A Bench Evaluation of the Effects of Circuit Configurations on Instrumental Dead Space During Home Mechanical Ventilation

Background: A number of circuit configurations can be proposed for implementing mechanical ventilation, most of which are likely to impact the instrumental dead space and thus, on alveolar ventilation. This bench study was designed to investigate the effects of circuit configuration on P CO 2 with constant ventilator settings ( iso-V T condition; V T : tidal volume ) and to evaluate the modifications of V T required to maintain stable alveolar ventilation with each circuit configuration ( iso- P ET CO 2 condition ; end-tidal CO 2 partial pressure being considered as a surrogate for alveolar ventilation). Methods: A total of 17 configurations were evaluated, including valve and leak circuits, heater humidifier or different heat and moisture exchangers, with or without catheter mount, and both invasive and noninvasive situations. All these evaluations were performed at 2 different respiratory rate, both in iso-V T and iso- P ET CO 2 conditions. Results: With valve circuits, modifications of instrumental dead space resulted in P ET CO 2 variations reaching up to 16 mm Hg from one configuration to another. The corresponding increase in the required V T to compensate for the additional dead space reached up to 120 mL. Leak circuits significantly reduced the effects of instrumental dead space compared with valve circuits. Conclusions: Any modification of the circuit configuration requires a systematic reevaluation of ventilation efficiency.