Gas Exhange

Pulse Oximetry

While the ability to perform pulse oximetry has been available for the past 30 years, it is only in the past 10 years that the cost of the technology has dropped to the point where it is accessible for the general population. A recent search on Amazon, revealed consumer grade pulse oximeters for $20. It is so prevalent a technology that it is built into almost all smart phones, watches and helath monitoring devices (i.e. Fitbits). Every outpatient practice I have been affilaited with appropriately considers SaO2 a routine part of the vital signs

All pulse oximeters on the market today rely on the diffential absorption of oxygenation and deoxygenation blood in the red and near infra-red region. In addition during the course of a cardiac cycle the amount of blood in the venous system is relatively static while the arterial blood volume changes with each systolic ejection. By measuring the changes in light absorption during the cardiac cycle, the pulse oximeter is able to isolate the arterial blood component and calculate the saturation of hemoglobin with oxygen (SaO2).

Having spent several decades in medical practice I have witnessed medical equipment evolve. One day many years ago we were clearing out a store room and came across an early pulse oximeter that was used in the clinic. The device required wall current, weight several pounds, and most likely had cost several hundreds of dollars when new. Now battery powered fingertip devices are the norm. Concern has been raised on the heels of the COVID epidemic about the accuracy of consumer grade pulse oximeters. There was a recent incident where a patient demanded an appointment because of her poor saturations at home. There was no function change in her respiratory capacity and when we checked her SaO2 in the office it was normal. She thankfully has brought into the office her device from home that she purchanced from a foreign website that promises to let 'you shop like a billionaire'. The device was giving eronous information and was not worth the $7 she paid for it. Desppite the example, there has been limited studies that have tested non-FDA approved devices and found many to be reasonably accurate.

A potential limitation of pulse oximetry is that we are measuring hemoglobin saturation, which is closely related but not synonomous with the partial pressure of oxygen in the blood stream. In 1904, a Danish physiologist, Christain Bohr, discovered a relationship between hemoglobin saturation and both metabolic and respiratory disturbances that bears his name.