What is the correct formula to calculate Raw?

• A. (PIP-Plat)/flow
• B. (PIP-Plat)*flow
• C. (PIP+Plat)/flow
• D. (Plat-PIP)/flow

Answer: (A)

Raw represents airway resistance and comes from the idea that the pressure drop driving flow equals flow times resistance. During a inspiratory hold, plateau pressure (PPlat) reflects alveolar pressure with no flow, while peak inspiratory pressure (PIP) includes the pressure needed to overcome both the lung/chest compliance and the airways. The difference PIP minus PPlat isolates the pressure loss across the airways themselves. Dividing that pressure difference by the inspiratory flow gives the resistance: Raw = (PIP − PPlat) / flow, with units of cm H2O per L/s. Using any other operation would not yield resistance (for example, multiplying by flow would not express pressure per unit flow, and simply adding or subtracting the pressures doesn’t reflect the pressure drop across the airways).