The first commercial noninvasive pulse oximeter appeared in 1981; before that, patients had to endure painful, expensive arterial blood gas draws with turnaround times of more than 15 minutes. From the operating rooms, pulse oximetry spread to the recovery room, then the ICUs. Now its use is widespread throughout the hospital. By reducing the incidence of unrecognized hypoxia, pulse oximetry has reduced inpatient mortality immeasurably.
The principle of pulse oximetry relies on the differences of oxygenated and deoxygenated red cells in absorption of red and infrared light wavelengths. Oxyhemoglobin and deoxyhemoglobin absorb these wavelengths in different amounts. The pulse oximeter senses the minute color changes in pulsating blood, and calculates how many oxygen molecules are bound to hemoglobin, or oxygen saturation. A strong, regular pulse is important to obtaining a good signal and accurate reading. Unfortunately, many critically ill patients do not have good pulses anywhere, resulting in inaccurate or fluctuating readings.
Despite its benefits, pulse oximetry has many limitations (Anaesthesia. 1991;46(4):291-5). Falsely high or low readings are common; good signal is often difficult to obtain; and patients frequently remove or dislodge the devices because they are annoying. Pulse oximeters report an average saturation over time, potentially creating lag. Other causes of errors include
- patient motion,
- some types of lighting,
- intravenous dyes,
- skin pigmentation and
- nail polish.
Carbon monoxide binds preferentially over oxygen to hemoglobin, creating carboxyhemoglobin that cannot bind oxygen. If 30% of hemoglobin binds carbon monoxide, then only 70% of hemoglobin can be saturated with oxygen. However, the pulse oximeter will read both hemoglobin saturations, producing an oxygen saturation reading of 100%, when in reality, the patient is severely hypoxemic. Pao2 can be misleading too, as it may be normal. Patients who may have been exposed to carbon monoxide must have carboxyhemoglobin levels measured with a co-oximeter.
Methemoglobin causes inaccuracies with the pulse oximeter readout, depending on the concentration of the abnormal hemoglobin. A cyanotic patient with oxygen saturations between 85% to 90% should have co-oximetry analysis performed.
It is essential to understand that a normal oxygen saturation does not rule out respiratory failure. Hemoglobin may remain saturated with oxygen despite hypoventilation causing rising levels of carbon dioxide. The oxygen saturation may not fall below 90% until the patient is already in serious trouble, especially if the patient is on supplemental oxygen. Respiratory rate, pCO2, and level of consciousness should all be assessed routinely. Do not rely on the oxygen saturation to warn you of impending respiratory failure.
Further information and reference material can be found online.