Tidal Volume Key to Identifying Post-op Hypoventilation

Author: Michael Vlessides

Anesthesiology News

When it comes to detecting low minute ventilation in the postoperative period, monitoring respiratory rate is not enough, a multicenter team of researchers has concluded.

Their study found that most hypoventilation episodes occurring in the first 48 hours after surgery were accompanied by decreases in tidal volume, while respiratory rate stayed within normal limits. These insights, they said, demand additional monitoring to stratify and protect patients at risk for respiratory depression after surgery.

“Using a respiratory volume monitor allows you to measure minute ventilation and tidal volume with 90% accuracy, and respiratory rate with 98% accuracy in a nonintubated patient,” she continued. “We can use this monitor to risk-stratify patients moving from the PACU to the floor, so those at higher risk are monitored more carefully for respiratory depression.”

The study comprised 353 patients (177 females; age, 48.8±15.3 years; body mass index, 26.5±4.8 kg/m²) recovering from abdominal surgery performed at the Cleveland Clinic, in Ohio. Each participant was monitored with a respiratory volume monitor (ExSpiron 1Xi, Respiratory Motion Inc.) in the PACU, and up to an additional 48 hours on the general hospital floor.

The device calculates minute ventilation, tidal volume and respiratory rate every five seconds. Minute ventilation is expressed as percent of predicted minute ventilation, based on patient sex, weight and height.

For the purposes of the trial, low minute ventilation events were defined as minute ventilation less than 40% of predicted minute ventilation for at least two minutes. Low tidal volume events were defined as tidal volume less than 3 mL/kg sustained for at least two minutes. Finally, low respiratory rate was defined as less than 8 breaths per minute (bpm) sustained for at least two minutes.

Patients were separated into two groups based on the number of low minute ventilation events they experienced in the last 60 minutes of their PACU stay: group A, 0 events and group B, one or more events. Group A was considered not to be at risk for respiratory depression on the general floor, and group B was considered to be at risk.

The researchers compared the groups on rates of low minute ventilation events, low tidal volume events and low respiratory rate events on the general floor, from the time of arrival on the floor to 48 hours.

Tidal Volume Versus Respiratory Rate

As Dr. Duke reported at the 2019 annual meeting of the International Anesthesia Research Society (abstract D140), patients were monitored for 2.9±1.5 hours in the PACU, followed by 42.2±10.4 hours on the floor. It was found that the 332 patients in group A experienced 0.004 low minute ventilation events per hour, significantly less than their 21 counterparts in group B, who experienced 1.37 low minute ventilation events per hour (P<0.001).

Similarly, the rate of low tidal volume events was significantly lower in group A, at 0.09 events per hour, than in group B with 1.42 events per hour (P<0.001).

Despite these differences, both groups proved comparable on the rate of low respiratory rate events. This rate decreased from approximately 0.25 per hour in the first 10 hours to less than 0.09 per hour on the second day of monitoring for both groups.

The researchers also explored the percentage of patients in each group with low minute ventilation events for each hour on the general floor. Not surprisingly, they found that the majority of patients in group A did not have any events for each hour. In contrast, approximately half the patients in group B had at least one event within the first 10 hours on the floor.

“These results suggest that tidal volume, not respiratory rate, is the greatest contributor to changes in minute ventilation,” Dr. Duke told Anesthesiology News. “What’s more, if a patient suffers a low minute ventilation event in the PACU, the monitor can risk-stratify them to allow clinicians to make the best decision regarding their care on the nursing floor.”

For Dr. Duke, the advent of respiratory volume monitoring offers an insight into patient care that was largely unavailable throughout her 35-year career. “I wish I had this earlier in my career,” she said.

“It’s not enough to come in and just check the patient’s respiratory rate and leave them sleeping. The respiratory rate is going to be totally normal the majority of the time, and yet the patient can be severely hypercarbic,” she continued. “For its part, the SpO₂ [peripheral capillary oxygen saturation] is very late in detecting respiratory depression because it’s an oxygenation monitor, not a ventilation monitor.

“The patient must have significant hypoxemia before the SpO₂ monitor alarms, and SpO₂ is notorious for having many false alarms,” she noted. “The minute ventilation monitor has almost no false alarms and it detects impaired ventilation very early to allow prevention of, rather than rescue from, serious consequences of respiratory depression. I think this kind of monitor offers a fabulous safety net for patients.”

Davide Cattano, MD, PhD, told Anesthesiology News that he thinks a monitor for minute ventilation is necessary. “What has been intriguing is the tendency and emphasis that many investigators—including the manufacturer of the device—have placed in trying to identify patients at risk of opioid-induced respiratory depression,” said Dr. Cattano, a professor of anesthesiology at McGovern Medical School at UT Health, in Houston.

“While the search for such a holy grail is necessary, I think the findings to date have been quite disappointing, except for the fact that obese patients and those with [obstructive sleep apnea] are not at increased risk compared to the general population,” Dr. Cattano said. “Although such results seem to contradict common sense, they are nonetheless very interesting.”