EVALUATION OF CARBON-DIOXIDE REBREATHING DURING PRESSURE SUPPORT VENTILATION WITH AIRWAY MANAGEMENT-SYSTEM (BIPAP) DEVICES

The purpose of this study was to evaluate whether carbon dioxide (CO2) rebreathing occurs in acute respiratory failure patients ventilated u sing the standard airway management system (BiPAP pressure support ven tilator; Respironics; Murrysville, Pa) with positive inspiratory airwa y pressure and a minimal level of positive end-expiratory pressure (PE EP) and whether any CO2 rebreathing may be efficiently prevented by th e addition of a nonrebreathing valve to the BiPAP system circuit. In t he first part of the study, the standard device was tested on a lung m odel with a nonrebreathing valve (BiPAP-NRV) and with the usual Whispe r Swivel connector (BiPAP-uc). With the BiPAP-uc device, the resident volume of expired air in the inspiratory circuit at the end of expirat ion (RVEA) was 55% of the tidal volume (VT) when the inspiratory press ure was 10 cm H2O and the frequency was at 15 cycles per minute. The B iPAP-NRV device efficiently prevented CO2 rebreathing but resulted in a slight decrease in VT, which was due to a significant increase in ex ternal PEEP (2.4 vs 1.3 cm H2O) caused by the additional expiratory va lve resistance. For similar reasons, both the pressure swing necessary to trigger pressure support and the imposed expiratory work were incr eased in the lung model when the nonrebreathing valve was used. In the second part of the study, seven patients weaned from mechanical venti lation were investigated using a randomized crossover design to compar e three situations: pressure support ventilation with a conventional i ntensive care ventilator (CIPS), BiPAP system use, and BiPAP-NRV. When we compared the BiPAP system use with the other two systems, we obser ved no significant effect on blood gases but found significant increas es in VT, minute ventilation, and work of breathing. These findings ar e experimental and are clinical evidence that significant CO2 rebreath ing occurs with the standard BiPAP system. This drawback can be overco me by using a nonrebreathing valve, but only at the expense of greater expiratory resistance.