Ventilator "cycling" refers to the mechanism by which the phase of the breath switches from inspiration to expiration. Modes of ventilation are time cycled, volume cycled or flow cycled. Time cycling refers to the application of a set "controlled" breath rate. In "controlled ventilation" a number of mandatory breaths are delivered to the patient at a predetermined interval.
The respiratory rate may be controlled by the operator or the patient. The patient may breathe spontaneously, and with modern ventilators these breaths are supported either by delivering facsimiles of the controlled breaths synchronously with the patient's effort or by allowing the patient more subjective control. Pressure support is a form of flow cycled ventilation in which the patient triggers the medical ventilator and a pressure limited flow of gas is delivered. The patient determines the duration of the breath and the tidal volume, which may vary from breath to breath.