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Anesthesiology:
doi: 10.1097/01.anes.0000290590.63556.58
Correspondence

Clarifying Equipment Specifications and Performance Characteristics

Preece, AnnMarie B.S.

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To the Editor:—

I have concern about the article entitled “Performance Characteristics of Five New Anesthesia Ventilators and Four Intensive Care Ventilators in Pressure-support Mode: A Comparative Bench Study,” which appeared in the November 2006 issue of Anesthesiology.1
Several statements made in the article are incorrect, and we appreciate an opportunity to set the record straight. In table 1 of the article, the Avance (GE-Datex-Ohmeda, Munchen, Germany—which would also apply to our Aestiva, Aisys, and Aespire 7900) is listed correctly as a flow-triggered system, but the specifications for flow triggering are incorrect. Flow triggering on the above GE systems is selectable from 0.2 to 10 lpm, to better address the needs of a pediatric population. Also, the inspiratory-to-expiratory cycle is listed as fixed, 25% of peak flow. Instead, pressure-support ventilation on all of the above systems is adjustable, from 5% to 50% of the peak inspiratory flow, again, to better meet the needs of a pediatric population.
A more serious error, however, is the following statement: “The best characteristics of the pressurization phase for the anesthesia ventilators were obtained with the Fabius, Primus, and Avance under all tested conditions and were comparable with those obtained with the ICU [intensive care unit] ventilators. The Fabius, Primus, and Avance are ‘piston ventilators,' which use an electric motor to compress gas in the breathing circuit, creating the driving force for mechanical insufflation to proceed. Therefore, they use no driving gas and may be used without depleting the oxygen cylinder in case of oxygen pipeline failure. These features may explain in part that these more recent anesthesia ventilators have comparable performance to modern ICU ventilators.”
The GE Healthcare Avance Anesthesia Carestation does not use a piston, nor do any of the other anesthesia systems from GE Healthcare. We use a microprocessor-driven flow control valve system like all major intensive care unit ventilators (including our own Engstrom critical care ventilator) marketed in the United States. The excellent performance of the Avance in this study is due to the rapid and frequent sensing (0.25 ms) of pressure in the patient's lungs via the flow sensors, and the rapid response of the flow valves in the ventilator.
The SmartVent system uses a variable orifice flow sensor on both the inspiratory and the expiratory side of the breathing system. These flow sensors incorporate pressure sensors on either side of a bidirectional Mylar flap. As gas flows through the sensors and encounters the flap, a pressure difference is created between the two sides of the flap. If there is a lot of gas flowing, the pressure difference between the two sides of the flap is more pronounced. If the gas flow is less, the pressure differential is less pronounced. The Mylar flap flexes more or less (hence the variable orifice attribute) depending on the flow, which makes the sensor accurate across the complete flow range. This also allows the SmartVent to cover the complete patient range from tiny neonates to obese adults. The SmartVent uses these pressure differential measurements on the inspiratory side to determine the total flow rate (fresh gas and flow from the bellows). This allows for tidal volume compensation, so that the correct tidal volume is delivered, regardless of fresh gas flow, oxygen flush, or compliance losses in the breathing system.
Because the flap is bidirectional, the SmartVent notices which side of flap has a greater pressure and so can determine flow direction, allowing for notification of the clinician if a reverse flow condition occurs in the circle system. The inspiratory flow sensor also communicates pressure changes in the patient's lungs to correctly deliver pressure-control ventilation, or to limit the airway pressure in volume-control ventilation. The inspiratory flow sensor also responds to negative flow during spontaneous modes of ventilation, such as pressure-support and synchronized mandatory ventilation. The inspiratory flow sensor, microprocessor, and flow valves give perioperative patients the advantages of critical care ventilation. The expiratory flow sensor is an independent monitor that reports the patient's exhaled tidal and minute volumes, and is not involved in volume compensation, ventilation calculations, or responses.
GE Healthcare Anesthesia Systems also have a multiple-breath, standing bellows, unlike the other anesthesia units cited, which gives visible confirmation of the integrity of the breathing system and the adequacy of fresh gas flow. Fresh gas is delivered directly to the inspiratory limb, not through the breathing bag, as is the design with a decoupled piston ventilator. Therefore, changes made by the user to fresh gas and anesthetic agent settings take effect at the patient much more quickly in the Avance Carestation than in fresh gas decoupled systems that use the breathing bag as a reservoir. The volume of the breathing circuit is only 2.7 l, so the Avance has less than half the volume (and time constant) of fresh gas decoupling systems, especially when those decoupled systems use a 3-l rebreathing bag.
AnnMarie Preece, B.S.
GE Healthcare, Madison, Wisconsin. annmarie.preece@med.ge.com
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Reference

1. Jaber S, Tassaux D, Sebbane M, Pouzeratte Y, Battisti A, Capdevila X, Eledjam J-J, Jolliet P: Performance characteristics of five new anesthesia ventilators and four intensive care ventilators in pressure-support mode: A comparative bench study. Anesthesiology 2006; 105:944–52

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