Maxim 3: A ventilator assists a breath using either pressure control (PC), or volume (flow) control (VC).

Equation of Motion.PNG

Regardless of patient effort (Pmusc), if the ventilator provides work to deliver a tidal volume, circuit pressure will rise (Pventilator).

Pressure Control:

In pressure control, the ventilator controls the set pressure within the circuit, and air flows towards the patient in response to this gradient. Part of that pressure would be used to overcome the resistance of the circuit and the patient's airway, and another part will be used to overcome the elastic recoil as tidal volume is being built up.

Whether the pressure differential applied is completely transmitted to the patient's alveolar spaces depends on the length of I-time in relationship to their lung mechanics (see the section on time constant).

Volume Control:

In volume control, the ventilator controls the flow into the circuit over the set I-time, effectively injecting a set tidal volume into the circuit and patient. Pressure within the circuit and the patient's lungs will build up across time depending on the patient's lung mechanics. Constant flow over I-time is the simplest VC implementation and leads to a square shaped inspiratory flow waveform. Other implementation of VC could use triangular flow waveforms and can be descending or ascending "ramps". Pressure in VC is a byproduct of pressure needed to overcome elastic recoil, and pressure built up as flow is forced through the circuit and the patient airways.

During VC, if after the tidal volume (VT) is delivered a pause is inserted as shown above *, pressure will drop to a plateau. The pressure drop represents the pressure needed to overcome circuit and patient airway resistance, while the horizontal plateau pressure reflects the alveolar pressures.

To ensure accurate volume delivery to the patient, modern ICU ventilators will often compensate for "compressible gas volumes". These are losses in VT due to the compliance of the circuit, and gas compression within components of the circuit. While this is reliable to a certain extent in larger patients it is not so for the smallest of them.

Time Control:

Some ventilator modes will control inspiratory and expiratory times, with pressure, flow, and tidal volume being relatively dependent variables on the lung mechanics. Examples include various high-frequency modes such as high-frequency oscillatory ventilation using a Carefusion 3100/3100A oscillator. On the oscillator, the ventilator controls inspiratory / expiratory times as a function of frequency and I:E ratio, and applies mechanical power to the circuit. Delivered pressure to the lungs depends on the circuit, airway resistance and lung compliance. The signal is attenuated as it pulses down the circuit, endotracheal tube, and the bronchial tree.