Chest
Volume 108, Issue 3, September 1995, Pages 772-778
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Clinical Investigations in Critical Care
Evaluation of Carbon Dioxide Rebreathing During Pressure Support Ventilation With Airway Management System (BiPAP) Devices

https://doi.org/10.1378/chest.108.3.772Get rights and content

The purpose of this study was to evaluate whether carbon dioxide (CO2) rebreathing occurs in acute respiratory 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 standard device was tested on a lung model with a nonrebreathing 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 CO2 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 H2O) 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 nonrebreathing valve was used. In the second part of the study, seven patients weaned from mechanical ventilation were investigated using a randomized crossover design to compare three situations: pressure support ventilation with a conventional intensive care ventilator (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 system. This drawback can be overcome by using a nonrebreathing valve, but only at the expense of greater expiratory resistance.

Section snippets

METHODS

This study comprised two parts, an experimental and a clinical study.

Experimental in Vitro Study

Table 2 shows the results obtained using the lung model, and Figure 2 depicts the pressure-volume loops recorded with the BiPAP-uc and the BiPAP-NRV. With the BiPAP-uc, the RVEA was 400 mL, ie, up to 55% of the VT. Use of the Sanders NRV-2 nonrebreathing valve fully eliminated rebreathing.

When the usual connector was replaced by a nonrebreathing valve, there was an increase in PEEP (2.4 vs 1.3 cm H2O) as a result of a rise in the time constant required to reach the equilibrium of the relaxed

DISCUSSION

Both our in vitro experiment (lung model) and our clinical randomized crossover study demonstrate that use of a standard BiPAP system to deliver inspiratory pressure support to ICU patients can result in a substantial amount of CO2 rebreathing with a marked increase in the WOB. Two simple factors were required to produce these adverse effects: (1) use of a standard tubing system with the widely used Whisper Swivel connector, and (2) a level of expiratory pressure set at the minimal value, ie,

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    The experimental part of this study was supported by a grant from the Caisse Nationale d'Assurance Maladie (CNAMTS); the clinical part by the Assistance Publique de Hopitaux de Paris.

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