The Science Journal of the American Association for Respiratory Care

2003 OPEN FORUM Abstracts

Bench Test of the Siemens Servoi Mechanical Ventilator with Heliox Mixtures.

Melissa K. Brown RRT-NPS Grossmont Community College, El Cajon, Ca; Jodette Brewer RRT, Sharp Memorial Hospital San Diego, Ca; David Willms MD, Sharp Memorial Hospital San Diego, Ca.

Background:
Due to its low density and high thermal conductivity, heliox mixtures can interfere with normal ventilator function and monitoring. We sought to determine if the Siemens Servoi ventilator could deliver heliox safely in both the volume control (VC) and pressure control (PC) mode. We also evaluated the accuracy of the spirometry, and sought to determine correction factors for both set and ventilator measured exhaled tidal volumes, as well as the accuracy of the oxygen blender and analyzer in the presence of heliox.

Methods
: An 80/20 heliox mixture was connected via the 50-psi air inlet of the ventilator. The ventilator was connected to a test lung (Adult, Infant TTL 1601, Michigan Instruments, Grand Rapids, MI). A pulmonary mechanics monitor (PF 3000, IMT Medical, Liechtenstein) was inserted into the inspiratory limb of the ventilator circuit to verify tidal volumes on 100% oxygen, determine overall system compliance, and independently verify the fraction of inspired oxygen. A Magnehelic pressure monitor (Dwyer Instruments, Michigan City, IN) was placed proximal to the test lung to measure plateau pressure. Tidal volume was determined as VT= Crs x P (measured at zero flow rate). Test lung compliance was set at .05 L/cmH2O and resistance set at 5 cmH2O/L/sec with room air, barometric pressure and humidity (ATPS). System compliance was measured on 100% oxygen for tidal volumes of 300 ml to 1000 ml in 100 ml increments in Volume Control (Square flow) with a 0.5 second inspiratory pause. In this lung model flow and tidal volume measurements are independent of gas density. The following parameters were compared: VT set, exhaled VT (VTexh) as determined by the ventilator's pneumotach, and effective VT delivered by the ventilator as determined by the lung model (VTdel). Measurements were taken at an FiO2 measured of 21%, 30%, 40%, and 50%. In VC VT's of 500ml, 750ml, and 1000ml were studied with ventilator settings 0 PEEP, rate 12, Peak Flow 40 LPM, I:E ratio 1:1. In PC, +15, +20, +30 were studied with PEEP 5, rate 12, I:E ratio 1:1. Six successive breaths were measured and analyzed for each VT and pressure.

Results
: The Servoi cycled consistently with all heliox mixtures in both VC and PC. The ventilator oxygen analyzer was within 3% of the external analyzer, and the blender within 1%. The Servoi exhaled VT display was erratic at 21% and 30% displaying less than 100ml at all VT's. Factors for determining VTdel by either the ventilator set VT or from the ventilator expired VT is in the table below.

VTdel = VT set x factor

VTdel= VTexh x factor

FiO2 set Set VT factor Exhaled VT factor
21% 0.95 None possible
30% 0.95 None possible
40% 0.95 1.37
50% 0.95 1.24


 
Conclusion: The Servoi ventilator can be safely used with heliox mixtures. In high concentrations of heliox the ventilator tidal volume spirometry dramatically underestimates VT. Clinicians will be unable to calculate effective VT del at 21% and 30% with factors. At the 500ml VT the low minute alarm was breeched and could not be disabled causing a constant alarm condition. More research needs to be done evaluating the function of the Servoi in the pediatric and infant volume ranges.

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