In PSV, what determines the delivered tidal volume?

• A. The set respiratory rate
• B. The patient’s sedation level
• C. The patient’s effort and the set pressure support
• D. The PEEP level

Answer: (C)

In pressure support ventilation, breaths are initiated by the patient and amplified by a preset pressure boost. The size of each breath, or tidal volume, comes from the interaction of two main factors: the patient’s own inspiratory effort and the amount of pressure support the ventilator provides. The lung and airways’ compliance and resistance also shape how much volume actually moves for a given pressure, so the same pressure support can yield different volumes in different patients or at different lung conditions.

That’s why the delivered tidal volume isn’t set by the respiratory rate, nor by PEEP alone, and not directly by sedation level. The rate only changes how often breaths occur; PEEP sets a baseline pressure to keep airways open but doesn’t determine how much air is drawn in each breath. Sedation can reduce effort and thus lower tidal volume, but the fundamental determinant remains the combination of the patient’s effort and the set pressure support.