Pressure controller: The ventilator maintains the same pressure waveform at the mouth regardless of changes in lung characteristics.
 
Flow controller: Ventilator volume delivery and volume waveform remain constant and are not affected by changes in lung characteristics. Flow is measured
 
Volume controller: Ventilator volume delivery and volume waveform remain constant and are not affected by changes in lung characteristics. Volume is measured
 
Time controller: Pressure, volume, and flow curves can change as lung characteristics change. Time remains constant.
 
When pressure is controlled, volume and flow can vary with each breath, depending on the compliance and resistance of the lungs. When flow or volume is controlled, the pressure varies. Current intensive care unit (ICU) microprocessor-controlled ventilators can change the way they function from breath to breath and even within the same breath.
 
Pressure-Controlled Breathing
When the ventilator maintains the pressure waveform in a specific pattern, the breathing is described as pressure controlled (also pressure limited or pressure targeted). The pressure wave­form is unaffected by changes in lung characteristics.
 
Volume-Controlled Breathing
When a ventilator maintains the volume waveform in a specific pattern, the delivered breath is volume controlled (volume limited, volume targeted). The volume and flow waveforms remain unchanged, but the pressure waveform varies with changes in lung characteristics.
 
Flow-Controlled Breathing
When the ventilator controls flow, the flow and volume waveforms remain unchanged, but the pressure waveform changes with alterations in lung characteristics. Flow can be controlled directly by a device as simple as a flowmeter or by a more complex mechanism, such as a solenoid valve. Any breath that has a set flow waveform also has a set volume waveform, and vice versa.
 
When the operator selects a flow waveform, the volume waveform is automatically established
(Flow = Volume change/Time; Volume = Flow x Time).
In practical terms, ventilator operators typically are more interested in volume and pressure delivery than the flow waveform.
 
Time-Controlled Breathing
When both the pressure and the volume waveforms are affected by changes in lung characteristics, the ventilator delivers a breath that is time controlled. Many high frequency jet ventilators and oscillators control time (both inspiratory and expiratory). Time-controlled ventilation is used less often than volume and pressure ventilation .
 
The ventilator typically measures variables in one of the three places:(1) at the upper or proximal airway, where the patient is connected to the ventilator; (2) internally, near the point where the main circuit lines connect to the ventilator; or(3) near the exhalation valve.