In this ventilator mode, the breath cycle is defined by which variable?

• A. Time
• B. Volume
• C. Pressure
• D. Flow

Answer: (A)

The breath cycle is defined by time. In time-cycled ventilation, a preset inspiratory time determines when the breath ends—after that fixed duration, the ventilator ends inspiration and starts expiration. This means the amount of air delivered can vary with lung mechanics, because the cycle ends by a timer, not by reaching a target volume, pressure, or flow.

If the cycle were defined by volume, it would end as soon as the set tidal volume is delivered; by pressure, it would end when the airway pressure hits the target; and by flow, it would end when inspiratory flow falls to a low threshold. The key idea here is that the end of inspiration is governed by elapsed time.