Closed system suctioning (CSS) is a method of removing secretions from the tracheobronchial tree of patients with mechanical ventilation without disconnecting the mechanical ventilator. The putative benefits of CSS include the maintenance of positive pressure ventilation, oxygen supply, and positive end-expiratory pressure (PEEP). However, some evidence indicates that negative airway pressure may develop during CSS if inappropriate ventilator settings are selected. The purpose of this in vitro, descriptive study was to delineate the airway pressures obtained during CSS with use of different ventilators and combinations of ventilator settings. Each ventilator was connected to a Vent Aid Training Test Lung and set at a ventilatory rate of 12/min and a tidal volume of 800 ml. For each ventilator, CSS was performed at peak inspiratory flow rates of 25, 40, 50, and 60 L/min, at sensitivities of 0.5, 1.0, 2.0, and 3.0 cm H2O, with and without PEEP of 10 cm H2O, and in modes of intermittent mandatory ventilation, assist control, and control. Airway pressure was measured at the Y-connector of the ventilator tubing. Two CSS trials were performed at each ventilator setting with the Ballard Trach Care System, a 14F in-line suction catheter, at a flow rate of 20 L/min for 10 seconds. During each suctioning trial, the peak positive and negative airway pressures were recorded. The negative airway pressure was above -10 cm H2O in most situations. In the control mode at all flow rates with or without PEEP, the Bennett MA1 and the Bourns-Bear 1 and 2 produced sustained peak negative airway pressure of less than -50 cm H2O.(ABSTRACT TRUNCATED AT 250 WORDS)