Selecting a tidal volume of 12 to 15 ml/kg may reduce atelectasis (S,h.┬ž.pir.o.,..199,4), improving ventilation-perfusion matching and enhancing oxygenation. However, an alternative method may be to use positive end-expiratory pressure (PEEP) to prevent alveolar collapse.

H4>Circuit compressible volume

The volume of gas within the ventilator circuit, which may include a humidifier, will be compressed by the positive pressure during inspiration. Therefore part of the inspiratory tidal volume set on the ventilator is 'lost', and this loss may be increased by compliant circuit tubing. The amount of tidal volume that is lost to this compression is proportional to the change in pressure during the respiratory cycle (peak inspiratory pressure minus positive end-expiratory pressure), and is approximately 3 to 5 ml for a pressure change of 1 cmH2O (Kacmarek.iandVeneg┬žsiJn98Z). Increases in circuit volume (e.g. magnetic resonance imaging suites) can increase the volume loss. Apart from the reduction in ventilation, this volume loss makes auto-PEEP measurement using an expiratory hold inaccurate.

In circumstances where a large compressible volume loss is possible, exhaled tidal volume should be measured as close to the patient's airway as possible to obtain accurate tidal and minute volumes.

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