Noninvasive respiratory volume monitoring within the first two hours after surgery may be able to identify patients at risk for opioid-induced respiratory depression up to 48 hours later.
According to an observational study that measured minute ventilation in patients after colorectal surgery, frequency of low minute ventilation at discharge from the PACU was predictive of respiratory issues on the general hospital floor.
“Based on these results, patients who are at risk for respiratory depression in the PACU are far more likely to be at risk up to two days later,” said Wael Saasouh, MD, who conducted this analysis with colleagues from the Department of Outcomes Research at Cleveland Clinic Anesthesiology Institute, in Ohio. “Identifying which patients would benefit from additional monitoring on the general hospital floor and modification of opioid dosing would help improve safety while best allocating resources to contain costs.”
As Dr. Saasouh reported, although opioids are a mainstay of postoperative pain management, they can also lead to opioid-induced respiratory depression, a potentially life-threatening condition. Additional monitoring for patients receiving opioids has therefore been advocated, but there is not a clear and easy way to detect respiratory depression and intervene outside the PACU.
“When patients are in the PACU, they’re receiving high-intensity monitoring, so if they crash, a nurse or an anesthesiologist can always intervene,” Dr. Saasouh explained. “The problem happens one or two days later when they’re on the general hospital wards. Several studies have shown that lack of monitors on the general wards allows clinically significant hypoxemia to be missed. Part of the problem is that continuous noninvasive respiratory monitoring is still not routinely performed.”
Tracking Patient Respiration
Dr. Saasouh and his colleagues used a respiratory volume monitor (ExSpiron, Respiratory Motion) to quantitatively measure tidal volume, respiratory rate and minute ventilation for up to 48 hours after abdominal surgery. The system consisted of a lightweight monitor, a patient cable and a disposable electrode pad set, which noninvasively provides real-time, continuous minute ventilation measurements.
“We wanted to know how many patients experience low minute ventilation events [LMVe] in the PACU postoperatively, and then we wanted to know how many of these patients would actually continue to have these events later on during their hospital stay,” Dr. Saasouh said.
Predicted minute ventilation was calculated for each patient based on body surface area. An LMVe was defined as minute ventilation less than 40% of predicted minute ventilation sustained for at least two minutes, and LMVe rate was calculated as the number of LMVe per hour. Patients were then subdivided into three risk groups based on their minute ventilation in the PACU.
As Dr. Saasouh reported at the International Anesthesia Research Society’s 2017 annual meeting (abstract 1309), most patients started off with very infrequent low minute ventilation. Of the 167 patients enrolled and monitored in the study, 67 started off in the “green zone,” indicating that they were relatively safe. These patients continued to stay at low risk for respiratory depression up to 48 hours after surgery. But a subset of patients who initially were found to be at risk, remained at risk throughout the course of the study.
“One might suspect that postoperative hypoventilation is limited to the PACU stay as a consequence of anesthesia and opioid administration, and would resolve by the time a patient is deemed fit to be transferred to the general hospital floor. Based on these results, however, that turns out not to be entirely true,” said Dr. Saasouh, who noted that at-risk patients continue to have LMVe at a rate of three to four per hour. “These patients are still at higher risk of having respiratory issues on the floor two days later, where there’s virtually no monitoring.
“Instead of monitoring every patient postoperatively,” he said, “physicians can at least start with the higher-risk group by keeping them on a respiratory volume monitor. Once those patients are out of the danger zone, the monitors can be removed.”
Beneficial Data on Ventilation
Results of the study also showed that the majority of patients maintained adequate minute ventilation, with few LMVe in the PACU and on the general hospital floor, showing that the respiratory volume monitor can provide useful data without unnecessary alarms.
“The monitors are noninvasive, so they’re not difficult for the patient to bear,” Dr. Saasouh added, “but they provide physicians a lot of information, and they’re an easy way to follow patients.”
Vikas O’Reilly-Shah, MD, PhD, assistant professor of anesthesiology at Emory University School of Medicine, in Atlanta, inquired about the study’s inclusion criteria.
“This is actually one arm of a bigger prospective trial that has been going on for about 2.5 years now and will continue for another year,” Dr. Saasouh replied. “The original study is for colorectal surgeries lasting at least two hours and for which all patients receive general anesthesia. The second and third arms of the study measure blood oxygen saturation and blood pressure, respectively.
“It’s difficult to see the whole picture without unblinding, but we are actively working on combining these minute ventilation data with the oxygen saturation data,” Dr. Saasouh continued. “We want to see if having lower than predicted minute ventilation translates to actual hypoxemia, and if it does, whether that leads to adverse events.”