The equation of motion in ventilator mechanics creates which pressure gradient?

• A. Alveolar pressure
• B. Peak pressure
• C. Transairway pressure
• D. Plateau pressure

Answer: (C)

The key idea is the pressure gradient that actually drives gas through the airways. In ventilator mechanics, the equation of motion shows that airway pressure is needed to overcome both airway resistance and the elastic recoil of the respiratory system, plus any baseline pressure like PEEP. The driving force for flow is the difference between the pressure at the airway opening (mouth/circuit) and the pressure inside the alveoli. That specific difference is the transairway pressure. It captures the pressure drop across the conducting airways that pushes gas from the ventilator into the lungs.

Alveolar pressure by itself is the pressure inside the gas spaces of the alveoli, not the gradient across the airways. Peak pressure is the maximum airway pressure during inspiration and includes contributions from both resistance and elasticity, but it is not the gradient across the airway. Plateau pressure is the alveolar pressure when there is no flow, again not the driving gradient. So the gradient that truly drives inspiratory flow through the airways is transairway pressure.