1995 OPEN FORUM Abstracts
SYSTEM DEAD SPACE (Vsys) AS A QUALITY CONTROL INDICATOR FOR HELIUM DILUTION LUNG VOLUMES
K White CRTT, K McCarthy RCPT. Pulmonary Function Laboratory. The Cleveland Clinic Foundation, Cleveland, Ohio 44195.
We undertook this project to determine the reproducibility of the system dead space (Vsys) measurement that is calculated when the technologist accepts the stable initial helium concentration at the start of the measurement of functional residual capacity. We collected 355 system dead space values from 5 different pulmonary function analyzers (Excel PFT Analyzers, Cybermedic, Inc. Boulder, CO) over a twelve month time period (convenience sample).
The value, Vsys, which includes the volume of the bellows, tubing, patient valve and CO2 absorbent reservoir, is affected by the initial bellows volume, the amount of oxygen initially added to the system, the amount of helium added to the system and the validity of the initial helium reading (true equilibration of helium). Values that fall outside of the anticipated range of the expected mean ± 2 SD are associated with improper bellows calibration, loose packing of the CO2 absorbent in its reservoir or failure of the circulating blower motor.
The mean (±SD) Vsys calculated just prior to lung volume measurement for all five units was 12.5 (±0.73) liters. The mean Vsys for individual units ranged from 12.5 to 12.7 liters and standard deviations ranged from 0.14 to 0.32 liters. These findings are summarized in the table below.
Unit 1 Unit 2 Unit 3 Unit 4 Unit 5 All Units
N100 137 306718352
Mean12.5 12.5 12.6 12.7 12.6 12.5
S.D.0.20 0.32 0.11 0.33 0.14 0.73
We conclude that Vsys is reproducible, particularly when one looks at values obtained from the same testing device. We propose that, when available, Vsys should be routinely used as a quality control indicator that can alert the pulmonary function technologist to equipment malfunction and prevent erroneous measurement of functional residual capacity by helium dilution.