Respiratory care 62(1) 78-85 doi 10.4187/respcare.05111
Accurately measuring the partial pressure of end-tidal CO2 (PETCO2 ) 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 PETCO2 of 40, 65, and 30 mm Hg. Both PETCO2 and inspired PCO2 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).
PETCO2 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. PETCO2 usually deteriorated more under simulated hypoventilation, regardless of the measurement method.
Both of the specially designed O2 capnography masks provided reasonably stable PETCO2 without significant CO2 rebreathing at the commonly used O2 flows. Because of their open design, PETCO2 measured at high O2 flows may produce artificially lower readings that may not reflect arterial CO2 levels compared with lower O2 flows.