Patients who lose too much blood during surgery can suffer heart attacks.

But measuring blood volume requires either inserting a catheter into the pulmonary artery, ordering an expensive echocardiogram, or resorting to guesswork.

Kirk ­Shelley, an anesthesiologist at Yale University, has devised a way to noninvasively measure blood loss using a pulse oximeter, a finger-clip device commonly used to measure pulse rate and blood oxygen levels in hospital patients. The pulse oxime­ter measures how much light of different wavelengths the blood absorbs.