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

Abstract

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.

From belief to knowledge: call it evidence if you prefer

“…While confirming the results previously obtained by Saatchi et al [5] using a different technique, the study by Fodil et al [6] extends the analysis to the helmet, an interface that, despite increasing popularity because of its excellent tolerability over time [7], still raises concern because of the high risk of re-breathing related to its large internal volume. Fodil et al [6] elegantly show that when the internal volume is much larger than the tidal volume, as occurs with the helmet, the importance of the convective flows is negligible and is only minimally affected by the breathing cycle.…”

“…This notion has been repeatedly challenged by physiological and clinical studies. In a bench investigation, Saatci et al [5] found that the increase in the actual dead space observed during spontaneous unassisted breathing consequent to the application of oro-nasal masks was reduced close to zero thanks to the continuous flow produced by turbine ventilators throughout expiration.…”

Patient-Ventilator Interaction During Noninvasive Ventilation

“…18 Nonetheless, for a respiratory rate of 20 breaths/min, this adds about 2 L/min of dead space. Saatci et al 19 found that a face mask increased dynamic dead space from 32% to 42% of tidal volume. The use of PEEP also reduces the effective dead space of the mask.…”

“…The use of PEEP also reduces the effective dead space of the mask. 19 For ventilator/interface combinations that use a leak port, CO 2 can be cleared from the mask by incorporating the leak port into the mask rather than the circuit. This effectively reduces the dead space of the mask.…”

“…If rebreathing occurs, this may increase respiratory drive and contribute to asynchrony. [30][31][32] Rebreathing is decreased if the leak port is in the mask rather than in the circuit, 19,20 if oxygen is titrated into the mask rather than into the circuit, 33 with a higher expiratory pressure, 30 and with a plateau exhalation valve. 30 Major determinants of rebreathing are the expiratory time and the flow through the circuit during exhalation.…”