What cycles inspiration in PSV?

• A. Volume delivered
• B. Flow
• C. Set pressure
• D. Time

Answer: (B)

In Pressure Support Ventilation (PSV), inspiration is cycled based on flow. This mode of ventilation assists the patient during their inspiration by delivering a set pressure to help achieve adequate tidal volume. The key factor in determining when to stop delivering this pressure support is the patient’s own inspiratory effort, specifically the flow rate of the exhaled air.

When the patient starts to exhale, the ventilator monitors the flow of air leaving the lungs. Once this flow falls below a predetermined threshold, the ventilator recognizes that the patient has completed their inspiratory effort and discontinues the pressure support. This flow-based cycling allows for a more responsive and adaptive mode of ventilation that accommodates varying patient demands during the respiratory cycle.

In contrast, other options, such as volume delivered, set pressure, and time, do not play a direct role in determining when inspiration ends in PSV. Volume may be a target for the overall treatment, but it does not dictate when to cycle off the assistance. Set pressure is the level of assistance provided during the breath but does not cycle the breath on or off. Time-based cycling, in other modes, is not applicable in PSV where the cycle is primarily driven by the patient's flow.