By Adam Marcus

A surprisingly large fraction of patients experiences prolonged periods of hypoxemia while recovering from surgery, new research shows. Although the implications of the findings for patients are not yet clear, experts said results suggest that efforts to monitor oxygen saturation on the ward are not nearly as effective as clinicians might assume.

“The way we’re doing it now is not providing physicians with what they really want, which is an early warning sign of respiratory distress,” said Daniel I. Sessler, MD, chair of the Department of Outcomes Research at Cleveland Clinic, in Ohio, who helped conduct the study. “I’m guessing that in 10 years, maybe even in five years, continuous pulse oximetry will be the standard of care on hospital wards because hypoxia is so common,” Dr. Sessler said.

The prospective observational study, which Dr. Sessler, Andrea Kurz, MD, and their colleagues presented at the 2013 annual meeting of the American Society of Anesthesiologists was a subanalysis of the VISION study, which looked at 40,000 patients over 45 years old undergoing noncardiac inpatient surgery. Patients were included if they were over age 45 and were admitted for inpatient procedures at Cleveland Clinic and a second hospital. The roughly 1,500 patients in the substudy had continuous pulse oximetry from the time they left the postanesthesia care unit or the intensive care unit for up to 48 hours.

Unlike previous studies of continuous pulse oximetry, the Cleveland Clinic researchers masked the monitors and muted their alarms to blind clinicians to their output. “They thus had no way of knowing the saturations we recorded,” Dr. Sessler said, although they were permitted to perform their own clinical routines.

That blinding was important, Dr. Sessler added, because “if alarms go off, people come in and do things. You can’t then ask how long patients would otherwise have remained hypoxic.”

The results, he said, were “pretty sobering”: Approximately 21% of patients averaged at least 10 minutes per hour with SpO2 values below 90%, and approximately 8% of patients averaged at least 20 minutes per hour. Approximately 8% of patients averaged at least 5 minutes per hour with SpO2 less than 85%.

Although the researchers did not evaluate clinical sequelae of hypoxic episodes—and the study wasn’t powered to do so—Dr. Sessler said the implications are concerning. “Most people don’t think that it’s a good thing to have prolonged periods of desaturation.” One likely consequence, he noted, is poor wound healing because adequate tissue oxygenation is key to both healing and fighting infection.

Opioids surely contribute to postoperative hypoxia, Dr. Sessler said. Another obvious cause is sleep apnea. Indeed, the Cleveland group is now evaluating the relationship between sleep apnea and hypoxia in hospitalized patients.

Dr. Sessler and his colleagues also are comparing nursing reports with their continuous pulse oximetry results. “We think there’s a huge discrepancy there. Nurses wake patients up and start taking vital signs, and by then people are breathing fine. Then they go back to sleep and start desaturating.”

Eugene Viscusi, MD, professor and director of acute pain management at Thomas Jefferson University Hospitals, in Philadelphia, said the study raised cause for concern. “Generally, we would think of 85% saturation as needing treatment. We consider these patients to be on the cusp of disaster, which I don’t deny. However, it is interesting that there were no catastrophic results.

“I see two lines of questions here,” Dr. Viscusi added. “One is the above: Just how risky is this hypoxia? Can we quantify the risk of these progressing to a bad outcome? The second question is how to predict which patients will become this hypoxic,” which boils down to demographics that aren’t yet clear.