Patient Monitoring

Respiratory Volume Monitoring

By Chris Vocopoulos, M.D., Duke University, Durham, NC.
Gary Mullen, M.D., M.B.A., Anesthesiologist, Vidant Medical Center, Greenville, NC.

Adequate breathing, also known as ventilation, is key to survival. Physicians have long sought a better way to continuously monitor ventilation, but limitations in technology and sensor design have made it difficult. Ventilation is an important physiologic parameter to measure, especially in hospitalized patients, because pain medicines in the opioid class have a significant impact on a patient’s rate and quality of breathing.   Opioid Induced Respiratory Depression (OIRD), alternately called OIVI (opioid-induced ventilatory impairment) continues to negatively affect patient safety, and “is frequently the most serious of the opioid-induced adverse effects”.[i]

There are many places in the hospital where ventilation is carefully monitored, yet the most notorious cases of OIRD tend to occur on hospital floors where patients are receiving opioids for pain relief but have infrequent or inadequate monitoring. When a patient is transferred from a high-acuity setting such as the PACU (Post Anesthesia Care Unit) to the floor, they are especially at risk because of the change in frequency of vital signs monitoring and a reduction in staffing ratios.   Other areas where sedation is used and patients are at risk for respiratory compromise include the endoscopy and cardiac catheterization theaters. These “offsite” locations frequently do not have an anesthesiologist or anesthetist supervising the administration of sedation; instead, the sedation is administered by nurses or proceduralists with a different skill set.  Additionally, a smaller number of respiratory problems will manifest after a patient is extubated (when the breathing tube is removed) after surgery, or when they are extubated in the ICU setting. It is impossible to predict when a particular patient will have a problem, because each patient responds differently to medications used to sedate them. Because patients being administered opioids cannot always be closely monitored by qualified staff, devices are needed that can quickly and objectively communicate respiratory data.

An increased emphasis on pain management over the past decade has driven awareness of the need for better respiratory monitoring, but monitoring the respiratory function has fallen well short of monitoring the cardiac function. For decades now, healthcare providers have had reliable cardiac monitoring and telemetry (telemetry is the process of sending the monitored information to a screen somewhere else, so that a doctor who is not next to the patient can see the same information displayed). Good cardiac monitoring was the result of having a reliable sensor – the EKG—as well as a substantial infrastructure that included both bedside and central nurse station monitoring. When a problem occurred, the information would be conveyed to a central monitoring station, and a healthcare provider could respond almost immediately. Until now, physicians have not had that capability for monitoring ventilation. There has been neither a reliable sensor nor a central infrastructure in place to carry the information.

When pulse oximetry came into use in the 1980s it was frequently thought of as a solution to prevent respiratory disasters, but oxygen saturation turned out to be a very late sign of respiratory depression. The sensor position on the finger was also problematic, leading to frequent false alarms from movement or low-blood-flow states. In patients receiving supplemental oxygen, the additional oxygen often masked declining respiratory function as the carbon dioxide would build up in the body (a condition called hypercapnea). A patient’s carbon dioxide level is affected by ventilation independent of the oxygen level, such that a patient can have a normal pulse-oximetry reading and a life-threatening elevation of carbon dioxide. Only appropriate ventilation can lower the CO2. In the 1990s, measuring exhaled carbon dioxide (end tidal CO2, ETCO2, or capnometry) came into widespread use. This technology worked well with patients on ventilators, but not as well for extubated patients. For these patients breathing on their own, the sampling cannula, placed at the mouth and nose, is easily dislodged, is prone to airflow disturbance (i.e., oxygen administration) and contributes to alarm fatigue.

Other monitors can be deceptive indicators of respiratory condition. Rate and apnea monitors may miss a diagnosis of respiratory depression because patients can have obstructed breaths that are counted as real breaths, or they can have ineffective, small breath sizes with adequate rate. Conversely, patients with respiratory pauses and sleep apnea have large “rescue breaths” that usually keep them safe. Better ventilation monitoring is needed for non-intubated patients.

To better monitor adverse events from opioid use, new guidelines, including March 2014 CMS guidelines, are in place that require hospitals to address better monitoring of breathing problems in post-operative patients using opioids. New and better technologies that can provide a better gauge of respiratory competence and which can be used to communicate succinctly a patient’s status will be helpful in understanding compromised breathing in non-intubated patients and in meeting these new guidelines.

A new technology has recently become available for clinical use that is an improvement on existing respiratory monitoring. Using impedance, and electrodes similar to the ECG, non-invasive Respiratory Volume Monitoring (RVM) can, for the first time, monitor minute ventilation (MV) in patients not on a ventilator. MV is the amount of air that goes in and out of the lungs in one minute. MV is the product of both the size of each breath (tidal volume), and the respiratory rate. This fundamental measurement of breathing is the same measurement anesthesiologists and critical care physicians see on their ventilators when they are controlling and monitoring breathing for mechanically ventilated patients. Until now, upon extubation, doctors could not easily or reliably measure MV. It is now possible to continue to monitor ventilation after extubation with the same level of accuracy and confidence as when the patient was on the mechanical ventilator.

Studies have shown the accuracy and clinical utility of RVM monitoring in a wide variety of adult patient populations. Continuous, real-time monitoring of MV in these settings can identify patients with respiratory depression and improve patient safety by alerting doctors and nurses as a problem is occurring instead of after it has occurred.

Physicians have observed that comparing MV measurements before and after opioids can help make this assessment and be predictive of recovery after opioid administration.[ii] There is currently no reliable way to distinguish those patients who are most at risk for an unexpected reaction to opioids. However, MV data potentially can be used in the PACU to help better identify those patients with sensitivity and provide them with a dosing regime that can keep them safe on the floor. A real-time decline in MV can be seen in some patients after their initial opioid dosing, and that data can be used to modify dosing or dictate changes is therapy.. Better identification of those patients who may be at risk for respiratory depression would be helpful to institutions who need to determine the patients requiring intervention such as CPAP or BiPap, reversal agents, or transfer to a more monitored bed.   Sending the at-risk patients, instead of all patients, to monitored beds improves patient safety, while substantially reducing hospital morbidity and associated costs.

Bringing respiratory monitoring up toward the level of cardiac monitoring has the potential to help reduce OIRD/OIVI and other respiratory deaths in the hospital.


[i] Jarzyna et al., American Society for Pain Management Nursing Guidelines on Monitoring for Opioid-Induced Sedation and Respiratory Depression. Volume 12, No. 3, 2011: pp. 118-145.

[ii] Voscopoulos, C, et al. Opioid-Induced Respiratory Depression—Evaluation of a New Protocol to Identify Patients at Risk. American Society of Anesthesiologists, October 2013, San Francisco, CA.

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