April 29, 2020 3 min read

What Is Pulse Oximetry?

In its most common mode, a medical professional will place the sensor device on a slim part of the patient's body, typically a fingertip or earlobe. If the patient is an infant, then they will place the Pulse Oximeter across their foot. The device then passes two wavelengths of light through the body part to a photodetector. The Pulse Oximeter then surveys the changing absorbance at each of the wavelengths, allowing the device to evaluate the absorbances due to the pulsing arterial blood.

Why Is Pulse Oximetry Important?

Pulse oximetry has transformed our capability to monitor oxygenation in a constant, dependable, and non-invasive way. Gathering this information is vital because it provides the conceptual foundation of comprehending its boundaries and knowing when Pulse Oximeter readings may be erroneous. 

How Does It Work?

The Pulse oximeter has revolutionized the medical industry with its ability to continuously observe the working oxygen overload of hemoglobin in arterial blood (SaO2). Pulse oximetry is so universally accepted in medical care that it is often regarded as a fifth vital sign. It is crucial to understand how the technology functions as well as its limitations. That is because erroneous interpretations can lead to unnecessary testing.

Numerous false alarms in the intensive care unit can also threaten patient safety by confusing caregivers. 

To fully grasp why settings in which pulse oximeter readings of oxygen saturation (SpO2) can end in false estimations of the true SaO2, an understanding of two fundamental principles of pulse oximetry is required: how oxyhemoglobin (O2Hb) is distinguished from deoxyhemoglobin (HHb) and how the SpO2 is determined only from the arterial compartment.

Pulse oximetry is centered around the principle that O2Hb and HHb differentially assimilate red light. Luckily, O2Hb and HHb have striking discrepancies in the absorption of red light because these two wavelengths penetrate tissues well. In contrast, blue, green, yellow, and far-infrared light are significantly absorbed by non-vascular tissues and water. O2Hb absorbs more prominent amounts of IR light and lower amounts of red light than does HHb; this is compatible with experience – well-oxygenated blood with its higher densities of O2Hb seems bright red to the eye because it dissipates more red light than does HHb. However, HHb absorbs more red light and appears less red. 

Utilizing this contrast in light absorption properties between O2Hb and HHb, pulse oximeters shoot two wavelengths of light, red at 660 nm and near-infrared at 940 nm from a set of minute light-emitting diodes located in one fin of the finger probe. The light that is conveyed through the finger is then detected by a photodiode on the opposite arm of the probe. Meaning, the relative amount of red and IR light absorbed is utilized by the pulse oximeter to define the proportion of Hb bound to oxygen sequentially.

Pulse Oximetry

In the photo above, you can see a schematic diagram of light absorbance by a pulse oximeter.

In diagram A In a patient with healthy cardiac function, the onset of the cardiac systole, as indicated by the emergence of the QRS complex, corresponds with the emergence in the raise of the arterial blood volume. The volume of red and IR light absorbed in the arterial compartment also rises and falls with systole and diastole individually thanks to the increase and decrease in blood volume. The volume that rises with systole is also classified as the pulsatile or "alternating current" (AC) compartment. The compartment in which the blood amount does not change with the cardiac cycle is distinguished as the non-pulsatile or "direct current" (DC).

In diagram B, a cross-sectional artery and a vein are illustrating the pulsatile (AC) and non-pulsatile (DC) compartments of the blood. Notice that only the artery has a pulsatile (AC) component. 

In diagram C, there is a calibration curve of the Red: IR Modulation Ratio concerning the SpO2. Increased red light absorbance is connected with increased deoxyhemoglobin meaning a lower SpO2.

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