Not too much, not too little. Titrating flow rate to minimise inspiratory effort during helmet CPAP: A bench study.

Background: Non-invasive helmet respiratory support is suitable for several clinical conditions. Continuous-flow helmet CPAP systems equipped with HEPA filters have become popular during the recent Coronavirus pandemic. However, HEPA filters generate an overpressure above the set PEEP.

Methods: A lung simulator was used to mimic patient respiratory mechanics and effort. Compared to room air spontaneous breathing, the additional inspiratory effort attributable to helmet CPAP (ΔPmusHelmet) was recorded at different continuous-flow rates (30-150 L/min), PEEP levels (5, 10, 12.5 cmH2O) and respiratory rates (15, 20, 25, 30 breaths/minute), both with and without a HEPA filter at the outlet port.

Results: Helmet pressure swings during inspiration largely explained ΔPmusHelmet variations (p<0.001, Spearman's Rho=0.964). The lowest ΔPmusHelmet levels (0.2 [0; 0.4] cmH2O) were frequently recorded (>70%) at a 90 L/min flow rate. Higher ΔPmusHelmet levels were recorded when the continuous-flow was lower than the peak inspiratory flow (3.7 [3.1; 5.6] cmH2O, p<0.001) or when a HEPA filter was used (2.7 [2.2; 3.5], p<0.001). Increasing the flow rate resulted in higher overpressure levels, particularly with a HEPA filter (p<0.001). Overpressure levels correlated with ΔPmusHelmet (p<0.001, Spearman's Rho=0.598).

Conclusions: Helmet pressure swings below PEEP lead to additional inspiratory efforts. The HEPA filter acts as a flow resistor, generating an overpressure leading to increased respiratory effort. The continuous-flow rate should be titrated high enough to slightly exceed the peak inspiratory flow; however, further flow increase is not recommended as it leads to an increase in overpressure and helmet pressure swings below PEEP.