Unloading of the work of breathing by proportional assist ventilation in a lung model

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Abstract
Objectives Proportional assist ventilation is devised to increase airway pressure in proportion to inspiratory effort. A systematic study of the performance of this new mode of ventilation has not been presented. We tested in the laboratory the capability of proportional assist ventilation to unload the work of breathing in proportion to ventilatory drive, under a variety of mechanical loads. Design During variations of "ventilatory drive" (i.e., tidal volume), unloading of the work of breathing by proportional assist ventilation was contrasted with unloading by pressure-support ventilation. Setting The respiratory laboratory of a university-affiliated teaching hospital. Subject A bellows-in-a-box lung model, powered by a sine wave air flow generator. Interventions Proportional assist and pressure-support ventilation were preset to provide comparable support at a baseline "ventilatory drive" of 0.7-L tidal volume. The set levels of proportional assist and pressure-support ventilation were subsequently applied to five tidal volumes, from 0.2 to 1.2 L. Three levels of inspiratory support and three settings of mechanical load were evaluated. Measurements and Main Results Proportional assist ventilation significantly (p < .05) reduced the work of breathing of the lung model at all but the lowest tidal volume (0.2 L). The preset proportion of ventilatory support (30%, 50%, and 70%) unloaded the work of breathing uniformly as ventilatory drive was varied at tidal volumes of >or=to0.5 L, but not always at tidal volumes of <or=to0.4 L. In contrast, pressure-support ventilation overassisted low tidal volumes and underassisted high tidal volumes (p < .05). Conclusions In a lung model, a prototype system delivering proportional assist ventilation provided uniform unloading of the work of breathing as the ventilatory drive was varied within a tidal volume range of 0.5 to 1.2 L. These findings confirm the theoretical modeling of proportional assist ventilation. This system, however, failed to properly unload low tidal volumes of 0.2 to 0.4 L. (Crit Care Med 1997; 25:267-272)