2001 OPEN FORUM Abstracts
VALIDATION OFA PISTON-DRIVEN VENTILATOR FOR THE ADMINISTRATION OF HELIOX THERAPY.
Justin P. Lafreniere;Douglas Oberly, MS, RRT; R. Fredrick Knauft, MD Hartford Hospital, Hartford, CT.
Background: In 1990Gluck et. al. showed that 60:40 heliox significantly reduced airway pressuresand carbon dioxide retention in severe asthmatics who had been intubated forrespiratory failure. However, the gas has also been shown to affect the accuracyof volumes delivered by certain mechanical ventilators. To our knowledge thereare few mechanical ventilators approved by the FDA for the administration ofheliox therapy. We conducted a bench validation of Mallinckrodt Nellcor PuritanBennett?s 760 ventilator because of its piston driven design. We hypothesizedthat this piston-driven design would avoid the technical problems that are associatedwith heliox and the computer controlled valves in other modern ventilators.We also compensated for the important density difference between heliox andstandard oxygen therapy by utilizing the CosmoPlus (Novametrix Medical Systems)external-monitoring device.
Methods: The benchvalidation was conducted using a 70:30 heliox mixture with MNPB?s 760 ventilator,the CosmoPlus monitoring device and the TTL test lung (Michigan Instruments).The 760 was set to maintain a rate of 10 bpm and an inspiratory time of 0.9sec. The test lung was adjusted to a ?normal? model (compliance of 70 mL/cmH2Oand a resistance of 5 cmH2O/L/sec) and an ?asthmatic? model (complianceof 30 mL/cmH2O and a resistance of 50 cmH2O/L/sec). TheCosmoPlus was positioned between the wye and the test lung in order to recordthe most accurate pressure and volume measurements. A set of 50 data pointswas collected during each interval of both the pressure control (Pi = 10-40cmH2O) and volume control (Vt= 200-1200 mL) modes of the 760. Tidalvolumes and distending pressures during both heliox and oxygen ventilation werecompared for validation. Per operators manual, the volume control validationwas allowed a difference of + 10 mL + 10% of setting, while the pressurecontrol validation was allowed a difference of + 3 cmH2O +2.5% of setting. The use of PEEP at 15 cmH2O was also validated inthe ?asthmatic? model.
Experience: Usingthe CosmoPlus external monitoring device and the TTL test lung we were ableto show that this ventilator is capable of delivering a 70:30 heliox mixtureas accurately as pure oxygen during both volume control and pressure controlventilation. Using the Student?s T-test for a difference in two means, we determinedthat the 760 exhibited no significant difference in accuracy between helioxand oxygen delivery (p=.05) in the ?normal? and ?asthmatic? model. Upon furtherexamination of the data we discovered some potential clinical benefits to theuse of heliox therapy. Comparing the mean tidal volumes of heliox and oxygendelivered by the ventilator at identical distending pressures, we found thatthe heliox tidal volumes were significantly greater than the oxygen tidal volumes.
Conclusion: Afterconducting our validation of MNPB?s 760, we are confident that while used inconjunction with the Novametrix CosmoPlus external monitoring device, this piston-drivenventilator is both safe and effective for use in the delivery of heliox therapy.We also found evidence that heliox therapy potentially presents a new lung protectivestrategy whereby respiratory care providers can maximize delivered volumes whilemaintaining minimal distending pressures. Further research involving human lungmodels is necessary to further validate the MNPB 760 with heliox.