Pulse oximetry has revolutionized the ability to monitor oxygenation in a continuous, accurate, and non-invasive fashion. Despite its ubiquitous use, it is our impression and supported by studies that many providers do not know the basic principles behind its mechanism of function. This knowledge is important because it provides the conceptual basis of appreciating its limitations and recognizing when pulse oximeter readings may be erroneous. In this review, we discuss how pulse oximeters are able to distinguish oxygenated hemoglobin from deoxygenated hemoglobin and how they are able to recognize oxygen saturation only from the arterial compartment of blood. Based on these principles, we discuss the various conditions that can cause spurious readings and the mechanisms underlying them.
Basic principles of function
The pulse oximeter has revolutionized modern medicine with its ability to continuously and transcutaneously monitor the functional oxygen saturation of hemoglobin in arterial blood (SaO2). Pulse oximetry is so widely prevalent in medical care that it is often regarded as a fifth vital sign.1 It is important to understand how the technology functions as well as its limitations because erroneous readings can lead to unnecessary testing. 2 Frequent false alarms in the intensive care unit can also undermine patient safety by distracting caregivers. To recognize the settings in which pulse oximeter readings of oxygen saturation (SpO2) may result in false estimates of the true SaO2, an understanding of two basic principles of pulse oximetry is required: (i) how oxyhemoglobin (O2Hb) is distinguished from deoxyhemoglobin (HHb) and (ii) how the SpO2 is calculated only from the arterial compartment of blood.
Pulse oximetry is based on the principle that O2Hb and HHb differentially absorb red and near-infrared (IR) light. It is fortuitous that O2Hb and HHb have significant differences in absorption at red and near-IR light because these two wavelengths penetrate tissues well whereas blue, green, yellow, and far-IR light are significantly absorbed by non-vascular tissues and water.3 O2Hb absorbs greater amounts of IR light and lower amounts of red light than does HHb; this is consistent with experience – well-oxygenated blood with its higher concentrations of O2Hb appears bright red to the eye because it scatters more red light than does HHb. On the other hand, HHb absorb more red light and appears less red. Exploiting this difference in light absorption properties between O2Hb and HHb, pulse oximeters emit two wavelengths of light, red at 660 nm and near-IR at 940 nm from a pair of small light-emitting diodes located in one arm of the finger probe. The light that is transmitted through the finger is then detected by a photodiode on the opposite arm of the probe; i.e., the relative amount of red and IR light absorbed are used by the pulse oximeter to ultimately determine the proportion of Hb bound to oxygen.
The ability of pulse oximetry to detect SpO2 of only arterial blood is based on the principle that the amount of red and IR light absorbed fluctuates with the cardiac cycle, as the arterial blood volume increases during systole and decreases during diastole; in contrast, the blood volume in the veins and capillaries as well as the volumes of skin, fat, bone, etc, remain relatively constant. A portion of the light that passes through tissues without being absorbed strikes the probe’s photodetector and, accordingly, creates signals with a relatively stable and non-pulsatile “direct current” (DC) component and a pulsatile “alternating current” (AC) component (Fig. 1A). A cross-sectional diagram of an artery and a vein during systole and diastole illustrates the non-pulsatile (DC) and pulsatile (AC) compartments of arteries and the relative absence of volume change in veins and capillaries (Fig. 1B). Pulse oximeters use amplitude of the absorbances to calculate the Red:IR Modulation Ratio (R)4; i.e., R = (Ared,AC/Ared,DC)/(AIR,AC/AIR,DC) where A = absorbance; in other words, R is a double-ratio of the pulsatile and non-pulsatile components of red light absorption to IR light absorption. At low arterial oxygen saturations, where there is increased HHb, the relative change in amplitude of the red light absorbance due to the pulse is greater than the IR absorbance, i.e., Ared,AC > AIR,AC resulting in a higher R value; conversely, at higher oxygen saturations, AIR,AC > Ared,AC and the R value is lower (Fig. 1C). A microprocessor in pulse oximeters uses this ratio (calculated over a series of pulses) to determine the SpO2 based on a calibration curve that was generated empirically by measuring R in healthy volunteers whose saturations were altered from 100% to approximately 70%5 (Fig. 1C). Thus, SpO2 readings below 70% should not be considered quantitatively reliable although it is unlikely any clinical decisions would be altered based on any differences in SpO2 measured below 70%.
Figure 1. Schematic diagram of light absorbance by a pulse oximeter. (A) In a person with good cardiac function, the onset of the cardiac systole, as denoted by the onset of the QRS complex coincides with the onset in the increase of the arterial blood volume. The amount of red and IR light absorbed in the arterial compartment also rises and fall with systole and diastole, respectively, due to the increase and decrease in blood volume. The volume that increases with systole is also known as the pulsatile or “alternating current” (AC) compartment and the compartment in which the blood volume does not change with the cardiac cycle is known as the non-pulsatile or “direct current” (DC) compartment. (B) A cross-sectional diagram of an artery and a vein displaying the pulsatile (AC) and non-pulsatile (DC) compartments of the blood vessels. Note that only the artery has a pulsatile (AC) component. (C) A diagram of a calibration (standard) curve of the Red:IR Modulation Ratio in relation to the SpO2. Increased red light absorbance (increased R) is associated with increased deoxyhemoglobin, i.e., lower SpO2. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)