The most appropriate immediate next step in management is to disconnect the endotracheal tube from the ventilator circuit and reconnect it after a few seconds. This patient has developed severe auto–positive end-expiratory pressure (auto-PEEP) as a result of breath stacking on the ventilator due to her obstructive lung disease. The intrathoracic pressure has risen to the point of impairing her central venous return, so her cardiac output is falling because of inadequate preload. Her oxygenation has worsened, owing to both her low cardiac output and to ventilation-perfusion mismatch in the lungs resulting from hyperinflation. The most immediate need is to release the pressure by disconnecting the ventilator circuit and allowing her to passively exhale for several seconds. The ventilator should then be reconnected and the settings changed to allow for more expiratory time.

Slowing the respiration rate, decreasing the tidal volume, and increasing the inspiratory flow rate while tolerating respiratory acidosis are strategies to increase the exhaled volume with each cycle. However, these interventions are not appropriate until after the increased intrathoracic pressure is released by temporarily disconnecting the ventilator circuit. This patient's unstable condition must be addressed before changes to the ventilator settings will be helpful.

Similarly, increased sedation and preparation for therapeutic paralysis may be useful for this patient, but her increased intrathoracic pressure must be released before other interventions are pursued.