The Science Journal of the American Association for Respiratory Care

1996 OPEN FORUM Abstracts

LABORATORY EVALUATION OF CONDENSATE PCO_{2} AS A PROXY FOR MIXED EXPIRED PCO_{2} DURING MECHANICAL VENTILATION.

Hideaki Imanaka, MD, Dean Hess, PhD, RRT, Luca Bigatello, MD, Ali Al-Himyary, MD, Robert M. Kacmarek, PhD, RRT, William E. Hurford, MD, Respiratory Care and Department of Anesthesia, Harvard Medical School, Massachusetts General Hospital, Boston; University of Nebraska, Omaha.

It has been suggested that the condensate PCO_{2} collected from the expiratory water trap correlates well with the mixed expired PCO_{2} collected using a Douglas bag (Chest 1992;101: 1601). We previously reported that the condensate PCO_{2} systematically overestimated the PCO_{2} from a Douglas bag and evaluated a temperature correction factor to compensate for this discrepancy (Am J Resp Crit Care Med 1996;153:A603). We conducted the present study to determine whether temperature correction would eliminate the discrepancy between condensate PCO_{2} and mixed expired PCO_{2} in a mechanically ventilated lung model. Methods: A 5-L anesthesia bag was connected to a 7200ae ventilator (Nellcor-Puritan-Bennett) and a humidifier (Cascade la). The temperature at the Y-piece was set to 37-38/C. A constant flow of CO_{2} was injected into the bag to establish 20, 30, 40, 50, 60, and 70 torr of end-tidal PCO_{2} measured at the Y-piece (Traverse Medical). After equilibration (>= 30 min), the condensate in the expiratory limb was drained into a water trap (Ballard) and its temperature measured with a digital thermometer (Fisher Scientific). The condensate was collected anacrobically and analyzed immediately for PCO_{2} using a calibrated blood gas analyzer (Coming 170) at 37/C. Mixed expired gas was collected simultaneously into a bag and its PCO_{2} was measured using the same analyzer. The ventilator was set at 20 breaths/min, PEEP 2 cm H_{2}O, FiO_{2} 0.21, tidal volume 0.5 or 1 L, I:E ratio of 1:0.6 or 1:2.2. Gas PCO_{2} was calculated from temperature (T) and condensate PCO_{2} using the following equation:

Gas PCO_{2}= Condensate PCO_{2} x 10^{-0010x\deltaT}, where \deltaT=37-T.

Results: The correlation between the mixed expired PCO_{2} by condensate (after temperature correction) and the actual mixed expired PCO_{2} (by gas collection) was: Condensate PCO_{2} = 1.02 x (Actual PCO_{2}) - 0.96 (R^{2}=0.96). The bias ± precision between condensate PCO_{2} and actual PCO_{2} was 0.3 ± 3.2 torr (limits of agreement, -6.1 to 6.8 torr). Conclusions: Even with temperature compensation, we failed to precisely predict mixed expired PCO_{2} from condensate PCO_{2}. This may be due to poor mixing in the expiratory limb. It is unlikely that condensate PCO_{2} is an accurate indicator for mixed expired PCO_{2}.

(See original for figure)

Reference: OF-96-099

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