In pressure-support ventilation (PSV), which factor most directly influences the delivered tidal volume?

• A. The set respiratory rate
• B. The patient’s inspiratory effort with the set pressure support
• C. The PEEP level
• D. The FiO2 setting

Answer: (B)

In PSV, the breath size (tidal volume) is driven by the patient’s own inspiratory effort in combination with the fixed pressure support the ventilator provides. The ventilator adds a set amount of pressure above PEEP during inspiration, but how much air actually moves in depends on how hard the patient works to inhale and on the lung’s mechanical properties. Stronger inspiratory effort or a higher pressure support raises the tidal volume, while weaker effort or poorer compliance/resistance lowers it. PEEP and FiO2 mainly influence oxygenation and baseline lung volume, not the volume delivered per breath, and the respiratory rate mainly affects how often breaths occur rather than how large each breath is.