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.

Design 

Lung model evaluation.

Setting 

Experimental laboratory of a large university-affiliated hospital.

Subjects 

A dual-chamber test lung was used to simulate the ventilatory mechanics of a patient with obstructive lung disease.

Intervention 

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.

Measurements and Main 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 (P_DR). Volume of CO₂ rebreathed was smaller during Facial-MEP compared with the other masks in all tested conditions (p < .001). The Vt and P_DR 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, P_DR 8.1 ± 0.1 cm H₂O/sec; Facial-WS, Vt 745 ± 9 mL, P_DR 10.2 ± 0.1 cm H₂O/sec; Total Face, Vt 790 ± 12 mL, P_DR 11.4 ± 0.2 cm H₂O/sec, p < .001).

Conclusion 

Facial-MEP with its exhalation port within the mask and the smallest mask volume demonstrated less rebreathed CO₂ and a lower P_DR 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.