Which pathophysiologic condition increases peak inspiratory pressure (PIP) while transairway pressure (PTA) remains the same?

• A. Atelectasis
• B. Pneumothorax
• C. ARDS
• D. Pulmonary edema

Answer: (C)

When peak inspiratory pressure rises but the pressure drop across the airway—the resistive component—stays the same, the problem is lung compliance, not airway resistance. PIP reflects both the pressure needed to overcome airway resistance and the elastic recoil of the lungs and chest wall. If the resistive portion (transairway pressure) isn't changing, then the increase in PIP must come from needing more pressure to expand a stiffer lung.

In ARDS, the lungs become markedly less compliant due to diffuse alveolar damage, edema, and collapse of alveolar units. This stiffens the respiratory system, so higher pressures are required to deliver the same tidal volume, which raises PIP (and typically plateau pressure) without necessarily changing the resistive pressure across the airway. That pattern—PIP up with PTA unchanged—fits ARDS best.

Other choices involve changes in airway resistance or more variable mechanics, where the resistive component would be affected along with or instead of the elastic component.