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Background There is growing interest towards the use of helmet noninvasive ventilation (NIV) for the management of acute hypoxemic respiratory failure. Gas conditioning through heat and moisture exchangers (HME) or heated humidifiers (HHs) is needed during facemask NIV to provide a minimum level of humidity in the inspired gas (15 mg H 2 O/L). The optimal gas conditioning strategy during helmet NIV remains to be established. Methods Twenty patients with acute hypoxemic respiratory failure (PaO 2 /FiO 2  &lt; 300 mmHg) underwent consecutive 1-h periods of helmet NIV (PEEP 12 cmH 2 O, pressure support 12 cmH 2 O) with four humidification settings, applied in a random order: double-tube circuit with HHs and temperature set at 34 °C (HH34) and 37 °C (HH37); Y-piece circuit with HME; double-tube circuit with no humidification (NoH). Temperature and humidity of inhaled gas were measured through a capacitive hygrometer. Arterial blood gases, discomfort and dyspnea through visual analog scales (VAS), esophageal pressure swings (Δ P  ES ) and simplified pressure–time product (PTP ES ), dynamic transpulmonary driving pressure (Δ P  L ) and asynchrony index were measured in each step. Results Median [IqR] absolute humidity, temperature and VAS discomfort were significantly lower during NoH vs. HME, HH34 and HH37: absolute humidity (mgH 2 O/L) 16 [12–19] vs. 28 [23–31] vs. 28 [24–31] vs. 33 [29–38],  p  &lt; 0.001; temperature (°C) 29 [28–30] vs. 30 [29–31] vs. 31 [29–32] vs 32. [31–33],  p  &lt; 0.001; VAS discomfort 4 [2–6] vs. 6 [2–7] vs. 7 [4–8] vs. 8 [4–10],  p  = 0.03. VAS discomfort increased with higher absolute humidity ( p  &lt; 0.01) and temperature ( p  = 0.007). Higher VAS discomfort was associated with increased VAS dyspnea ( p  = 0.001). Arterial blood gases, respiratory rate, ΔP ES , PTP ES  and ΔP L  were similar in all conditions. Overall asynchrony index was similar in all steps, but autotriggering rate was lower during NoH and HME ( p  = 0.03). Conclusions During 1-h sessions of helmet NIV in patients with hypoxemic respiratory failure, a double-tube circuit with no humidification allowed adequate conditioning of inspired gas, optimized comfort and improved patient–ventilator interaction. Use of HHs or HME in this setting resulted in increased discomfort due to excessive heat and humidity in the interface, which was associated with more intense dyspnea. Trail Registration  Registered on clinicaltrials.gov (NCT02875379) on August 23rd, 2016.

annals of intensive care

Gas conditioning during helmet noninvasive ventilation: effect on comfort, gas exchange, inspiratory effort, transpulmonary pressure and patient–ventilator interaction