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Dynamic dead space in face masks used with noninvasive ventilators: a lung model study

¹ Division of Engineering, King's College London, ² Dept of Anaesthetics, Guy's, King's and St Thomas' School of Medicine, Guy's Hospital, and ³ Dept of Respiratory Medicine, King's College Hospital, London, UK

CORRESPONDENCE: D.M. Miller, Dept of Anaesthetics, Guy's, Kings and St. Thomas' School of Medicine, Guy's Hospital, London, SE1 9RT, UK. Fax: 44 2079554050. E-mail: donald.miller@kcl.ac.uk

Keywords: dynamic dead space, face masks, lung model, noninvasive ventilators

Received: April 8, 2003
Accepted July 23, 2003

This study was supported by a grant from the Guy's and St Thomas' Hospital Charitable Foundation, and the Dept of Health National Health Service New and Emerging Applications of Technology Programme.

The aim of this study was to determine what the influence of different designs of face masks and different noninvasive ventilator modes would be upon total dynamic dead space.

Using a spontaneous breathing model, total dynamic dead space was measured when using 19 commercially available face masks and a range of ventilators in various ventilation modes.

Total dynamic dead space during spontaneous ventilation was increased above physiological dead space from 32% to 42% of tidal volume by using face masks. The use of noninvasive ventilation modes such as bilevel and continuous positive airway pressure, with continuous pressure throughout the expiratory phase, reduced total dynamic dead space to approach physiological dead space with most face masks. Pressure assist and pressure support ventilation decreased total dynamic dead space to a lesser degree, from 42% to 39% of tidal volume. Face masks with expiratory ports over the nasal bridge resulted in beneficial flow characteristics within the face mask and nasal cavity, so as to decrease total dynamic dead space to less than physiological dead space from 42% to 28.5% of tidal volume.

Exhaust ports over the nasal bridge in face masks effect important decreases in dynamic dead space provided positive pressure throughout the expiratory phase is used.

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