Respiratory Volume Monitors Track Ubiquitous Apneas

Author: Chase Doyle

Anesthesiology News

Although patients frequently experience cessation of breathing in the postoperative setting, these apneas seldom lead to sustained low minute ventilation over clinically relevant time scales, a study has concluded.

Data collected from noninvasive respiratory volume monitors in the PACU and on the general floor have shown that compensatory breaths following apneas generally restored minute ventilation to near pre-apnea levels. Nevertheless, researchers emphasized that some apneas can become dangerous when ignored, as when sedation decreases compensatory breath size.

“Apneic periods are very common, but they do not reflect oxygen saturation, which seems to be the last thing to drop,” said M. Alparslan Turan, MD, an associate professor of anesthesiology in the Department of Outcomes Research at the Cleveland Clinic, in Ohio. “However, very early signs of respiratory depression can be seen by looking at decreases in minute ventilation. Data from respiratory volume monitors provide a better overall metric of respiratory competence, driving better assessment of patient risk and individualization of care.”

To assess adequacy of ventilation in the postoperative setting, Dr. Turan and colleagues used a respiratory volume monitor (ExSpiron 1Xi, Respiratory Motion) to track minute ventilation in patients experiencing apnea in the PACU and on the general floor. The researchers continuously monitored minute ventilation for 48 hours following elective abdominal surgery. Minute ventilation was expressed as percentage of minute ventilation predicted based on body surface area and sex. Low minute ventilation or hypoventilation was defined in previous work as minute ventilation less than 40% of the predicted value. For each apnea incident, defined as a pause in breathing longer than 10 seconds, researchers calculated the patient’s minute ventilation over 30-, 60-, 90- and 120-second windows following the start of the apnea.

What Then Is the Relevance of Apnea?

As Dr. Turan reported at the 2018 annual meeting of the European Society of Anaesthesiology, 216 patients (110 men; body mass index, 26.7 kg/m²) were monitored for an average of 42 hours. The researchers recorded a total of 49,985 apneic incidents ranging in duration from 10 to 117 seconds, with 99% of patients experiencing at least one apnea.

“Since almost everybody gets apnea, there’s not much clinical relevance,” said Dr. Turan. “These results suggest that apnea is a poor indicator of respiratory insufficiency because patients typically compensate with higher tidal volume, resulting in a minimal change in oxygen saturation.”

The data showed that apneas lasting 10 to 18 seconds decreased instantaneous minute ventilation by as much as 30%, but their effect over the course of one minute was less than 10%. On a two-minute time scale, even 60-second apneas led to low minute ventilation just 20% of the time.

Nevertheless, the authors noted that respiratory volume monitors could still be useful in detecting critical events, as previous research from the Cleveland Clinic has shown that approximately 65% of critical desaturations go undetected on the general floor.

“There are so many devices available to detect differences in patients, but for the most part, these devices only provide information once a critical event has already occurred,” said Dr. Turan. “Respiratory volume monitors can predict and maybe help prevent some of the critical events that are going to happen on the floor.”

According to Dr. Turan, however, given the high cost of these devices, patient selection will be important if these devices are going to achieve more widespread use. “It will be necessary to choose patients who are more prone to critical events. Patients who are opioid dependent or who require large amounts of opioids on the PACU and general floor are our main safety issue. Future research is needed to see if predictions made by these devices are associated with meaningful outcomes.”