During positive pressure ventilation, which phase has positive pleural pressure?

• A. Expiration
• B. Inspiration
• C. Both
• D. None

Answer: (B)

During the inspiratory phase of positive pressure ventilation, the ventilator applies positive pressure to the airway, and this pressure is transmitted to the pleural space. As the chest wall and lungs are pushed outward to deliver the tidal volume, the intrapleural pressure rises from its normal negative value and can become positive, especially with higher airway pressures or with PEEP. In expiration, the airway pressure falls toward atmospheric, and the pleural pressure returns toward its negative baseline, so it is no longer positive. Therefore, positive pleural pressure occurs during inspiration.