Purpose: Low tidal volume (V(T)) delivered by flow-targeted breaths reduces ventilator-induced lung injury but may increase patient breathing effort because of limited flow. We hypothesized that a variable-flow, pressure-targeted breath would improve breathing effort versus a fixed flow-targeted breath.
Materials and methods: We compared pressure assist-control ventilation and volume assist-control ventilation (VACV) in 12 patients with acute respiratory failure receiving 6 to 8 mL/kg V(T). Backup frequency, V(T), inspiratory time, applied positive end-expiratory pressure and fraction of inspired oxygen were held constant. Patient breathing effort was assessed by airway pressure (Paw) drop below baseline 0.1 second after the breath initiation (P(0.1)), the maximal Paw drop during the triggering phase (Ptr), the magnitude of ventilator work during flow delivery, and the presence of an active expiratory effort during cycling and air trapping judged by the magnitude of residual flow at end-expiration.
Results: Compared with VACV, pressure assist-control ventilation decreased P(0.1), Ptr (by 25% and 16%, respectively), and evidence for trapped gas but not ventilator work during flow delivery or cycle dys-synchrony. Peak inspiratory flow was comparable between the 2 modes.
Conclusions: In patients receiving small V(T) VACV with increased breathing effort, variable-flow, pressure-targeted ventilation may provide more comfort by decreasing respiratory drive during the triggering phase.