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

Accurately measuring the partial pressure of end-tidal CO₂ (P_ETCO ) in non-intubated patients is problematic due to dilution of expired CO₂ at high O₂ flows and mask designs that may either cause CO₂ rebreathing or inadequately capture expired CO₂. We evaluated the performance of 2 capnographic O₂ masks (Cap-ONE and OxyMask) against a clinically expedient method using a standard O₂ 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.

An ASL-5000 simulator was attached to a manikin face with a catheter port, through which various CO₂/air mixtures were bled into the ASL-5000 to achieve a P_ETCO of 40, 65, and 30 mm Hg. Both P_ETCO and inspired P_CO were measured at O₂ flows of 5, 10, 15, and 20 L/min (adult model) and 2, 4, 6, 8, and 10 L/min (pediatric model).

P_ETCO decreased to varying degrees as O₂ 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 CO₂ rebreathing. P_ETCO usually deteriorated more under simulated hypoventilation, regardless of the measurement method.

Both of the specially designed O₂ capnography masks provided reasonably stable P_ETCO without significant CO₂ rebreathing at the commonly used O₂ flows. Because of their open design, P_ETCO measured at high O₂ flows may produce artificially lower readings that may not reflect arterial CO₂ levels compared with lower O₂ flows.