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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.
OF-02-062
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