title: Carbon dioxide rebreathing during non-invasive ventilation delivered by helmet: a bench study.
abstract: OBJECTIVE
  To define how to monitor and limit CO(2) rebreathing during helmet ventilation.
  
  
  METHODS
  Physical model study.
  
  
  METHODS
  Laboratory in a university teaching hospital.
  
  
  METHODS
  We applied pressure-control ventilation to a helmet mounted on a physical model. In series 1 we increased CO(2) production (V'CO(2)) from 100 to 550 ml/min and compared mean inhaled CO(2) (iCO(2),mean) with end-inspiratory CO(2) at airway opening (eiCO(2)), end-tidal CO(2) at Y-piece (yCO(2)) and mean CO(2) inside the helmet (hCO(2)). In series 2 we observed, at constant V'CO(2), effects on CO(2) rebreathing of inspiratory pressure, respiratory mechanics, the inflation of cushions inside the helmet and the addition of a flow-by.
  
  
  RESULTS
  In series 1, iCO(2),mean linearly related to V'CO(2). The best estimate of CO(2) 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, hCO(2) 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-l/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'CO(2), inversely related to MVtotal and can be lowered by increasing inspiratory pressure or adding a flow-by.
authors.affiliation: Servizio di Anestesia e Rianimazione I, Fondazione IRCCS Policlinico S. Matteo, Piazzale Golgi 2, Pavia, Italy. f.mojoli@smatteo.pv.it
authors.forename: Francesco
authors.initials: F
authors.lastname: Mojoli
authors.forename: Giorgio A
authors.initials: GA
authors.lastname: Iotti
authors.forename: Maddalena
authors.initials: M
authors.lastname: Gerletti
authors.forename: Carlo
authors.initials: C
authors.lastname: Lucarini
authors.forename: Antonio
authors.initials: A
authors.lastname: Braschi
mesh_terms: D002245:Carbon Dioxide / Q000378:metabolism*; D004867:Equipment Design; D006260:Head Protective Devices; D007755:Laboratories, Hospital; D008962:Models, Theoretical; D011175:Positive-Pressure Respiration / Q000295:instrumentation*; D015656:Respiratory Mechanics
chemical_list: D002245:Carbon Dioxide