Respiratory rate is one of the traditional vital signs that every patient should have monitored. However, it has been described as being the “neglected vital sign.”1 As part of an initial evaluation for a patient, particularly those exhibiting any form for respiratory distress, respiratory rate provides critical information to the bedside clinician. In addition, during more extended periods of patient care, for example, during the postoperative course of a patient following major surgery, respiration rate is measured, charted, and followed by involved clinicians to some extent. Frequently, the respiratory rate is either not recorded or constantly denoted as 16 or 20 respirations per minute. Respiratory rate is an important indicator of wellbeing of a patient, is an early sign of serious illness, and is an early warning of a poor outcome.2–4 Respiratory abnormalities, and in particular, tachypnea, have been frequently documented in patients prior to both intensive care unit transfer and cardiopulmonary arrest.2,3,5 Extremes of respiration rate (i.e., less than 6 and greater than 30 breaths per minute) were found to be independent predictors of in-hospital mortality for patients on the general care floor.4 Increasing respiratory rate and declining respiratory rate may be associated with early sepsis and opioid-induced ventilatory depression.6
The appreciation of deterioration in vital signs as a harbinger of poor patient outcome has been a major impetus for the implementation of rapid response teams. In evaluating the effectiveness of these programs, it has been observed that respiration rate monitoring is the most frequently absent recorded vital sign.7
The body maintains a healthy state by increasing respiratory rate in response to hypoxia, hypercarbia, and acidosis. Therefore, why has the respiratory rate become poorly monitored? The nurse counting respiratory rate is time consuming, it is not continuous, cannot be audited and, to be sure there is adequate ventilation associated with chest movements, requires a stethoscope.8 The stethoscope was invented by Rene Laennec in the 1840s, and this allowed breath sounds and rate to be characterized. Several decades ago it was very common to see anesthesiologists with a monaural stethoscope, with a custom-made earpiece, taped to a patient’s chest continually monitoring ventilation during an anesthetic. This has given way, during general anesthesia, to continuous monitoring of pulse oximetry, capnography, and minute volume. An esophageal stethoscope may be present but is rarely used continuously. In the unintubated patient, pulse oximetry, capnography, and impedance plethysmography obtained from the electrocardiogram are most frequently used to assess ventilation. In the postanesthesia care unit and general ward, continuous pulse oximetry may be used on specific patients but the majority of patients rely on the nurse intermittently counting respiratory rate.
Enhanced continuous physiologic monitoring in the form of pulse oximetry has been evaluated as a strategy to improve patient safety.9,10 Pulse oximetry is frequently relied on to assess ventilation as well as oxygenation, and yet it is a very late detector of hypoventilation when supplemental oxygen is being administered.11 However, given the limitations of pulse oximetry, particularly in patients receiving supplemental oxygen, the addition of respiration rate monitoring may add additional benefit to these monitoring strategies. The Joint Commission, Anesthesia Patient Safety Foundation, and the Centers for Medicare and Medicaid Servicesa have now published advisories that patients at risk for respiratory depression from postoperatively administered opioids should have oxygenation continually monitored and if supplemental oxygen is being given ventilation should be monitored as well.12,13
A range of technologies is available to help achieve this goal, but these technologies all have limitations under certain conditions. Capnography is most frequently used to monitor ventilation, and expired carbon dioxide is sampledvia nasal or oral-nasal cannula interfaces. The drawbacks of this technology are that hypoventilation may cause a reduction in the end-tidal carbon dioxide value, instead of an increase, as alveolar gas is no longer adequately exchanged. Some patients, and especially children, will dislodge the sampling interfaces and the technology will no longer be monitoring the patient. Transthoracic impedance plethysmography measures the respiratory rate from changes in impedance as the chest wall expands and relaxes. This technology may not detect obstructed breathing as the chest wall may continue to move with little or no passage of air. Recently, there has been the development of an enhanced impedance technology that can detect tidal volume and minute volume with approximately 90% accuracy.14
In this issue of Anesthesia & Analgesia, Atkins and Mandel15 document the performance of one of the available technologies for continuous respiration rate monitoring. In a group of 53 patients undergoing general anesthesia with a laryngeal mask airway, they compared the respiration rate measures from abioacoustic device to the measures derived from continuous spirometry provided by a pneumotachograph. Because of the inherent variability of respiration rate, they utilized a mathematical technique to provide an instantaneous assessment of respiration rate and compared these values to those provided by the acoustic device. The performance assessment included 3 different elements: precision of the acoustic device measurement versus the standard, delay time between the instantaneous rate determination and the device, and finally, if respiration rate values were different by >4 breaths per minute, the duration of the discrepancy. Atkins and Mandel found the acoustic device to have a precision of approximately 2 breaths per minute over an observed range from 7 to 48 breaths per minute. In the setting of changing respiration rate, the acoustic device demonstrated a median delay of 45 seconds relative to the instantaneous assessment method. Finally, 90% of the disparate respiration rate measures resolved within 33 seconds. Atkins and Mandel concluded that the respiratory acoustic monitor provides accurate estimates of respiratory rate changes during spontaneous ventilation during general anesthesia.15 This technology may provide a solution for the accurate and continuous monitoring of respiratory rate for hospitalized patients in other areas of care.
There have now been a number of studies evaluating bioacoustics sensor technology, in which an integrated acoustic transducer that collates respiratory vibrations to detect inspiratory and expiratory air flow displays the acoustic signal and the respiratory rate continuously.16–19 This is bringing health care professionals back to listening to breathing again.20 As the analysis of the auditory signals from air movement improves, vital information about respiratory health may potentially be determined from bioacoustics monitors and reviewed remotely.21 Obstructed, noisy breathing maybe observed as a wide amplitude signal. Wheezes have a dominant frequency usually >100 Hz and a duration of <100 milliseconds. Rhonchi are >250 milliseconds in duration and of a lower pitch and frequency of <200 Hz. In the future, we maybe listening to breathing from a visual monitor, while respiratory rate is counted accurately and continuously.
Studies of the performance accuracy of other emerging devices, such as respiration rate derived from the pulse oximeter photoplethysmograph signal, have utilized capnography-based measures of respiration rate. Signal processing monitoring devices such as pulse oximetry and the bioacoustics respiratory monitor calculate a steady stream of values and yet only present a single value on the monitor screen. These monitors automatically average their values over a window of time. Atkins and Mandel have provided a new tool with their approach of utilizing instantaneous respiration rate estimates to characterize performance of these devices.
What remains to be established, however, is the level of performance (accuracy, response time, persistence of discrepancy) required for these devices to have clinical benefits in different areas of care. For example, a different performance requirement may exist for these devices in the operating room versus the postanesthesia care unit versus the procedural sedation suite. The ability of these devices to provide an adequate substitute for the range of physiologic information provided by capnography apart from respiration rate will need to be determined. However, given the current lack of respiration rate monitoring on the general care floor, it is likely that the available devices are sufficiently accurate for implementation into continuous monitoring care strategies. The critical evidence needed is to assess whether the addition of continuous respiration rate monitoring can further enhance patient safety and outcomes when utilized. Given the clear role of respiration rate as a “vital” sign, this evidence will likely be provided in the near future.
Name: Scott D. Kelley, MD.
Contribution: This author helped write the manuscript.
Attestation: Scott D. Kelley approved the final manuscript.
Conflicts of Interest: Scott D. Kelley worked for Covidien and has equity interest in Covidien.
Name: Michael A. E. Ramsay, MD, FRCA.
Contribution: This author helped write the manuscript.
Attestation: Michael A. E. Ramsay approved the final manuscript.
Conflicts of Interest: Michael A. E. Ramsay received research funding from Masimo.
This manuscript was handled by: Maxime Cannesson, MD, PhD.
a Center for Clinical Standards and Quality/Survey & Certification Group. Ref: S & C: 14-15-Hospital. Available at: http://cms.gov/Medicare/Provider-Enrollment-and-Certification/SurveyCertificationGenInfo/Policy-and-Memos-to-States-andRegions-Items/Survey-and-Cert-Letter-14–15.html.Accessed July 23, 2014.
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