Do Filters Affect Humidity Levels Within Helmet-CPAP Powered by the Venturi System? Insights from a Bench-top Study

Helmet continuous positive airway pressure (H-CPAP) systems powered by Venturi devices are widely used for noninvasive respiratory support in acute hypoxaemic respiratory failure. Although mechanical filters are often added to reduce noise and improve environmental safety, their impact on gas humidification, flow, and FiO₂ delivery remains unclear. This bench-top study aimed to evaluate how the use of different filter types and configurations affects the humidity within the helmet. The secondary objective was to assess the changes in FiO₂ and flow associated with filter placement.

A bench-top study was conducted at the Respiratory Physiology Laboratory of the Italian University Hospital using a medium-sized CPAP helmet connected to a Venturi flow generator. Seven mechanical filters (5 HEPA and 2 HME) were tested in 3 configurations: no filter, 1 filter between the Venturi flowmeter and helmet gas-in, and 2 filters (post-flowmeter and Venturi air inlet). Temperature, relative humidity, flow, and FiO₂ were measured under 3 FiO₂ settings (0.35, 0.55, 0.75).

The addition of filters led to a statistically significant reduction in the absolute humidity inside the helmet (P<0.05). At FiO₂ 0.75, the absolute humidity decreased from 7.31 mgH₂O/L without filters to 6.64 mgH₂O/L with 2 filters. The use of filters led to a small yet notable rise in FiO₂ (from 0.46±0.11 with no filter to 0.48±0.12 with 2 filters; P=0.001) and a corresponding decrease in flow (from 1.37±0.34 L/s to 1.17±0.30 L/s; P=0.003). By setting the FiO2 to <0.55 and incorporating a single filter into the Venturi air port, external water vapour was allowed to enter the helmet, approaching the recommended level of 10 mgH2O/L.

Mechanical filters in H-CPAP systems reduce humidification and flow while increasing FiO₂. These effects should be carefully considered in clinical practice to maintain patient comfort and therapeutic effectiveness, particularly in settings without active humidification.