Effect of location and type of exhalation valve on carbon dioxide rebreathing during noninvasive positive pressure ventilation: a experimental study

Abstract

To investigate the influence of exhalation valve location as well as its type on carbon dioxide (CO2) rebreathing during noninvasive positive pressure ventilation (NPPV). With a standardized NPPV experimental model system, the exhalation valve was respectively installed between the ventilator tube and mask (position II), or on the mask (position II). This study included four groups according to the position and type of exhalation valve, namely: single-arch exhalation valve was installed on the position I (A group), and position II (C group, the distal end of single-arch exhalation valve was blocked); plateau exhalation valve was installed on the position I (B group) and position II (D group, the distal end of plateau exhalation valve was blocked). Under standard experimental condition, the pressure of end-tidal carbon dioxide (P(ET)CO2) 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 (V(T), V(T) was set at 300, 400, 500 mL). Leakage of exhalation (1) Under standard experimental condition, when EPAP was 5 cmH2O, P(ET)CO2 (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 P(ET)CO2 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 position II could induce significantly lower P(ET)CO2 in the trachea and mask than that being installed on position I (all P < 0.05). For the same expiratory valve position, plateau exhalation valve produced significantly lower P(ET)CO2 than single-arch valve (all P < 0.05). (2) As the V(T) increased, the P(ET)CO2 in the trachea of each group was reduced obviously. When V(T) was 500 mL, P(ET)CO2 (mmHg) was significantly lower than V(T), 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; all P < 0.05). (3) When exhalation valve type was placed in position I, 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. 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 V(T) is beneficial for CO2 discharge and CO2 rebreathing prevention.