Ptp equals Palv minus Ppl. This defines which pressure?

• A. Transpulmonary Pressure
• B. Transrespiratory Pressure
• C. Transairway Pressure
• D. Transthoracic Pressure

Answer: (A)

Transpulmonary pressure is the pressure difference across the lung wall, defined as alveolar pressure minus pleural pressure (Ptp = Palv − Ppl). This gradient is what actually distends and keeps the lungs inflated. When pleural pressure becomes more negative during inspiration or alveolar pressure rises with inspiration, the transpulmonary pressure increases, driving lung inflation. In mechanical ventilation, this distending pressure is the driving force that determines how much the lungs expand, so understanding it helps assess lung stretch and risk of overdistension.

Other pressure terms describe gradients across different interfaces (airway to atmosphere, across the airways themselves, or across the chest wall) and do not quantify the pressure across the lung wall, which is why they aren’t the correct match for this equation.