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October 17-21, 2009 New Orleans, LA Home Abstract Archive Search Abstracts 2009 Session Grid 2009 Meeting Website Policy Statements ASA Website Feedback 
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A1292
October 20, 2009 2:00 PM - 4:00 PM Room Area M |
| Evaluation of Breath Rate Measurement by Capnometry in Non-Intubated Sedated Volunteers |
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* Joseph A. Orr, Ph.D., Lara M. Brewer, M.S., Dwayne R. Westenskow, Ph.D., Ken B. Johnson, M.D. Anesthesiology, University of Utah, Salt Lake City, Utah |
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Introduction:
Capnometry is used to monitor respiration in non-intubated, spontaneously breathing patients undergoing procedural sedation, patient controlled analgesia and other procedures where respiration may be compromised. Accurate breath rate measurement using capnometry in non-intubated patients is more prone to false breath detection and missed breaths because of difficulties in acquiring an adequate alveolar gas sample, sample site location changes, irregular breathing, patient movement, etc. Capnometers detect a breath when a change in the CO
2
concentration is observed without regard for the volume of gas responsible for creating this change. We evaluated the accuracy of capnometry breath rate detection algorithms in volunteers administered a combination of target controlled infusions of a sedative hypnotic (propofol) and a short acting opioid (remifentanil).
Methods: We compared the accuracy of two breath detection/calculation algorithms (1) a simple threshold, and (2) a rule based as implemented in the capnometer (Capno5/LoFlo, Respironics, Wallingford CT) against breath rate as measured by a pneumotach system. The threshold algorithm detects a breath each time the capnogram exceeds 10 mm Hg and subsequently goes below 5 mm Hg. The rule based algorithm incorporates a complex set of heuristics for detecting breaths and rejecting spurious changes in CO 2 concentration. Both algorithms were compared against breath rate calculated by a pneumotach. Smaller gas movements (< 200 ml) that did not clear the airway dead space were not counted as breaths. 24 volunteers were fitted with a tight fitting sealed mask connected to a combination flow and CO 2 sensor. Each volunteer received 15 effect site concentration combinations of propofol and remifentanil delivered as target controlled infusions. The breath rate as calculated using the threshold algorithm and the rule based algorithm were separately compared against the breath rate as measured with the reference pneumotach system. Results: Using the threshold algorithm, the average detected breath rate was 3 breaths per minute (bpm) higher than the rate detected using the pneumotach. The standard deviation of the difference was 4.7 bpm. Using the rule-based algorithm, the detected breath rate was 0.97 bpm higher than the reference device (pneumotach) with a standard deviation of the difference of 2.92 bpm.[figure1] Discussion: While both algorithms detected more breaths than measured by the pneumotach, the embedded algorithm was substantially more accurate. False breath detects were often attributed to cardiogenic oscillations detected during periods of apnea and small breath attempts that occurred during periods of airway obstruction. Both of these conditions may cause the capnometer to fail to identify periods of respiratory failure during sedation. The high standard deviation observed when using the threshold algorithm could also lead to false high respiratory rate alarms. From Proceedings of the 2009 Annual Meeting of the American Society Anesthesiologists. |
Figure 1
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