Effect of location and type of exhalation valve on carbon dioxide rebreathing during noninvasive positive pressure ventilation：a experimental study / 中华危重病急救医学

ABSTRACT

ObjectiveTo investigate the influence of exhalation valve location as well as its type on carbon dioxide (CO2) rebreathing during noninvasive positive pressure ventilation (NPPV).Methods With a standardized NPPV experimental model system, the exhalation valve was respectively installed between the ventilator tube and mask (positionⅠ), or on the mask (positionⅡ). This study included four groups according to the position and type of exhalation valve, namely single-arch exhalation valve was installed on the positionⅠ (A group), and positionⅡ (C group, the distal end of single-arch exhalation valve was blocked); plateau exhalation valve was installed on the positionⅠ (B group) and positionⅡ (D group, the distal end of plateau exhalation valve was blocked). Under standard experimental condition, the pressure of end-tidal carbon dioxide (PETCO2) was monitored in the trachea or the mask through adjusting the expiratory positive airway pressure (EPAP, EPAP was set at 5 cmH2O and 10 cmH2O, 1 cmH2O = 0.098 kPa) and tidal volume (VT, VT was set at 300, 400, 500 mL). Leakage of exhalation valve was monitored when single-arch exhalation and plateau exhalation valves were respectively placed in the positionⅠ through adjusting the inspiratory positive airway pressure (IPAP at 5, 10, 15, 20 cmH2O respectively). Results① Under standard experimental condition, when EPAP was 5 cmH2O, PETCO2 (mmHg, 1 mmHg = 0.133 kPa) in the trachea was 69.6±3.4, 61.4±2.7, 54.8±1.5, 49.8±1.3 in A, B, C, D groups respectively; and it was 24.8±1.9, 21.8±1.6, 2.8±0.8, 1.8±0.8 in the mask, respectively. When EPAP was 10 cmH2O, the PETCO2 in the trachea was 64.2±3.6, 57.2±3.7, 48.8±2.6, 41.8±2.6 in A, B, C, and D groups respectively; and it was 23.0±1.6, 20.2±1.6, 2.2±0.8, 1.2±0.8 in the mask, respectively. For the same exhalation valve type, exhalation valve being installed on positionⅡ could induce significantly lower PETCO2 in the trachea and mask than that being installed on positionⅠ (allP< 0.05). For the same expiratory valve position, plateau exhalation valve produced significantly lower PETCO2 than single-arch valve (allP< 0.05).② As the VT increased, the PETCO2 in the trachea of each group was reduced obviously. When VT was 500 mL, PETCO2 (mmHg) was significantly lower than VT, which were 300 mL and 400 mL (A group 51.4±2.7 vs. 72.8±2.9, 69.6±3.4; B group 44.8±2.4 vs. 65.4±2.1, 61.4±2.7;C group 36.8±1.9 vs. 59.0±1.6, 54.8±1.5; D group 28.8±1.9 vs. 52.6±2.0, 49.8±1.3; allP< 0.05).③ When exhalation valve type was placed in positionⅠ, the air leakage of single-arch exhalation valve was increased to (15.8±1.9), (20.2±1.9), (23.8±2.8), (28.0±1.6) L/min, and the plateau exhalation valve was essentially unchanged to (24.2±1.6), (23.8±1.6), (25.2±1.6), (25.2±1.6) L/min as the IPAP was increased from 5, 10, 15, to 20 cmH2O. Conclusions Exhalation valve fixing on mask is more appropriate for CO2 discharge than that fixed on tube-mask valve. Plateau exhalation valve as well as moderately increasing VT is beneficial for CO2 discharge and CO2 rebreathing prevention.