The Science Journal of the American Association for Respiratory Care

2003 OPEN FORUM Abstracts

DEVELOPING A CORRECTION FACTOR TO DETERMINE THE SET TIDAL VOLUME WHEN USING HELIUM THROUGH A DRÄGER DURA 2 VENTILATOR

Diane Jereb, RRT, Kathy Bauer, RRT, James E. Martin, RRT, Departments of Pulmonary Services, and Pediatrics, MetroHealth Medical Center, Cleveland, Ohio

INTRODUCTION: It is well established that introducing Helium into a ventilator designed to operate with conventional gas sources alters the ability of the ventilator to accurately measure exhaled delivered tidal volume. In most cases, the flow sensing capabilities must be disabled to prevent audible and visual nuisance alarms. This bench study was designed to develop a mathematical correction factor used to calculate the tidal volume (VT) setting to achieve the desired VT when using a 70/30 Helium/Oxygen (Heliox) gas mixture. A correction factor was developed for the following ventilator oxygen settings of 21%, 30%, and 35% to allow for various helium concentrations. 

Methods:
All measurements were made with the Dräger Dura 2 ventilator in the CMV mode with auto flow off. The flow sensor and oxygen analyzer were turned off. A pre-mixed 70/30 Heliox gas mixture was connected directly to the 50-PSI air inlet. A Michigan Instruments TTLO model 2600i (Michigan Instruments, Inc. Grand Rapids, MI) set at compliance 0.05 L/cm H2O, resistance Rp 20 was used to determine the delivered tidal volume. Using this model, flow and volume measurements are independent of the physical properties of the gas mixture (Tassaux, AJRCCM 1999;160). Respiratory rate 12 bpm, inspiratory time 1 second, I:E ratio 1:4, and 0 cmH2O PEEP were held constant for the measurements. Tidal volumes were increased by 50 mL from 300-800 mL at the three ventilator oxygen settings. The delivered FIO2 was confirmed with a calibrated oxygen analyzer. A series of 12 breaths were measured for each tidal volume set at the three oxygen settings. The average difference between the set tidal volume and the measured tidal volume was used to determine a correction factor for the set oxygen used. 

RESULTS: In all cases the tidal volume delivered was greater than the set tidal volume. The set oxygen concentration of 21%, 30%, and 35% yielded a Heliox concentration of 70/30, 65/35, and 60/40 respectively. The table below illustrates the set oxygen, the actual delivered oxygen and helium concentrations, and the correction factor to determine the set tidal volume. 

Ventilator Set O2 Actual Delivered O2 Delivered Helium Correction Factor
21% 30% 70% 1.38
30% 35% 65% 1.34
35% 40% 60% 1.30


CONCLUSION: When using a 70/30 Helium/Oxygen gas mixture the set tidal volume can be determined by dividing the desired tidal volume by the correction factor which corresponds to the oxygen concentration set on the ventilator. Additional work needs to be done to validate the correction factor for use in SIMV, and when using autoflow. 

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