Bilevel positive airway pressure ventilation: factors influencing carbon dioxide rebreathing.

Abstract

Use of bilevel positive airway pressure (BLPAP) ventilators for noninvasive ventilation (NIV) is an established treatment for both acute and chronic ventilatory failure. Although BLPAP ventilator circuits are simpler than those of conventional ventilators, one drawback to their use is that they allow variable amounts of rebreathing to occur. The aim of this work is to measure the amount of CO(2) reinsufflated in relation to the BLPAP ventilator circuit in patients, and to determine predictive factors for rebreathing. Eighteen adult patients were ventilated on pressure support, either by intubation or with mask ventilation, during a weaning period. The mean inspiratory fraction of CO(2) (tidal FiCO(2)) reinsufflated from the circuit between the intentional leak and the ventilator was measured for each breath. The influence of end-tidal CO(2) concentration (ETCO(2)), respiratory rate (RR), percentage of inspiratory time (T (i)/T (TOT)), application of expiratory positive airway pressure (EPAP), and inspiratory tidal volume on magnitude of tidal FiCO(2), as well as the influence of intubation versus NIV, were studied by univariate comparisons and logistic regression analysis. In a total of 11,107 cycles, tidal FiCO(2) was 0.072 +/- 0.173%. Of fractions measured, 8,976 (81%) were under 0.10% and 2,131 (19%) were over 0.10%, with mean values of 0.026 +/- 0.027% and 0.239 +/- 0.326%, respectively. ETCO(2), EPAP, NIV versus intubation, and RR had significant predictive value for tidal FiCO(2) >0.10%. BLPAP ventilators present a specific rebreathing risk to patients. However, that risk remains modest, even in intubated patients, provided that EPAP is applied.