The expression (PIP minus PLAT) divided by Flow equals which ventilator parameter?

• A. Airway Resistance
• B. Compliance
• C. Peak Pressure
• D. Tidal Volume

Answer: (A)

Understanding how pressure is split during inspiration helps. The peak inspiratory pressure (PIP) is the total pressure needed to push gas into the lungs and through the airway, combining the elastic pressure of the lungs (related to compliance) and the pressure needed to overcome airway resistance. The plateau pressure (PLAT) is measured during an inspiratory hold when there’s no flow, so it reflects only the elastic pressure of the lungs and chest.

The difference between PIP and PLAT isolates the pressure drop caused by airway resistance at the actual inspiratory flow. When you divide that pressure drop by the inspiratory flow, you get airway resistance (Raw), with consistent units of cmH2O·s/L.

So this expression specifically yields airway resistance.

Why the others aren’t the answer: compliance involves the relationship between tidal volume and the elastic pressure (often PLAT minus PEEP), not a pressure difference divided by flow. Peak pressure is the total pressure at the airway opening and includes both resistive and elastic components, not just the resistive part. Tidal volume is the amount of air delivered, unrelated to this pressure-flow ratio.