When patients undergo surgery, it’s inevitable that they will lose some blood, so surgical teams strive to replenish patients’ fluids over the course of an operation. But the most common technique to track blood volume—catheters inserted through the heart that provide a readout on a monitor—are invasive and not particularly accurate.

According to Kirk H. Shelley, M.D., Ph.D., associate professor of anesthesiology, the flaws of catheter-based monitoring often engage operating room personnel in a delicate clinical balancing act with very high stakes. “Too little fluid can put a tremendous amount of stress on the kidneys, the cardiovascular system and the central nervous system. Organs need a certain amount of blood, and you’re risking a patient going into shock,” Shelley says. “But if you give too much fluid for the heart to pump, it backs up, causing bloating and pulmonary edema. Every day in the operating room, we try to find the right balance between these two extremes.”

Now Shelley, who as chief of ambulatory surgery takes part in about 8,000 surgeries a year at Yale-New Haven Hospital, has found a possible solution to this daily surgical dilemma that’s already very close at hand—or more precisely, clipped to patients’ fingers—in hospitals around the world. By combining a clinical insight from the 1870s with data provided by the modern pulse oximeter, a clothespin-like clip placed on a fingertip, ear or toe to measure the oxygen level in the blood, Shelley has discovered a noninvasive, precisely quantified method to monitor blood loss and guide difficult decisions in the operating room.

The pulse oximeter has become a common sight in hospitals since it was first introduced in the 1980s. The clips contain light-emitting diodes that shine both visible red and infrared light through the skin. Because deoxygenated hemoglobin allows infrared light to pass but absorbs red light, while oxygenated hemoglobin allows red light to pass and absorbs infrared, the oximeter can detect changes in the blood’s oxygen saturation by calculating the relative absorption of red and infrared light.

Shelley, who changed specialties from internal medicine to anesthesiology in the late 1980s, began a residency in his new field just as pulse oximeters appeared on the scene. In those early days, Shelley discovered that oximetry clips generated exceedingly complex waveforms that were “cleaned up” by oximeter manufacturers in favor of clear, simple signals. But Shelley’s curiosity about the wealth of information produced by early oximeters—“One man’s artifact is another man’s signal,” he says—prompted him to devise software to sift through the raw oximetry signal for potentially valuable clinical information.

In 1873 an observant German physician, Adolf Kussmaul, coined the term “pulsus paradoxus” for a phenomenon in which blood flow drops slightly after a deep breath, a dip caused when blood remains in the lungs and doesn’t reach the heart. Shelley discovered that pulsus paradoxus produced by the mechanical ventilation that accompanies general anesthesia could be detected in the raw oximetry waveform, and that this information could be combined with other data in the waveform to precisely manage fluid replacement in surgical patients.

L. Alan Carr, Ph.D., a senior licensing associate in Yale’s Office of Cooperative Research who shepherded the discovery through a patent application, says that Shelley found treasure where others saw trash. “There’s all sorts of wild, raw data that comes off the pulse oximeter that companies have worked hard to eliminate, because it has been seen as just noise,” Carr says. “What’s ironic is that the background data actually had useful information in it.”

As a member of an active research group headed by Professor of Anesthesiology David G. Silverman, M.D., which is devoted to noninvasive monitoring, Shelley is now adapting his method for use in non ventilated patients suffering from blood loss, such as trauma patients arriving at emergency departments. He plans to mine the pulse oximeter signal for more clinical riches, explaining that his affinity for noninvasive medical gadgetry stems from watching Star Trek’s Dr. McCoy in action.

“McCoy would pass his devices over the patient and would know exactly what to do with the patient,” Shelley says. “I really think the newer generations of the pulse oximeter and the new information we’re going to get out of them are going to be like that. We’re going to continue stepwise, evolving this.”