A Comparison of Measurements of Change in Respiratory Status in Spontaneously Breathing Volunteers by the ExSpiron Noninvasive Respiratory Volume Monitor Versus the Capnostream Capnometer
Authors: Williams George W. , George Christy A. , Harvey Brian C. , Freeman Jenny E. 
Published: July 2016

BACKGROUND:
Current respiratory monitoring technologies such as pulse oximetry and capnography have been insufficient to identify early signs of respiratory compromise in nonintubated patients. Pulse oximetry, when used appropriately, will alert the caregiver to an episode of dangerous hypoxemia. However, desaturation lags significantly behind hypoventilation and alarm fatigue due to false alarms poses an additional problem. Capnography, which measures end-tidal CO2 (Etco
2) and respiratory rate (RR), has not been universally used for nonintubated patients for multiple reasons, including the inability to reliably relate Etco
2 to the level of impending respiratory compromise and lack of patient compliance. Serious complications related to respiratory compromise continue to occur as evidenced by the Anesthesiology 2015 Closed Claims Report. The Anesthesia Patient Safety Foundation has stressed the need to improve monitoring modalities so that “no patient will be harmed by opioid-induced respiratory depression.” A recently available, Food and Drug Administration–approved noninvasive respiratory volume monitor (RVM) can continuously and accurately monitor actual ventilation metrics: tidal volume, RR, and minute ventilation (MV). We designed this study to compare the capabilities of capnography versus the RVM to detect changes in respiratory metrics.

METHODS:
Forty-eight volunteer subjects completed the study. RVM measurements (MV and RR) were collected simultaneously with capnography (Etco
2 and RR) using 2 sampling methods (nasal scoop cannula and snorkel mouthpiece with in-line Etco
2 sensor). For each sampling method, each subject performed 6 breathing trials at 3 different prescribed RRs (slow [5 min−1], normal [12.6 ± 0.6 min−1], and fast [25 min−1]). All data are presented as mean ± SEM unless otherwise indicated.

RESULTS:
Following transitions in prescribed RRs, the RVM reached a new steady state value of MV in 37.7 ± 1.4 seconds while Etco
2 changes were notably slower, often failing to reach a new asymptote before a 2.5-minute threshold. RRs as measured by RVM and capnography during steady breathing were strongly correlated (R = 0.98 ± 0.01, bias = Capnograph-based RR – RVM-based RR = 0.21 ± 1.24 [SD] min−1). As expected, changes in MV were negatively correlated with changes in Etco
2. However, large changes in MV following transitions in prescribed RR resulted in relatively small changes in Etco
2 (instrument sensitivity = ΔEtco
2/ΔMV = −0.71 ± 0.11 and −0.55 ± 0.11 mm Hg per 1 L/min for nasal and in-line sampling, respectively). Nasal cannula Etco
2 measurements were on average 4 mm Hg lower than in-line measurements.

CONCLUSIONS:
RVM measurements of MV change more rapidly and by a greater degree than capnography in response to respiratory changes in nonintubated patients. Earlier detection could enable earlier intervention that could potentially reduce frequency and severity of complications due to respiratory depression.