title: Position of exhalation port and mask design affect CO2 rebreathing during noninvasive positive pressure ventilation.
abstract: OBJECTIVE
  Noninvasive positive pressure ventilation may be considered a first line intervention to treat patients with hypercapnic respiratory failure. However, CO2 rebreathing from the ventilator circuit or mask may impair CO2 elimination and load the ventilatory muscles. This study was conducted to evaluate the effect of exhalation port location and mask design on CO2 rebreathing during noninvasive positive pressure ventilation.
  
  
  METHODS
  Lung model evaluation.
  
  
  METHODS
  Experimental laboratory of a large university-affiliated hospital.
  
  
  METHODS
  A dual-chamber test lung was used to simulate the ventilatory mechanics of a patient with obstructive lung disease.
  
  
  METHODS
  Hypercapnic respiratory failure (end-tidal CO2 of 75 mm Hg) and obstructive lung disease were simulated in a double-chamber lung model. A facial mask (inner volume of 165 mL) with exhalation port within the mask (Facial-MEP) or the same mask with exhalation port in the ventilator circuit (Facial-WS) and a total face mask with exhalation port within the mask (inner volume 875 mL, Total Face) were tested during continuous positive airway pressure and pressure support ventilation provided by a single-limb circuit ventilator at the same frequency and tidal volume.
  
  
  RESULTS
  A capnometer and a flow transducer were placed in the lung model upper airway to measure the volume of CO2 rebreathed and tidal volume (Vt). The inspiratory load was estimated from the pressure variation in the lung model driving chamber (PDR). Volume of CO2 rebreathed was smaller during Facial-MEP compared with the other masks in all tested conditions (p <.001). The Vt and PDR necessary to decrease end-tidal CO2 20% (from 75 to 60 mm Hg) was different among the tested masks (Facial-MEP, Vt 701 +/- 9 mL, PDR 8.1 +/- 0.1 cm H2O/sec; Facial-WS, Vt 745 +/- 9 mL, PDR 10.2 +/- 0.1 cm H2O/sec; Total Face, Vt 790 +/- 12 mL, PDR 11.4 +/- 0.2 cm H2O/sec, p <.001).
  
  
  CONCLUSIONS
  Facial-MEP with its exhalation port within the mask and the smallest mask volume demonstrated less rebreathed CO2 and a lower PDR than either the Facial-WS or Total Face masks. Additional studies are necessary to confirm if mask design can clinically affect patient's inspiratory effort during noninvasive positive pressure ventilation.
authors.affiliation: Department of Anesthesia and Critical Care and Respiratory Care, Massachusetts General Hospital, Harvard Medical School, Boston, 02214, USA.
authors.forename: Guilherme P P
authors.initials: GP
authors.lastname: Schettino
authors.forename: Sunisa
authors.initials: S
authors.lastname: Chatmongkolchart
authors.forename: Dean R
authors.initials: DR
authors.lastname: Hess
authors.forename: Robert M
authors.initials: RM
authors.lastname: Kacmarek
mesh_terms: D002245:Carbon Dioxide / Q000378:metabolism*; D004867:Equipment Design; D006801:Humans; D008397:Masks; D011175:Positive-Pressure Respiration / Q000295:instrumentation*; D012121:Respiration, Artificial; D012129:Respiratory Function Tests; D012131:Respiratory Insufficiency / Q000378:metabolism* / Q000628:therapy; D015656:Respiratory Mechanics
chemical_list: D002245:Carbon Dioxide