The following is one in a series of columns by guest authors illustrating the importance of vital signs to the practice of hospital medicine.
Imagine you are a physician in 1975. A 65-year-old man walks into the ED appearing mildly dyspneic. Vital signs reveal a temperature of 37 degrees Celsius, a pulse of 85 beats/min, a blood pressure of 155/85 mm Hg, and a respiratory rate of 20 breaths/min. How sick is this man?
You may immediately be thinking that something very important,vital even, is missing: The oxygen saturation is not listed. But in 1975, the pulse oximeter was not routinely available yet. Arterial blood oxygen saturation required an arterial puncture, which is quite painful to the patient, not to mention the anxiety it induces in the medical trainee attempting arterial cannulation for the first time.
It will likely surprise younger physicians to know that the modern pulse oximeter was not invented until the early 1970s and did not become commercially available until the 1980s. Even in the late 1990s there was still debate regarding the utility of routine pulse oximetry for ED patients. In 1988, Dr. Thomas Neff suggested that we should consider oxygen saturation by pulse oximetry as a “fifth vital sign.” This concept definitely took hold, and now the small but complex device is available in most primary care offices around the country. But even with its widespread availability, most physicians today probably can't explain how this powerful tool functions.
A pulse oximeter is able to non-invasively measure the amount of oxygen-saturated hemoglobin (oxyhemoglobin) in a patient's arterial blood. The science behind this amazing feat was mostly understood by 1852 when German physicist August Beer proved that the amount of light transmitted through a solution varies based on the concentration of solute. Practically applying this idea was much more difficult.
In 1939, Karl Matthes, a German physician, made a device that showed that oxyhemoglobin saturation could be measured in the ear. He made use of 2 different wavelengths of light, red and infrared, because infrared is absorbed more by oxygenated hemoglobin and red more by deoxygenated hemoglobin. One early application of this device was when American physiologist Glenn Millikan made an ear oximeter to alert pilots in World War II that their oxygen saturation was low. For his device, Millikan coined the term oximeter.
The concept can be employed flawlessly to measure oxygen in a test tube full of blood. It is only a simple comparison of the amount of red absorbed to the amount of infrared absorbed. The ear oximeter, however, actually performed very poorly, because the light absorbed by the ear is affected very little by arterial blood. A vast majority is absorbed by other tissues, including skin, fat, and connective tissue.
In London, J. R. Squire helped to partially solve this problem by using pneumatic pressure to squeeze the blood from the web space of the hand, allowing calculation of the baseline absorbance without blood. This idea was made more practically feasible when Earl Wood, an American physiologist/cardiologist, began to use the combination of an ear oximeter and pneumatic pressure. Dr. Wood, who is best known for his invention of the G-suit (which allowed pilots to survive high altitudes), continued to expand on the science and engineering of pulse oximetry throughout the mid-20th century.
The last major breakthrough occurred by serendipity, as so many major discoveries do. Takuo Aoyagi, a Japanese electrical engineer, was trying to use a version of Wood's ear oximeter to measure the dilution of dye for purposes of measuring cardiac output. He kept having difficulty because of the constant artifact created by pulsations. It was this difficulty, however, that prompted him to realize that the changes created with a pulsation represented only the change in arterial blood. Therefore, the time when a pulse was not present could be used as a baseline absorption, eliminating the need for pneumatic pressure. Unfortunately, the business Dr. Aoyagi worked for did not recognize the potential of his invention, and other companies eventually produced the first pulse oximeters.
Like most medical devices, the pulse oximeter is not perfect. It has been found to provide inaccurate results in certain settings (which are easily apparent when you understand how the device works). One major issue involves the presence of carboxyhemoglobin and methemoglobin, such as in carbon monoxide poisoning and methemoglobinemia. These molecules have similar absorbance patterns to oxyhemoglobin and therefore cause falsely elevated readings. Other color-related issues can cause problems, including certain dyes (like methylene blue), acrylic nails, and skin color.
The last important “limitation” of the pulse oximeter is that it is only a tool, meant to be used by a thinking medical professional. One important aspect of this rule is that oxygenation is not ventilation. Many patients have a normal oxygen saturation but still have a breathing problem in that they cannot clear carbon dioxide. Therefore, the invasive blood gas measurement has not been entirely replaced. Still, the next time you use a quick clip-on device to obtain the oxygen saturation information you need, remember how much pain you are saving the patient (and intern).