If you are a nurse or are studying to become a nurse, it is essential to know what physical quantity fingertip oximeter or a "finger pulse ox" measures, what could go wrong with the measurement and how to verify the accurate readings. On the other hand, and this is highly unlikely to learn in the nursing school, it is also highly beneficial to know more detailed principles of fingertip pulse oximeter operation. One reason - what are you going to say when an inquiring mind of a patient asks you that? The other reason - by knowing the principles of operation, you will better understand the advantages and limitations of finger pulse oximeters and blood oxygen meters in general, and will be better able to use the devices appropriately, and replace them with a more appropriate tool when the circumstances warrant it.

Fingertip oximeters were invented by the company called Nonin just a few decades ago. A repeated series of two distinct pulses, a red pulse, and an infrared pulse, is sent through a patient's finger. It turns out that the oxygenated hemoglobin in blood transmits a different amount of red and infrared light than the non-oxygenated hemoglobin. Therefore, by calibrating, and measuring the relative absorption of both red and infrared light, the ratio of oxygenated to non-oxygenated red blood cells is determined.

Advantages of fingertip oximeter

Clearly, the fingertip pulse oximetry procedure, when compared to drawing blood, has two advantages.

First, it is non-invasive, and the related infection issues and blood handling issues are few.

Second, since the light pulses are extremely fast, the readout can be had very quickly, in ten seconds or so. This is important in critical moments such as in an emergency medical situation, or for high performance athletes during the game, and for pilots flying above 10,000 feet where the oxygen in air becomes diluted.

Disadvantages of fingertip oximeters

The main disadvantage stems from the fact that, by just sending light pulses through the fingertip, it is elusive to determine the exact volume of the blood that is being sampled by the light. The measurement of oxygen saturation can therefore only be relative - it only gives the relative ratio of oxygenated hemoglobin concentration versus the non-oxygenated hemoglobin concentration. While the normal ratio is considered to be between 97% to 100%, the manufacturers warn that the reading accuracy goes down when the saturation reaches 70% or so.

When the readout percentage goes as low as 70% it could be due to several factors. One factor could be the oxygenated hemoglobin deficiency, and the other could be the hemoglobin or red blood cells deficiency itself.

So whenever you come across a low reading on the oximeter, check that everything is in good order with the device, and if so, a blood test is in order. The advantage of a blood test versus the fingertip oximetry is that in a blood test, the amount of the blood sample is known exactly, and therefore, the absolute hemoglobin concentrations can be determined. These are more relevant for the well-being of the patient than just the oxygen saturation ratio returned by the fingertip oximeter.