2002 OPEN FORUM Abstracts
THE EFFECT OF HELIUM-OXYGEN GAS MIXTURES ON OXYGEN AND TIDAL VOLUME DELIVERY IN FOUR PEDIATRIC VENTILATORS.
John W. Berkenbosch1, Ryan Grueber2, Osauma Dabbagh3, Andrew McKibben3. Departments of Child Health1, Respiratory Care2, and Internal Medicine3, The University of Missouri-Columbia, Columbia, MO, USA
INTRODUCTION: Interest is growing regarding the application of helium-oxygen (heliox) mixtures during mechanical ventilation. While the beneficial effects of heliox derive primarily from it?s decreased density, resulting in decreased resistance to airflow, these same physical properties could have a significant impact on ventilator function. However, little investigation has been undertaken to define these effects, particularly in ventilators commonly used for pediatric applications.
Methods: We investigated the effects of heliox during volume and pressure ventilation on the function of 4 pediatric ventilators, the Bird VIP and VIP Gold and the Siemans Servo 900C and Servo 300. Heliox was administered as an 80:20 mixture of helium-oxygen through the air inlet of the ventilator. Tidal volumes (VT) or inspiratory pressures were chosen to deliver VT?s of 20-250 mL during ventilation with an FiO2 of 1.0. FiO2 values of 0.2, 0.4, 0.6, 0.8, and 1.0 were tested. Inspiratory VT displayed on the ventilator (displayed VT) was compared with the inspiratory VT displayed during ventilation with 100% oxygen (expected VT Dis). Delivered VT was measured with a Neonatal Bicore connected to the side port of a ?bag-in-box? spirometer, making measurements independent of gas properties. This volume was compared with the VT delivered during ventilation with 100% oxygen (expected VT Del). For each ventilator, ratios of displayed VT or delivered VT/expected VT (Dis or Del) at each FiO2 were calculated and compared with a Kruskal-Wallis analysis of variance. A p value <0.05 was significant.
Results: All four ventilators functioned in the presence of helium. The delivered FiO2 was less than set FiO2 with the VIP, VIP Gold, and Servo 900C. During volume ventilation, VT displayed were decreased in helium with the VIP Gold (4%, p<0.001) but were unaltered with the VIP, Servo 900C and Servo 300. Delivered VT increased in helium with the VIP (87%), VIP Gold (82%), and Servo 900C (34%) (p<0.0001). In contrast, delivered VT decreased 12% in helium with the Servo 300 (p<0.001).
During pressure ventilation, VT displayed were decreased in helium with the VIP (32%), VIP Gold (36%) and Servo 900C (18%) (p<0.0001). VT displayed increased 8% in helium with the Servo 300 (p<0.0001). Delivered VT increased in helium with the VIP (28%), VIP Gold (17%) and Servo 900C (6%). Delivered VT decreased 6% in helium with the Servo 300 (p<0.005). For al ventilators, the magnitude of change in VT displayed was increased and that of VT delivered was decreased compared to during volume ventilation.
DISCUSSION: While each ventilator functioned in helium, the actual volumes both displayed and delivered were significantly altered by helium. These effects were both ventilator- and ventilation mode-specific and are most likely related to the gas delivery system. Ventilator parameters and VT delivery were least affected by helium in the Servo 300. Additionally, VT delivery was affected less during pressure-controlled versus volume-controlled ventilation, suggesting that pressure-controlled modes may be more desirable when ventilating with heliox. Further study to determine the presence and/or magnitude of these effects in a branched lung model would be relevant and valuable.