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Do new anesthesia ventilators deliver small tidal volumes accurately during volume-controlled ventilation?

Patricia R Bachiller, Joseph M McDonough, Jeffrey M Feldman
Anesthesia and Analgesia 2008, 106 (5): 1392-400, table of contents
18420850

BACKGROUND: During mechanical ventilation of infants and neonates, small changes in tidal volume may lead to hypo- or hyperventilation, barotrauma, or volutrauma. Partly because breathing circuit compliance and fresh gas flow affect tidal volume delivery by traditional anesthesia ventilators in volume-controlled ventilation (VCV) mode, pressure-controlled ventilation (PCV) using a circle breathing system has become a common approach to minimizing the risk of mechanical ventilation for small patients, although delivered tidal volume is not assured during PCV. A new generation of anesthesia machine ventilators addresses the problems of VCV by adjusting for fresh gas flow and for the compliance of the breathing circuit. In this study, we evaluated the accuracy of new anesthesia ventilators to deliver small tidal volumes.

METHODS: Four anesthesia ventilator systems were evaluated to determine the accuracy of volume delivery to the airway during VCV at tidal volume settings of 100, 200, and 500 mL under different conditions of breathing circuit compliance (fully extended and fully contracted circuits) and lung compliance. A mechanical test lung (adult and infant) was used to simulate lung compliances ranging from 0.0025 to 0.03 L/cm H(2)O. Volumes and pressures were measured using a calibrated screen pneumotachograph and custom software. We tested the Smartvent 7900, Avance, and Aisys anesthesia ventilator systems (GE Healthcare, Madison, WI) and the Apollo anesthesia ventilator (Draeger Medical, Telford, PA). The Smartvent 7900 and Avance ventilators use inspiratory flow sensors to control the volume delivered, whereas the Aisys and Apollo ventilators compensate for the compliance of the circuit.

RESULTS: We found that the anesthesia ventilators that use compliance compensation (Aisys and Apollo) accurately delivered both large and small tidal volumes to the airway of the test lung under conditions of normal and low lung compliance during VCV (ranging from 95.5% to 106.2% of the set tidal volume). However, the anesthesia ventilators without compliance compensation were less accurate in delivering the set tidal volume during VCV, particularly at lower volumes and lower lung compliances (ranging from 45.6% to 100.3% of the set tidal volume).

CONCLUSIONS: Newer generation anesthesia machine ventilators that compensate for breathing circuit compliance and for fresh gas flow are able to deliver small tidal volumes accurately to the airway under conditions of normal and low lung compliance during volume-controlled ventilation. Accurate VCV may be a useful alternative to PCV, as volume is guaranteed when lung compliance changes, and new strategies such as small volume/lung protective ventilation become possible in the operating room.

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