Monthly Archives: March 2014

Hypoxia After Surgery Much More Common Than Previously Believed



Study finds high rate of prolonged bouts of desaturation on wards

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 (abstract BOC12) 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.”

“Sobering” Results

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.

Elizabeth A. M. Frost, MD, clinical professor of anesthesiology at Icahn School of Medicine at Mount Sinai, in New York City, wondered if transient hypoxia is really so harmful.

“Does desaturation lead to respiratory arrest? I suppose at the end of the day it would, but how often are patients somewhat hypoxic after surgery and nothing happens?” Dr. Frost asked. Evidence suggests that patients who do not receive oxygen during transport to the postanesthesia care unit arrive there hypoxic. “But there do not seem to be any adverse consequences,” she noted.

Finally, she said, “We know that REM/NREM [rapid eye movement/non-REM] sleep patterns are disturbed for several days postoperatively. Is this what causes hypoxia? And how many patients actually desaturate during sleep normally, without surgery? Are they at risk for arrest?”

—Adam Marcus




For Sciatic Blocks, Lower Dose of Local Provides Ample Anesthesia


San Francisco—Where 30 mL once stood as a standard dose, 10 mL now does the job.

So concluded a recent study by Illinois researchers of sciatic nerve block in patients undergoing total knee arthroplasty, which found that although 2.5 and 5 mL doses of local anesthetic provided suboptimal analgesia, amounts between 10 and 30 mL were comparably effective when paired with ultrasound guidance.

“We do a lot of regional techniques for total knee replacements, and we performed this study because we’re always looking for the minimum dose that will still offer the benefit we’re aiming for,” said Jessica J. Buren, MD, a fellow in regional anesthesia and acute pain medicine at Northwestern University Feinberg School of Medicine, in Chicago.

Dr. Buren and her colleagues enrolled 139 adults undergoing total knee arthroplasty into the trial. Each underwent a sciatic nerve block using the infragluteal-parabiceps approach under ultrasound guidance and nerve stimulator assistance.

“We use ultrasound guidance to visualize the nerve sheath, which we call the common investing extraneural layer [CIEL],” said Dr. Buren, who presented the findings at the 2013 annual meeting of the American Society of Anesthesiologists (abstract 1315). “With the nerve stimulator, the minimum current needed to get a response is very different if the needle is outside the CIEL or inside the CIEL.”

Participants were randomized into 14 groups receiving 0.5% bupivacaine or 0.5% ropivacaine with epinephrine in amounts ranging from 2.5 to 30 mL. After local anesthetic administration, a blinded clinician assessed sensory and motor function every two minutes for 10 minutes, and every five minutes thereafter until one hour before surgery, researchers said.

After surgery, the clinician assessed the patient’s motor and sensory block every four hours until the anesthetic was completely resolved. A block was considered successful if it produced complete motor block and insensitivity to pinprick at the one-hour mark in the sural, tibial, deep and superficial peroneal nerves.

“We found that in subjects where we used 5 mL or less of local anesthetic, we had 30 of 40 subjects with a successful block,” Dr. Buren told Anesthesiology News. “By comparison, in the patients who received 10 mL or greater of local anesthetic, 97 of 99 had a complete block.

“What’s more,” she continued, “the patients who received 10 mL and greater had less pain behind the knee in the first 24 hours after surgery. They also had a longer duration of motor block [up to 56 hours], of course, which is not what we were aiming for, but is what we expected.”

These findings are consistent with much of current literature on the subject, said Chad M. Brummett, MD, assistant professor of anesthesiology at the University of Michigan Health System, in Ann Arbor, who moderated the poster session. “This is probably going to be the biggest change in regional anesthesia practice in the coming years: volume and concentration. It will be interesting to see how important a role concentration begins to play as we drop volume. Certainly we can expect to see local anesthetic toxicity drop off.”


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