Mean airway pressure rises with increases in all of the following except which?

• A. Total cycle time
• B. Peak inspiratory pressure
• C. Positive end expiratory pressure
• D. Respiratory rate

Answer: (A)

Mean airway pressure is the average pressure in the airways over the entire breath. It rises when the airway is exposed to higher pressures for more of the cycle. Increasing peak inspiratory pressure raises the pressure during inspiration, pushing the average up. Increasing end-expiratory pressure raises the baseline pressure maintained at the end of expiration, also increasing the mean. Increasing the fraction of time spent under pressure—by shortening the total cycle time or increasing the respiratory rate with the same inspiratory time—also raises mean airway pressure. However, lengthening the total cycle time means inspiration occupies a smaller portion of the cycle, which lowers the average pressure. Therefore, extending the total cycle time would not raise MAP.