Cycling is the method for how a emergency ventilator knows to give a breath and stop a breath. Cycling is the governing system for how a breath will ultimately be applied. Parameters vary but rate, I:E and other similar parameters are almost always set by the clinician alongside the cycle.
Volume controlled
Volume controlled systems of ventilation are based on a measured volume variable that is set by the clinician. When the emergency ventilator detects the set volume having been applied, the emergency ventilator cycles to exhalation. This is measured various ways by each brand and model. Some emergency ventilators measure using a flow sensor at the circuit wye while some measure where the expiratory circuit plugs into the expiratory port on the emergency ventilator body, which means that the emergency ventilator will give all support to take breath to the patient.
Pressure-controlled cycling is based on an applied positive pressure that is set by the clinician. In pressure-controlled modes, the total volume is variable, as the emergency ventilator is using only the pressure as a measurement for cycling. Most emergency ventilators calculate pressure at the expiratory circuit though some measure near the circuit with a proximal pressure line.
Spontaneously controlled
Spontaneously controlled cycling is a flow-sensed mode dependent on a spontaneously breathing patient to cycle. Spontaneously controlled ventilation is typically only in reference to continuous spontaneous ventilation, also called continuous positive airway pressure (CPAP).
Negative pressure-controlled
Negative pressure ventilation cycles by producing a negative pressure around the chest and abdomen. Negative pressure moves across the chest and diaphragm and causes air to move into the lungs in the normal fashion.When the negative pressure stops being applied, the chest returns to atmospheric pressure and the inspired air then is exhaled.