Objective: To evaluate the operation of a continuous positive airway pressure system by using tracheal airway pressure (PT) as the control signal for system operation (i.e., tracheal pressure control).
Design: Repeated measures.
Setting: University research laboratory.
Subjects: Twelve anesthetized, spontaneously breathing swine.
Interventions: Subjects were intubated and connected to a tracheal pressure control system (5 cm H2O continuous positive airway pressure). Varying inspiratory flow demands and degrees of partial endotracheal tube occlusion (25%, 50%, and 75%) were studied. Tracheal pressure control was compared with a conventionally controlled system (pressure from breathing circuit Y-piece [PY] used as control signal) during endotracheal tube occlusion.
Measurements and results: Imposed resistive work of breathing (work to spontaneously inhale through endotracheal tube and ventilator circuit), work by ventilation system assisting inhalation, PT, PY, tidal volume, and inspiratory flow demands were measured. As inspiratory flow demands increased (range, 0.2-2.3 L/sec), pressure assist increased automatically (range, 5-40 cm H2O) as well as work of breathing by ventilation system assisting inhalation (range, 0.2-2.5 J/L). Imposed resistive work of breathing was nullified at the lower and was negligible at the higher flow demands. During endotracheal tube occlusion with a conventionally controlled system, PY was unchanged, whereas PT decreased (up to -15 cm H2O) and imposed resistive work of breathing increased (up to 1.05 J/L). With tracheal pressure control, PY increased automatically (range, 8-52 cm H2O), whereas PT varied slightly (range, 2 to -4.6 cm H2O). Imposed resistive work of breathing was negligible (range, 0-0.2 J/L). Breathing circuit pressure (PY), not pulmonary airway pressure (PT), increased significantly during tracheal pressure control.
Conclusions: Tracheal pressure control results in automatic and variable levels of pressure assist to decrease imposed resistive work of breathing under conditions of varying spontaneous inspiratory flow demands and endotracheal tube occlusion. Conventional systems are potentially flawed when PY is used as the control signal because they do not function in this manner and do not accurately assess pulmonary airway pressure.