Monitoring for low minute ventilation during the 45 minutes before anticipated PACU discharge can be useful in identifying patients at risk for postoperative opioid-induced respiratory depression on the general hospital floor. Investigators concluded that monitoring patients with a respiratory volume monitor may help prevent this potentially devastating postoperative event.
“We’ve all been concerned about opioid-induced respiratory depression,” said Roman Schumann, MD, a professor of anesthesiology at Tufts University School of Medicine, in Boston. “So the real question is, how can we clinically identify it? Is there an objective measure that actually indicates respiratory depression? There are several scores that can be applied in clinical medicine, but they don’t seem to provide us with what we really need in this regard.”
Dr. Schumann and his colleagues employed a noninvasive respiratory volume monitor (ExSpiron 1Xi, Respiratory Motion Inc.), which assesses minute ventilation, tidal volume and respiratory rate. “This monitor has a high degree of accuracy in spontaneously breathing adults as well as in children who are not on a ventilator,” he said.
Patients were categorized as either low risk (zero low minute ventilation events) or high risk (one or more low minute ventilation events) in the final 45 minutes before PACU discharge. Any low minute ventilation event that occurred within 45 minutes of IV opioid administration was considered opioid-induced respiratory depression.
In characterizing the two groups, Dr. Schumann noted that 105 of the 119 patients (88%) were deemed to be low risk. “I feel it is somewhat reflective of what we see in our practice,” he said. Of interest, body mass index was greater in the low-risk patients (37.6 kg/m2) than their high-risk counterparts (32.4 kg/m2). STOP-BANG scores, on the other hand, were 3.2±1.6 in low-risk patients and 3.9±2.0 in high-risk patients.
As Dr. Schumann reported at the 2017 annual meeting of the American Society of Anesthesiologists (abstract JS06), 38% of low-risk and 43% of high-risk patients received IV opioids on the general hospital floor. Although the likelihood of experiencing opioid-induced respiratory depression was only 3.4% in low-risk patients, it was 44.4% in high-risk patients (P<0.001). In the cohort that did not receive IV opioids on the general hospital floor, high-risk patients had six times more low minute ventilation events per hour than low-risk patients (P<0.00001).
“We also analyzed minute ventilation over time,” Dr. Schumann continued. As demonstrated in Tables 1 and 2 and Figures 1 and 2, significant differences were found between the low- and high-risk groups, both in the PACU and on the general hospital floor.
|Table 1. Low- and High-Risk Differences in the PACU|
|Low-Risk Patients||High-Risk Patients||P Value|
|Predicted minute ventilation, %||114||77||0.008|
|Low minute ventilations per hour||0.19||1.24||<0.00001|
|Low minute ventilation duration, minutes||3.8||4.3||0.40|
|Monitored time without low minute ventilation, %||98.6||91.5||<0.00001|
|Table 2. Low- and High-Risk Differences on the General Hospital Floor|
|Low-Risk Patients||High-Risk Patients||P Value|
|Predicted minute ventilation, %||122||65||0.0006|
|Low minute ventilations per hour||0.23||1.51||<0.00001|
|Low minute ventilation duration, minutes||4.0||5.0||0.014|
|Monitored time without low minute ventilation, %||98.2||85.7||<0.00001|
“These tables illustrate a significant difference in minute ventilation between groups in the PACU and on the floor,” Dr. Schumann explained. “And it shows that maybe we are onto something here with the criteria we applied because the high-risk group seems to be consistently at less than 80% of predicted minute ventilation, while the low-risk group is near 100%.” Although minute ventilation for low-risk patients on the floor continued to improve, it declined in high-risk patients to levels below those measured in the PACU.
These results, Dr. Schumann explained, offer some fairly telling insights into the frequency of potentially dangerous respiratory events. Indeed, 14% of the study cohort had at least one low minute ventilation event within 45 minutes after PACU discharge. What’s more, high-risk patients who experienced one of these low-minute ventilation events near PACU discharge had a 10-fold increase in the incidence of opioid-induced respiratory depression after opioid administration on the general hospital floor. There was also a sixfold increase in respiratory depression in high-risk patients, even in the absence of IV opioids on the general hospital floor.
But as Dennis Fisher, MD, the professor emeritus of anesthesia at the University of California, San Francisco noted, there’s more to measuring minute ventilation than just a snapshot of one single point in time. “It was shown many years ago that it’s not just the average ventilation over a period of time that’s important; it’s also the pattern. Does the device look at patterns within that period of time or does it just give an aggregate?”
“It looks at real time, and also at trending over time,” Dr. Schumann replied.