1996 OPEN FORUM Abstracts
AIRWAY RESISTANCE CHANGES WHEN DELIVERING HELIUM MIXTURES THROUGH AN INFANT VENTILATOR: A BENCH STUDY
James G. Bisguard RRT, Augusto Sola MD. Intensive Care Nursery, University of California at San Francisco, San Francisco Ca.
Introduction: Helium has long been recognized for its' therapeutic value in treating various forms of pulmonary disease. The low density inherent to the physical properties of this gas may lend particular clinical value in the treatment of conditions associated with increased airway resistance (RAW). At our institution, helium gas mixtures have been introduced through mechanical ventilators to treat various forms of both acute and chronic lung disease. Past bench studies by our staff have shown this practice to be safe in the clinical arena. However, since these ventilators have been designed to operate with conventional gas sources (ie oxygen, compressed air), we decided to further investigate if the use of helium would affect airway resistance and other important values associated with mechanical ventilation.
Methods: A Sechrist IV-100 time cycled, pressure limited ventilator was used to deliver various mixtures of medical gases. The ventilator was set to deliver PIP 22cmH20, PEEP 5cmH20, RR 20/min, Ti. 50sec, Flow 10 lpm and FiO2.40. Baseline data values were obtained using conventional gas sources of oxygen (02) and compressed air (02,N2). Helium was introduced to the ventilator gas supply by means of connecting the compressed air gas inlet hose directly to a pressure regulated H cylinder of Heliox (He 80%,02 20%). Compliance of the test lung was set to remain fixed at .002 1/cmH20. RAW was set at 200 cm/1/s and later increased to 400 cm/1/s. RAW data was measured by a Bicore CP-100 pulmonary monitoring device. Flow transducers to Bicore unit were placed at the ventilator circuit/test lung interface. When heliox was in use, specially calibrated flow transducers were used to ensure reliable test results at all studied concentrations of gas mixtures. Oxygen concentration (FiO2) was continuously monitored by a Teledyne Oxygen Analyzer. All devices were calibrated according to manufacturers' specification. Results: When using conventional gas sources through the ventilator with inline resistance fixed at 200 cmH20/1/s, the Bicore monitor measured a RAW value of 120 cmH20/1/s. With helium introduced to our gas supply, this value dropped significantly to 90 cmH20/1/s. When inline resistance was increased to 400 cmH20/1/s, RAW of conventional gas source ventilation was recorded at 177 cmH20/1/s. This value decreased to 133 cmH20/1/s when helium was used. At both levels of fixed inline circuit resistance, monitored values reflected a decrease in RAW equal to 25% when heliox was in use.
FiO2 Added Fixed RAW Measured RAW Change
Helium cm/1/s cm/1/s w/Helium
.40 0 200 120 -
.40 .60 200 90 25.0%
.40 0 400 177 -
.40 .60 400 133 24.9%
Conclusions: The use of helium gas mixtures delivered through a time cycled, pressure limited ventilator significantly decreased measured Airway Resistance in a test lung model. We therefore suggest, that the clinical usage of this gas can be effectively and safely used to treat pulmonary disease associated with increased airway resistance.