The Effect of Filters on CPAP Delivery by Helmet

When helmet CPAP is performed using a Venturi system, filters are frequently interposed in the respiratory circuit to reduce noise within the helmet. The effect of the interposition of these filters on delivered fresh gas flow and the resulting F_IO is currently unknown.

In a bench study, 2 different Venturi systems (WhisperFlow and Harol) were used to generate 3 different gas flow/F_IO combinations (80 L/min-F_IO 0.6, 100 L/min-F_IO 0.5, 120 L/min-F_IO 0.4). Different combinations of filters were applied at the flow generator input line and/or at the helmet inlet port. Two types of filters were used for this purpose: a heat and moisture exchanger filter and an electrostatic filter. The setup without filters was used as baseline. Gas flow and F_IO were measured for each setup.

Compared to baseline, the interposition of filters reduced the gas flow between 1-13% (P < .001). The application of a filter at the Venturi system or at the helmet generated a comparable flow reduction (-3 ± 2% vs -4 ± 2%, P = .12), whereas a greater flow reduction (-7 ± 4%) was observed when filters were applied at both sites (P < .001). An increase in F_IO up to 5% was observed with filters applied. A strong inverse linear relationship (P < .001) was observed between the resulting gas flow and F_IO .

The use of filters during helmet CPAP reduced the flow delivered to the helmet and, consequently, modified F_IO . If filters are applied, an adequate gas flow should be administered to guarantee a constant CPAP during the entire respiratory cycle and avoid rebreathing. Moreover, it might be important to measure the effective F_IO delivered to the patient to guarantee a precise assessment of oxygenation.