotension and hypothermia all cause inaccurate pulse oximeter readings. Anemia causes an inaccurate oxygen saturation in the blood due to the lack of available haemoglobin for oxygen to bind to. Hypotension and hypothermia cause inaccurate readings as well due to reduced blood flow to extremities.
Methemoglobin is an altered form of haemoglobin that does not carry oxygen molecules. Normally, a small amount of methemoglobin is found in the blood stream, but an amount larger than 1 to 2 per cent causes a pulse oximeter to provide an inaccurate, lower oxygen saturation reading.
Other Causes of Inaccuracy
There are many other reasons why a pulse oximeter gives inaccurate readings. These reasons include not being properly attached to the patient's finger or ear lobe correctly, being placed on a patient who is shivering or frequently moving, being placed on skin with dark pigmentation, being attached to cold fingers or fingers with fake nails, bright room lighting and nail polish obstructing the laser.