Comparison of patient–ventilator interfaces based on their computerized effective dead space

Abstract

Purpose: Non-invasive ventilation is largely used to treat acute and chronic respiratory failure. This ventilation encounters a nonnegligible rate of failure related to the used interface/mask, but the reasons for this failure remain unclear. In order to shed light on this issue and to better understand the effects of the geometrical design of interfaces, we aimed to quantify flow, pressure and gas composition in terms of CO 2 and O 2 at the passage through different types of interface (oronasal mask, integral mask and helmet). In particular, we postulated that due to specific gas flow passing throughout the interface, the effective dead space added by the interface is not always related to the whole gas volume included in the interface. Methods: Numerical simulations, using computational fluid dynamics, were used to describe pressure, flow and gas composition during ventilation with the different interfaces. Results: Between the different interfaces the effective dead spaces differed only modestly (110-370 ml), whereas their internal volumes were markedly different (110-10,000 ml).

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.…”

“…In this issue of Intensive Care Medicine, Fodil et al [6] present a new evaluation of the actual dead space of four interfaces in common clinical use (two oro-nasal masks, one full face mask, and a helmet) that have different internal volumes, ranging from about 100 ml to more than 10 l. The authors used mathematical simulations made with a computational fluid dynamics software analyzing the pressure field/flow pattern and variations in gas composition inside the interface. It was found that, in spite of the large variation of their internal volume and consequent to the streaming effect of gas flowing through the interface, the actual dead space of the four interfaces differed to a relatively moderate extent, ranging from 110 to 370 ml.…”

“…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. The helmet's actual dead space is thus about 50% of the tidal volume in all instances.…”

“…The technique presented by Fodil et al [6] might be important also for manufacturers, by allowing them to predict the behavior of their devices in different conditions. In this regards, we wonder whether it is time for the producers of NIV interfaces to provide standardized bench data obtained with a reproducible technique, like the one presented in this elegant study.…”

Avaliação do desempenho de diferentes interfaces para ventilação não invasiva em modelo mecânico simulando paciente com DPOC

“…Além do volume interno da máscara medido in vitro, o padrão de fluxo dentro da máscara, a presença de vazamentos intencionais ou não intencionais 29 , variações de volume corrente e esforço do paciente, e uso de válvulas de exalação 30,31 também influenciam o espaço morto efetivo da interface e a reinalação de CO 2 , podendo influenciar os resultados obtidos com a VNI. Um estudo recente em pacientes com insuficiência respiratória, mas estáveis o suficiente para tolerar turnos seqüenciais em quatro interfaces diferentes, mostrou que interfaces de alto volume podem ter desempenho comparável a interfaces de volume bem menor 32 .…”