The Science Journal of the American Association for Respiratory Care

2008 OPEN FORUM Abstracts

DESIGN AND EVALUATION OF A SEMI-CLOSED CIRCUIT FOR NONINVASIVE VENTILATION WITH HELIOX

Karel Roubik1, Adela Strnadova1, Vladimir Zabrodsky2, Roman Zazula1



Background: Heliox (helium and oxygen mixture) reduces work of breathing due to its low density. The current way of heliox application using systems without rebreathing makes ventilation with heliox expensive. The aim of the study is to design a semi-closed circuit rebreather system for heliox application and to evaluate its suitability for patients suffering from acute asthma or COPD exacerbation.

Methods: A semi-closed rebreather system for heliox has been designed. It is a modified semi-closed anesthesia system consisting of adjustable heliox and oxygen sources with calibrated flow meters, two one-way valves, an adjustable expiratory valve, an inspiratory safety valve, two bacterial filters, a carbon dioxide absorber, a reservoir bag and a face mask. Partial pressures of oxygen, carbon dioxide and helium are measured in the inspiratory limb. Helium fraction is measured especially for monitoring of denitrogenation process and for a proper evaluation of heliox effect. A real-time monitoring system records pressure and flow curves and computes various breath characteristics. As the semi-closed circuit has a certain airflow resistance, it represents an extra workload for a patient. The resistances of the inspiratory and expiratory limbs were measured using air and heliox 80:20 at different airflows from 20 to 60 L/min in steps of 10 L/min. The results were compared with a usual value of airway resistance in patients suffering from COPD exacerbation.

Results: The constructed system has been tested and resistances have been measured. The resistance of the inspiratory limb is 367 ± 18 Pa.s/L for air and 279 ± 47 Pa.s/L for heliox. The expiratory limb resistance is 392 ± 39 Pa.s/L for air and 287 ± 34 Pa.s/L for heliox. As bacterial filters represent important parts of these resistances (approx. 120 Pa.s/L for air and 90 Pa.s/L for heliox), their elimination can reduce the resistances significantly. The final resistances of the circuit limbs when compared with a common airway resistance in patients suffering from COPD exacerbation (approx. 1 000 Pa.s/L) seem to be low enough for preserving the benefits of heliox in the airways.

Conclusion: A low-resistance semi-closed circuit for noninvasive ventilation with heliox has been assembled offering a cost effective heliox application in acute asthma or COPD exacerbation patients. Its efficacy should be confirmed by an intended clinical trial.

Acknowledgment: The study was supported by grant MSM 6840770012