The Science Journal of the American Association for Respiratory Care

1996 OPEN FORUM Abstracts

COMPARISON OF CLOSED AND OPEN METHODS OF MEASURING FUNCTIONAL RESIDUAL CAPACITY BY HELIUM DILUTION IN PARALYZED, SEDATED DOGS.

A McKibben, AB Adams, S Becker, T Takahashi, JJ Marini. University of Minnesota and St. Paul Ramsey Medical Center. Minneapolis/St. Paul, MN.

The knowledge of functional residual capacity (FRC), its absolute value and changes with Intervention, may assist optimal ventilator management. For the measurement of FRC in the clinical setting both open and closed system methods have been described using inert gas dilution or washout. Equipment for such methods requires technical expertise, is expensive and bulky. In an animal study requiring FRC measurements, we evaluated two relatively simple methods of helium (He) dilution for obtaining FRC values. The open "expiratory washout" method involved the Infusion of He in the inspiratory limb during the inspiratory phase only. After a steady state was confirmed by a constant He concentration detected in the expiratory limb, the He infusion was stopped and exhalate captured in a Douglas bag for 4 minutes. He concentration and exhalate volume were determined. The closed "equilibration" method used a super syringe with a known volume and concentration of He. At end exhalation a valve to the animal was opened to connect the airway to the syringe, which was used to apply and withdraw 10 manual inflations at an peak airway pressure approximating 20 cm H_{2}O. Methods: In each paralyzed, sedated animal study, FRC was measured by both methods prior to study intervention. The animals were ventilated at 10 mL/Kg and f=10 while on 5 cm H_{2}O PEEP. The open method was followed by the closed method with a 15 minute Interval of fresh gas flushing between. Five comparison sessions were made in 4 dogs whose mean weight was 22.0±1.3 Kg. Results: Reproducible values were found by both methods. The mean FRC by the closed method was 940 mL, 18% less than the mean FRC by the open method which was 1146 mL. Conclusion: Although further evaluation of these He dilution methods for FRC determination is necessary, both methods provide reproducible FRC measurements within the range of previously reported studies. We speculated that minor differences between methods may result from incomplete gas equilibration, leading to a systematic under-estimation of FRC by the closed method.

Reference: OF-96-185

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