Evaluation of Carbon Dioxide Rebreathing During Pressure Support Ventilation With Airway Management System (BiPAP) Devices* Frederic Lofaso, MD; Laurent Brochard, MD; Dominique Touchard; Thierry Hang; Main Harf, MD; and Daniel Isabey, PhD The purpose of this study was to evaluate whether carbon dioxide (CO2) rebreathing occurs in acute res¬ piratory failure patients ventilated using the standard airway management system (BiPAP pressure support ventilator; Respironics; Murrysville, Pa) with positive inspiratory airway pressure and a minimal level of positive end-expiratory pressure (PEEP) and whether any CO2 rebreathing may be efficiently prevented by the addition of a nonrebreathing valve to the BiPAP system circuit. In the first part of the study, the stan¬ dard device was tested on a lung model with a nonre¬ breathing valve (BiPAP-NRV) and with the usual Whisper Swivel connector (BiPAP-uc). With the BiPAP-uc device, the resident volume of expired air in the inspiratory circuit at the end of expiration (RVEA) was 55% of the tidal volume (Vt) when the inspiratory pressure was 10 cm H2O and the frequency was at 15 cycles per minute. The BiPAP-NRV device efficiently prevented C02 rebreathing but resulted in a slight decrease in Vt, which was due to a significant increase in external PEEP (2.4 vs 1.3 cm H20) caused by the additional expiratory valve resistance. For similar reasons, both the pressure swing necessary to trigger pressure support and the imposed expiratory work were increased in the lung model when the nonre¬ breathing 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 compare three situations: pressure support ventilation with a conventional intensive care ventila¬ tor (CIPS), BiPAP system use, and BiPAP-NRV. When we compared the BiPAP system use with the other two systems, we observed no significant effect on blood gases but found significant increases in Vt, minute ventilation, and work of breathing. These findings are experimental and are clinical evidence that significant CO2 rebreathing occurs with the standard BiPAP sys¬ tem. This drawback can be overcome by using a non¬ rebreathing valve, but only at the expense of greater expiratory resistance. (CHEST 1995; 108:772-78) BiPAP-uc=BiPAP system using the usual Whisper Swivel connector; BiPAP-NRV=BiPAP system using a nonre¬ breathing valve; CIPS=conventional intensive care ven¬ tilator; CPS=conventional pressure support ventilation; IPS=inspiratory pressure support; N-CPAP=nasal-contin- uous positive airway pressure; PEEP=positive end-expi¬ ratory pressure; Pes=esophageal pressure; RVEA=resi- dent volume of expired air in the inspiratory circuit at the end of expiration; Vt=tidal volume; WOB=work of breath¬ ing; WOBE-L=expiratory work imposed on the lung model Key words: bilevel positive airway pressure; CCVrebreath- ing; pressure support; work of breathing T^Toninvasive ventilation with a pressure support ^ ^ ventilator is an attractive method for the treat¬ ment of acute ventilatory failure and has been used widely, with or without positive end-expiratory pres¬ sure (PEEP). Clinicians can choose from a wide array of pressure support devices with fairly marked differ- *From the Service de Physiologie-Explorations Fonctionelles and Institut National de la Sante et dela Recherche Medical (IN- SERM) U 296 (Drs. Lofaso, Harf, and Isabey and Ms. Touchard); Reanimation Medicale (Dr. Brochard); Hopital Henri Mondor, Creteil, France; and the Association d'Entraide des Polios et Handicapes (Mr. Hang), Puteaux, France. The experimental part of this study was supported by a grant from the Caisse Nationale d'Assurance Maladie (CNAMTS); the clini¬ cal part by the Assistance Publique de Hopitaux de Paris. Manuscript received October 12,1994; revision accepted February L, 199o. Reprint requests: Dr. Lofaso, INSERM 296; 8 rue du General Sar- rail, 94010 Creteil Cedex, France ences in design.1"7 Relatively simple devices, initially designed for home mechanical support, may be both easy to use and less expensive than sophisticated ven¬ tilators. On the other hand, patients admitted for acute ventilatory failure may have different ventilatory re¬ quirements from those of patients treated at home, and home ventilators may have technical limitations. In this study, we examined one of these limitations, which may have a major negative impact on efficacy.8'9 The airway management system (BiPAP; Respiron¬ ics; Murrysville, Pa) is a nasal-continuous positive air¬ way pressure (N-CPAP) equipped with a solenoid valve system to permit separate adjustment of expira¬ tory positive airway pressure and inspiratory positive airway pressure. This device provides both inspiratory pressure support (IPS) and PEEP. It was first used to 772 Clinical Investigations in Critical Care Downloaded From: http://journal.publications.chestnet.org/pdfaccess.ashx?url=/data/journals/chest/21720/ on 06/26/2017