A Laboratory Evaluation of 2 Mechanical Ventilators in the Presence of Helium-Oxygen Mixtures
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A Laboratory Evaluation of 2 Mechanical Ventilators in the Presence of Helium-Oxygen Mixtures Melissa K Brown RRT-NPS and David C Willms MD BACKGROUND: Helium-oxygen (heliox) mixtures are being used more frequently with mechan- ical ventilators. Newer ventilators continue to be developed that have not yet been evaluated for safety and efficacy of heliox delivery. We studied the performance of 2 previously untested venti- lators (Servo-i and Inspiration) during heliox administration. METHODS: We measured tidal volume (VT) delivery, gas blending, gas analyzing, and pressure stability in the presence of heliox. A heliox (80% helium/20% oxygen) tank was attached to the 50-psi air inlet. We compared the set VT (ie, set on the ventilator) and the exhaled VT (measured by the ventilator) to the delivered VT (measured with a lung model). Pressure measurements were also evaluated. We also compared the ventilator-setting fraction of inspired oxygen (F ) to the F measured by the ventilator and the IO2 IO2 F measured with a supplemental oxygen analyzer. RESULTS: Heliox significantly affected both IO2 the exhaled VT measurement and the actual delivered VT (p < 0.001) with both the Servo-i and the Inspiration. Neither peak inspiratory pressure (in the pressure-controlled ventilation mode) nor positive end-expiratory pressure were adversely affected by heliox with either ventilator. Introduc- ing heliox into the gas-blending systems caused only a small error in F delivery and monitoring. IO2 CONCLUSIONS: Both ventilators cycled consistently with heliox mixtures. In most cases, actual delivered V can be reliably calculated if the F and the set V or the measured exhaled V is T IO2 T T known. With the Servo-i, at high helium concentrations the exhaled VT measurement was unreliable and caused a high-priority alarm condition that couldn’t be disabled. A supplemental oxygen analyzer is not necessary with either device for heliox applications. Key words: helium, heliox, tidal volume, mechanical ventilation. [Respir Care 2005;50(3):354–360. © 2005 Daedalus Enterprises] Introduction panded from asthma2–5 and airway obstruction6–9 to bron- chiolitis10–12 and chronic obstructive pulmonary dis- The medical use of helium-oxygen mixture (heliox) was ease.13–15 The advantage of heliox is primarily its lower first described by Barach in 1935, to treat asthma and density than either air or oxygen. The lower density re- airway obstruction.1 Recently, heliox applications have ex- duces airway resistance and thus reduces patient work of breathing and the driving pressure required to achieve ad- equate ventilation. The mechanisms of improved ventila- Melissa K Brown RRT-NPS is affiliated with the Respiratory Therapy tion efficiency may be by conversion of turbulent flow to Program, Department of Health Sciences, Grossmont Community Col- laminar flow and, perhaps, improved gas mixing in distal lege, El Cajon, California, and with the Chest Medicine and Critical Care airways. Medical Group, Pulmonary Department, Sharp Memorial Hospital, San With the recent increased interest in heliox for critically Diego, California. David C Willms MD is affiliated with the Department of Pulmonary Medicine, Sharp Memorial Hospital, San Diego, Califor- ill patients, heliox delivery methods have evolved from nia. masks to mechanical ventilators. Because of its low den- Melissa K Brown RRT-NPS presented a version of this paper at the OPEN FORUM of the 49th International Respiratory Congress, held in Las Ve- gas, Nevada, December 8–11, 2003. Correspondence: Melissa K Brown RRT-NPS, Chest Medicine and Crit- eVent Medical Ltd supplied heliox for this study. ical Care Medical Group, Pulmonary Department, 4th Floor, Sharp Me- morial Hospital, 7901 Frost Street, San Diego CA 92123. E-mail: David C Willms MD is a consultant for eVent Medical. [email protected]. 354 RESPIRATORY CARE • MARCH 2005 VOL 50 NO 3 EVALUATION OF 2MECHANICAL VENTILATORS USING HELIUM-OXYGEN MIXTURE sity and high thermal conductivity, heliox can interfere manufacturer’s specifications and inserted into the inspira- with normal ventilator function and monitoring. For ex- tory limb of the ventilator circuit to verify VT on 100% ample, with heliox, a screen pneumotachometer may un- oxygen, determine overall system static compliance, and derestimate flow (and thus volume), because of the lower independently verify the F , peak inspiratory pressure IO2 resistance across the screen. Hot-wire type pneumotachom- (PIP), and PEEP. A pressure monitor (Magnehelic, Dwyer eters will grossly malfunction, because heliox’s thermal Instruments, Michigan City, Indiana) (which has a bellows conductivity is markedly higher than that of air. Accurate flat spring dial gauge with an operating range of 0–80 cm measurement of flow is essential for mechanical ventila- H2O) was calibrated according to the manufacturer’s spec- tors to control tidal volume (VT) delivery, monitor inspira- ifications and placed proximal to the test lung to measure tory and expiratory volumes, and blend and analyze gas plateau pressure. System static compliance was measured mixtures. Accurate information is clinically valuable and on 100% oxygen for VT of 300–1,000 mL, in 100-mL necessary to ensure patient safety. increments, during volume-controlled ventilation, constant There are few published studies of the effect of heliox flow, with a 0.5-s inspiratory pause. Measured compliance on specific mechanical ventilators. The most comprehen- was linear over the tested range of VT. sive review, by Tassaux et al,16 does not address ventila- tors more recently introduced. The present study was de- Measurement of VT and PEEP signed to determine how 2 newer ventilators (the Servo-i and the Inspiration) function in the presence of heliox. We The following variables were compared: set VT (ie, the were interested in testing these ventilators’ performance VT ventilator setting), exhaled VT (measured by the ven- during heliox administration, to determine whether they tilator’s pneumotachometer), and delivered VT (measured could deliver gas accurately during volume-controlled and by the lung model). Measurements were taken at measured pressure-controlled ventilation. We also sought to deter- F values of 0.21, 0.30, 0.40, 0.50, and 1.0. With F IO2 IO2 mine (1) correction factors for measured volumes, (2) values Ͻ 1.0, helium constituted the balance of the gas whether the set positive end-expiratory pressure (PEEP) is mixture (eg, with F 0.21, helium constituted 79% of the IO2 maintained, and (3) if the delivered fraction of inspired gas mixture). oxygen (F ) was within the normal error range during If there was a discrepancy between the set F and the IO2 IO2 heliox administration. measured F , the blender was adjusted to deliver the IO2 desired F . Smaller intervals were selected in the lower IO2 Methods F range, to focus data collection on the concentrations IO2 most widely used in clinical practice. The ventilation mode Two ventilators were tested: the Inspiration (eVent Med- was volume-control. We studied VT settings of 500, 750, ical Ltd, Galway, Ireland) and the Servo-i (Maquet Critical and 1,000 mL. For each F , the same preset V was used, IO2 T Care, Solna, Sweden). The devices were calibrated accord- regardless of actual delivered VT, with the following ven- ing to the manufacturers’ specifications. An 80% helium/ tilator settings: PEEP zero, respiratory rate 12 breaths/min, 20% oxygen mixture, in an H-size tank, was connected via peak flow 40 L/min, inspiratory-expiratory ratio 1:1, trig- Ϫ the 50-psi air inlet of the ventilator. The ventilator was ger sensitivity 2cmH2O. connected to a test lung (Michigan Instruments, Grand Six successive breaths were measured and analyzed for Rapids, Michigan) that was cross-calibrated independently each VT, with 2 min allowed after each change for system from the manufacturer. equilibration. We collected data on actual PEEP levels The test lung is built around a chamber, and the com- (measured by the pulmonary mechanics monitor) at PEEP pliance and “airway” resistance of the chamber are deter- of 5, 10, and 15 cm H2O, at the settings used for the mined by precision spring-loading and variable cross-sec- 750-mL VT, to determine PEEP stability with a heliox ϭ tion resistors. The delivered VT is determined as VT mixture. Set PEEP was compared to measured PEEP in the respiratory-system compliance ϫ pressure (measured at lung model chamber at F of 0.21 (balance helium) and IO2 zero flow). The calculated VT is verified against a cali- 1.0. brated volume scale attached to the device. Test-lung com- pliance was set at 0.05 L/cm H2O; resistance was set at 5 Measurement of PIP in Pressure-Controlled cm H2O/L/s with room-air temperature, barometric pres- Ventilation sure, and humidity (ie, ambient temperature and pressure saturated). In this lung model, flow and VT measurements Pressure-control levels of 15, 20, and 30 cm H2O were are independent of gas density, as described by Tassaux et studied to determine if heliox affected the ventilator’s control al.16 of PIP in the pressure-control mode. Measurements were A pulmonary mechanics monitor (PF-300, IMT Medi- taken at F of 0.21, 0.30, 0.50 (balance helium), and 1.0. IO2 cal, Vaduz, Liechtenstein) was calibrated according to the We used the fastest available rise time, and the following RESPIRATORY CARE • MARCH 2005 VOL 50 NO 3 355 EVALUATION OF 2MECHANICAL VENTILATORS USING HELIUM-OXYGEN MIXTURE settings: PEEP 5 cm H2O, respiratory rate 12 breaths/min, exhaled VT measured by the ventilator’s pneumotachom- Ϫ inspiratory-expiratory ratio 1:1, and trigger sensitivity 2cm eter. The magnitude of the delivered-versus-exhaled VT Ͻ H2O. We collected data on PIP (measured by the supplemen- discrepancy was significant (p 0.001), inversely related tal monitors), inspiratory V (measured by the pulmonary to F , and linear (r2 ϭ 0.99, p Ͻ 0.001).