A Comparison of Different Techniques for Interfacing Capnography With Adult and Pediatric Supplemental Oxygen Masks

BACKGROUND: Accurately measuring the partial pressure of end-tidal CO 2 (P ETCO 2 ) in non-intubated patients is problematic due to dilution of expired CO 2 at high O 2 flows and mask designs that may either cause CO 2 rebreathing or inadequately capture expired CO 2 . We evaluated the performance of 2 capnographic O 2 masks (Cap-ONE and OxyMask) against a clinically expedient method using a standard O 2 mask with a flow-directed nasal cannula used for capnography (CapnoLine) in a spontaneous breathing model of an adult and child under conditions of normal ventilation, hypoventilation, and hyperventilation. METHODS: An ASL-5000 simulator was attached to a manikin face with a catheter port, through which various CO 2 /air mixtures were bled into the ASL-5000 to achieve a P ETCO 2 of 40, 65, and 30 mm Hg. Both P ETCO 2 and inspired P CO 2 were measured at O 2 flows of 5, 10, 15, and 20 L/min (adult model) and 2, 4, 6, 8, and 10 L/min (pediatric model). RESULTS: P ETCO 2 decreased to varying degrees as O 2 flow increased, depending upon the breathing pattern. Although all devices appeared to perform reasonably well under normal and hyperventilation conditions, the clinically expedient method was associated with substantially more CO 2 rebreathing. P ETCO 2 usually deteriorated more under simulated hypoventilation, regardless of the measurement method. CONCLUSIONS: Both of the specially designed O 2 capnography masks provided reasonably stable P ETCO 2 without significant CO 2 rebreathing at the commonly used O 2 flows. Because of their open design, P ETCO 2 measured at high O 2 flows may produce artificially lower readings that may not reflect arterial CO 2 levels compared with lower O 2 flows.