Carbon dioxide rebreathing during non-invasive ventilation delivered by helmet: a bench study

Objective: To define how to monitor and limit CO2 rebreathing during helmet ventilation. Design: Physical model study. Setting: Laboratory in a university teaching hospital. Interventions: We applied pressure-control ventilation to a helmet mounted on a physical model. In series 1 we increased CO2 production (V'CO2) from 100 to 550 ml/min and compared mean inhaled CO2 (iCO(2), mean) with end-inspiratory CO2 at airway opening (eiCO(2)), end-tidal CO2 at Y-piece (yCO(2)) and mean CO2 inside the helmet (hCO(2)). In series 2 we observed, at constant V'CO2, effects on CO2 rebreathing of inspiratory pressure, respiratory mechanics, the inflation of cushions inside the helmet and the addition of a flow-by. Measurements and results: In series 1, iCO(2), mean linearly related to V'CO2. The best estimate of CO2 rebreathing was provided by hCO(2): differences between iCO(2), mean and hCO(2), yCO(2) and eiCO(2) were 0.0 +/- 0.1, 0.4 +/- 0.2 and - 1.3 +/- 0.5%. In series 2, hCO2 inversely related to the total ventilation (MVtotal) delivered to the helmet - patient unit. The increase in inspiratory pressure significantly increased MVtotal and lowered hCO(2). The low lung compliance halved the patient: helmet ventilation ratio but led to minor changes in MVtotal and hCO(2). Cushion inflation, although it decreased the helmet's internal volume by 33%, did not affect rebreathing. A 8-1/min flow-by effectively decreased hCO(2). Conclusions: During helmet ventilation, rebreathing can be assessed by measuring hCO(2) or yCO(2), but not eiCO(2). It is directly related to V'CO2, inversely related to MVtotal and can be lowered by increasing inspiratory pressure or adding a flow-by.