To an anesthesiologist, silence in an operating room means danger. The pulse oximeter’s soothing tone gives us real-time information about a patient’s oxygen saturation; without it, the danger of occult hypoxemia lurks. Most modern anesthesiologists would struggle to imagine performing an anesthetic without a pulse oximeter (fig. 1). But like all technology on which we rely, we must understand how a pulse oximeter works—and recognize its limitations—to maximize its utility.

Fig. 1.

Girl with a pulse-ox earring.

Fig. 1.

Girl with a pulse-ox earring.

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Pulse oximetry records data to produce a value that the observer—in this case, a physician—uses to form an assumption about reality. First, an emitter produces red and infrared wavelengths of light that are then absorbed by oxyhemoglobin and deoxyhemoglobin. Second, a detector records how much of this light has been absorbed. Third, the pulse oximetry software inputs the data into a variation of the Beer–Lambert law to calculate a value for the percentage of arterial hemoglobin that is bound to oxygen. Fourth, the observer uses this output value to form an assumption about the proportion of hemoglobin that is bound to oxygen, and, by extension, the delivery of oxygen to a patient’s cells. Though the wavelengths emitted by the pulse oximeter are also absorbed by skin, bones and soft tissue, this interference is distinguished from blood as its absorption is fixed, rather than pulsatile, like the flow of blood. In other words, it is the pulsatile flow created by a patient’s beating heart that makes the pulse oximeter’s calculation possible (as we all know from being in the operating room with a patient on cardiopulmonary bypass, the pulse oximeter does not provide a value).

The pulse oximeter uses light’s behavior to communicate something vital, as does art, where light is also a tool to tell us something about our world. Johannes Vermeer, a seventeenth-century Dutch painter, was a master of using light to communicate. His hyperrealistic portraits capture bright and darkness so accurately that, at first glance, they resemble photographs. Many speculate he used optical tools such as mirrors, lenses, or a camera obscura in the creation of his paintings, including the inspiration for this photograph, Girl with a Pearl Earring (c. 1665; fig. 2). Did Vermeer use his paints to simply trace light he projected onto a canvas via a camera obscura? Or is Girl with a Pearl Earring a version of reality that he saw with his eyes, interpreted in his occipital cortex, and then transmitted with his brush? Is one method superior to the other?

Fig. 2.

Girl with a Pearl Earring.

Fig. 2.

Girl with a Pearl Earring.

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The similarity between the science of the pulse oximeter and the art of Vermeer’s paintings points to the enduring universality of humans measuring and manipulating light to drive emotion and action. Gazing at Girl with a Pearl Earring fills the viewer with a sense of curiosity, intrigue, and a longing to know more. A pulse oximeter may not be art in the way we usually think, but what it communicates to us still creates emotion—worry or fear when the value is low, relief or happiness when the value is high. We will never know what the girl represented in Girl with a Pearl Earring truly looked like; we trust that what we see now is what Vermeer saw then. Similarly, when interpreting a pulse oximeter’s calculated value, we must remember that we cannot personally observe what is happening at the molecular level. For example, the pulse oximeter cannot reliably herald a decline in oxygen content in hyperoxemic states; its output will not be as representative of true hemoglobin saturation in persons with more melanated skin tones; it will falsely reassure us with a normal value in patients painted cherry red with carboxyhemoglobinemia.

Understanding how our tools work makes us better anesthesiologists. No single tool will be able to tell us, with complete certainty, what is really happening in and outside of our patient’s cells. Our job is to use all the data to triangulate something closest to the truth. And that, perhaps, is art.

Figure 1 is a photograph of Dr. Eckman Basiri, taken by Dr. Fabiszak. Figure 2 is a public domain image of Girl with a Pearl Earring by Johannes Vermeer, provided by the Mauritshuis art museum, The Hague, The Netherlands (https://www.mauritshuis.nl). No generative artificial intelligence tools were employed.

Supported by National Institutes of Health (Bethesda, Maryland) grant No. R35 GM145319 and departmental support.

Dr. Connor has consulted for Teleflex, LLC (Wayne, Pennsylvania), on issues regarding airway management and device design and for General Biophysics, LLC (Wayland, Massachusetts), on issues regarding inhalational kinetics. These activities are unrelated to the material in this article. The other authors declare no competing interests.