Assessment of errors when expiratory condensate PCO2 is used as a proxy for mixed expired PCO2 during mechanical ventilation.
OBJECTIVE
We designed a series of experiments to determine whether expiratory water condensate (PconCO2) can be used as a proxy for mixed expired gas collection.


METHODS
In 18 adult mechanically ventilated patients with ARDS (40 samples), simultaneous collections of arterial blood, expiratory water trap condensate, mixed expired gas, and minute ventilation were used to calculate VCO2 and VD/VT. To assess the effect of temperature, a constant gas flow (PCO2 10-30 mm Hg) was bubbled through water at temperatures of 19.5-37 degrees C. Gas and water samples were collected, immediately analyzed for PCO2, and a temperature correction factor was calculated. A lung model was constructed using a 5 L anesthesia bag connected to a mechanical ventilator with a heated humidifier. Temperature at the Y-piece was set to approximately 37 degrees C and CO2 was injected into the bag to establish an end-tidal PCO2 of 20-70 mm Hg. After equilibration, condensate was collected, PCO2 was measured, and the temperature-corrected PCO2 was compared to PECO2. The capnogram at points along the expiratory limb circuit was used to evaluate gas mixing.


RESULTS
There was an over-estimation of PECO2 by PconCO2 (p < 0.001) for the patient data, resulting in an underestimation of VD/VT (p < 0.001) and an overestimation of VCO2 (p < 0.001). The temperature correction factor for PCO2 in water was -0.010 (about half of the factor used for whole blood). The bias between temperature-corrected PconCO2 and PECO2 was 0.3 +/- 3.2 mm Hg in the lung model. Mixing in the expiratory limb was poor, as evaluated by the capnogram.


CONCLUSIONS
Even with temperature correction, we failed to precisely predict PECO2 from PconCO2. For measurement of VD/VT and VCO2, we do not recommend methods that use PconCO2.
Imanaka
H
H
Department of Anaesthesia, Harvard Medical School, Boston, Massachusetts, USA.
Hess
D
D
al-Himyary
A
A
Bigatello
L M
LM
Ritz
R
R
Kacmarek
R M
RM
Hurford
W E
WE
D000328:Adult; D000368:Aged; D015982:Bias; D001831:Body Temperature; D019296:Capnography / Q000295:instrumentation / Q000379:methods*; D002245:Carbon Dioxide / Q000032:analysis* / Q000097:blood / Q000378:metabolism; D005544:Forecasting; D006801:Humans; D006813:Humidity; D008168:Lung / Q000378:metabolism; D008875:Middle Aged; D008954:Models, Biological; D012119:Respiration; D012121:Respiration, Artificial; D012128:Respiratory Distress Syndrome / Q000097:blood / Q000378:metabolism / Q000628:therapy; D013696:Temperature; D013990:Tidal Volume; D012122:Ventilators, Mechanical; D014867:Water
D014867:Water; D002245:Carbon Dioxide